Skywatcher DC Motor drive for PST etalon.

Frankenscope? Let's see it!***be advised that NOTHING in this forum has been safety tested and you are reading and using these posts at your own peril. blah, blah, blah... dont mess around with your eyesight when it comes to solar astronomy. Use appropriate filtration at all times...
Post Reply
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

I don't want to hijack the present threads on motor driven, etalon tuning any more than I already have.
Th builder's deserve the admiration of those of us who value their contribution to "hands-off" or remote imaging.

My task is to drive the standard PST etalon adjustment band reliably using a Skywatcher [clone] 12V DC gearbox motor.

The PST etalon adjustment band is 60mm is diameter.
I wrapped a thin cord, Prusik loop around the etalon band to measure its resistance to turning.
This shows that 2.8 lbs or slightly over 1.25kg is enough for reliable rotation at a tangent to the etalon band.

The Skywatcher [clone] motor can easily lift 1.5 kg or 3.3 lbs via toothed, timing belt.
I attached the same Prusik loop cord to the belt and loaded the cord with known weights.
There was no hesitation [at all] in lifting this weight when 12V DC power was applied briefly.

A typical timing pulley for this little, gearbox, DC motor would be about 9mm in diameter.
So there is a ratio of at least 10:1 in favour of the drive motor in turning the etalon band.
Ignoring losses, reduced speed equals increased torque.

The PST etalon band is structured rather than toothed. It is also rather "slick" or slippery.
Suggesting that increased surface grip will reduce the need for excessive, timing belt tension.
A suitable rubber band stretched over the etalon band [or the smooth, metal drum beneath it] should suffice.

I have ordered a couple of Skywatcher DC focuser motors.
The other motor will be applied in exactly the same manner to the 2" long throw, helical focuser of my modified 6" f/8 H-a 'scope.

While I await the arrival of the new motors I shall use my existing motor to experiment with the PST etalon drive.
I need to find a suitable [continuous] timing belt to allow the motor to clear the PST adapter on which it will be mounted.


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Merlin66 »

Look forward to seeing your results.....


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
User avatar
marktownley
Librarian
Librarian
Posts: 42272
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Has thanked: 20437 times
Been thanked: 10245 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by marktownley »

Indeed, sounds interesting.


Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

Thank you both.

The Skywatcher DC focuser motor surprised me in its lifting power with the hefty FT 3.5 + 2" Lacerta prism.
This gives me confidence that the same can be achieved with the PST etalon and helical focuser.
High rotational speed is not required so the lowest possible gear ratio makes most sense.
I like to build things which others can easily copy at minimal expense while avoiding any machining.
To that end I'm still looking for affordable, standard tube rings or clamps to fit the AOK etalon adapters.
No point in reinventing the wheel.

The present run of grey weather needs constructive work for idle hands.
I'm suffering from solar, sensory deprivation. That usually means burning plastic! ;)


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
User avatar
Martin_S
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1839
Joined: Sat Mar 16, 2019 12:34 am
Location: Brisbane , Australia
Has thanked: 979 times
Been thanked: 2992 times

Re: Skywatcher DC Motor drive for PST etalon.

Post by Martin_S »

Chris, I also read your blog regarding this topic, you should put some pictures up showing your progress. The motorized etalon tuning on my Lunt scope has been a real game changer for me as I don't do visual solar observing. The ability to Focus my telescope and tune my etalons remotely makes my imaging sessions a breeze. I wonder if the design principles of your modification would be feasible on pressure tuned etalons. The more types of solar scopes we can adapt to remote operation will make solar observing and Imaging a lot more user friendly and hopefully entice more amateur astronomer's over from the darkside. I wish you all the best in your endeavor .

Martin


H alpha : ,Skywatcher 120mm F8 open frame refractor, a tilted 2" 7nm H-alpha filter as a sub energy rejection filter, Baader TZ4 telecentric focal extender, Player One energy rejection filter, Combo Quark Chromosphere, Naked PST etalon for double stacking, ASI74mm camera
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

Hi Martin and thanks. Your setup all sounds great.

As has been pointed out earlier the standard PST etalon adjustment is very stiff compared with other types.
First trials showed I needed a lot of belt tension to get the PST etalon band to move.
I have discovered that the thin, O-rings, between the two etalon rings are where all the friction lies.

Alternative O-ring lubricants [silicone] don't seem to help very much at all.
Remove the O-rings altogether and everything becomes very sloppy.

I tried wrapping the inner etalon drum with PTFE/Teflon plumber's tape in place of the O-ring.
Friction vanished but the outer ring/band was still too sloppy.

After lunch I shall try a wrap, or two, of neatly applied, electrical tape to see if it provides a firmer bed for the outer ring.
Hopefully I can find a nice balance between low friction and high stability without returning to the O-rings.

The etalon drive screw retains the outer ring but stability is still important for belt driving.
The belt will ensure that the etalon doesn't retune involuntarily despite the lack of friction.

I can well understand the choice of O-rings in the original design but think they overdid the friction aspect.
It was so hard to turn mine that I actually believed I was distorting the etalon housing by gripping the band between my finger and thumb!

I usually photograph my projects to death but I haven't had anything worth sharing yet.


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

PST etalon drive Update: Electrical tape is too thick even in a single layer!

Teflon plumbing tape is fine if enough is used and laid as wide, flat and smooth as possible from the reel.

Very little remaining friction and enough stability to offer effortless driving by the motor and belt.

O-rings not needed.

Yet to find suitable, etalon adapter, sized clamps @ 61mm Ø to hold the motor brackets neatly.


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

Another update: Both timing belts proved to be too long.
I ordered 280 and 300mm GT2 assuming 70mm C-to-C between motor and etalon.
The stockist had no belts in a slightly smaller size. 250-260mm.
I have ordered some from eBay but must wait for delivery again.

The etalon band now rotates effortlessly with a wrap of Teflon plumbing tape on the inner drum.
I had run out of tape and bought some more while I was out looking for suitable clamps.

Again I drew a blank on clamps. They had no 63-65mm in stainless steel hose clamps.
So I bought 60mm and just managed to trap the provided motor bracket at the thin end.
I can find bigger hose clamps easily enough elsewhere and they don't look too awful in slim SS.
Attachments
P1400302 rsz 600.JPG
P1400302 rsz 600.JPG (63.18 KiB) Viewed 7780 times


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
User avatar
marktownley
Librarian
Librarian
Posts: 42272
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Has thanked: 20437 times
Been thanked: 10245 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by marktownley »

Interesting to see how this develops


Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!
User avatar
Martin_S
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1839
Joined: Sat Mar 16, 2019 12:34 am
Location: Brisbane , Australia
Has thanked: 979 times
Been thanked: 2992 times

Re: Skywatcher DC Motor drive for PST etalon.

Post by Martin_S »

How many degrees of rotation does the etalon need to move to bring it on band?


This link though slightly off topic might be tp your advantage

https://www.engineersedge.com/calculato ... vers_1.htm


Martin


H alpha : ,Skywatcher 120mm F8 open frame refractor, a tilted 2" 7nm H-alpha filter as a sub energy rejection filter, Baader TZ4 telecentric focal extender, Player One energy rejection filter, Combo Quark Chromosphere, Naked PST etalon for double stacking, ASI74mm camera
User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Merlin66 »

The full travel of the PST etalon tuning is about 160 deg.

Rusted,
Looks promising....not sure how to fit it to a standard PST.....not too many members will be game to dismantle their PST etalon to apply Teflon tape.


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

Relax! ;) The etalon mods are very simple indeed. There is nothing to break if you use watchmakers screwdrivers.
Though I am assuming that the entire etalon housing has already been removed from the PST for further mods.
It need not be removed, but it will be very awkward to work on a standard PST still in one piece.
Just unscrewing the gold tube from the etalon will help enormously.
PSTs are notorious for being glued together with [rock] hardened locking fluid.
It needs good quality rubber clamps to get the PST apart without damage.
These band type tools are normally used to remove tight oil filters from car engines.

Meanwhile back at the PST etalon housing:

You have to lift the ribbed rubber adjustment band off first. I use a small screwdriver. Slide it under the edge of the knurled band.
Then move the screwdriver around the etalon's circumference to slide the band off little by little.

Now remove the small, etalon drive screw. It's head is deliberately hidden under a piece of white paper tape for security.
No need to remove the tape. Just poke about with a small cross-head screwdriver until you find the screw head's socket.

Once the [12mm long?] drive screw is removed the outer shell will be free to slide off the etalon housing.
It is quite stiff to remove because of the hidden O-rings. These are where the friction lies in normal use.

Once you have the outer shell free you have to remove the O-rings.
Poke a flat bladed watchmakers screwdriver under each ring in turn and lever the O-ring gently upwards and off the etalon housing.

Now you have a bare, metal, inner drum with a slot for the adjustment screw to drive the hidden etalon.
Don't move the etalon arc in the slot without noting its position first!
If you do move the etalon arc with a screwdriver or cocktail stick it should be returned to its original position.
Otherwise you may never reach proper tuning because the drive screw will hit the end of the slot too soon.

I'll take some pictures this morning to show each step if it stops raining.
I need proper daylight to capture some decent pictures of these largely, black anodized components.


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Merlin66 »

Rusted,
In a previous thread I’ve detailed the PST etalon construction and the replacement of worn O rings.
viewtopic.php?f=9&t=25461
Getting a semi remote PST etalon tuning will be of interest to many, an easy addition of a drive would be welcome.


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

Thanks.

I have added two, new, heavily illustrated, "chapters" to my blog showing the dismantling of the outer shell of the PST etalon.

The "innards" of the delicate etalon group remain completely untouched and unmodified.
Think of this mod. as simply removing the O-ring packing and then Teflon wrapping the inner shell.

The Teflon tape takes up the space between the inner and outer shells and removes the heavy O-ring friction.
The vital drive screw holds the whole lot together just as it did with the O-rings in place.

I can't imagine why anyone with a standard PST would need remote tuning.
So have assumed the mod is aimed at Stage 2 PST Modifiers.

I can add my photos here but they must all be resized smaller to avoid slow downloads.
My symmetrical, 200Mbps fibre Internet means instant downloads but not all are remotely so lucky.

On my blog I make the images "small" so the pages download quickly.
Only those images of interest need be enlarged individually with a left click.
On the forum it's strictly WYSIWYG on image size. So I am being far more cautious.
I think 400 pixels max would probably do it.
Attachments
P1400315 rsz 400.JPG
P1400315 rsz 400.JPG (32.9 KiB) Viewed 7754 times


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

Some resized images of the PST etalon in various states of undress.
For those who haven't heard of them before the large etalon adapters are from AOK Swiss.
These have male and female 2" push fits and matching etalon threads as appropriate.

Before proceeding further I intend to try one O-ring on the unsupported [non-taped] side.
The Teflon tape wrapping obviously can't cover the activation slot or the drive screw will be blocked.
The present outer shell stability is adequate [after wrapping] but it is worth checking with one O-ring.

An alternative to Teflon tape wrapping would be to buy slightly thinner O-rings of the same major diameter.
This would be a more mechanically "sophisticated" option [than tape wrapping] if they are available.
All the motor mod requires is a reduction in friction from the standard PST level.

A third alternative: I might try thinning the existing O-rings by fitting them snugly on a turned dowel, pipe or bar.
Then wrapping them with a tube of abrasive paper to flatten the active surfaces slightly.
This would require frequent trial fittings on the etalon to avoid "overdoing" the thinning.
The O-rings should remain oriented correctly provided they aren't stretched too tightly on the sanding mandrel.

Those with a lathe [like myself] can groove a suitable sanding cylinder which exactly matches the diameter of the inner etalon shell.
Cleaning up the O-rings between "sandings" would be sensible.
Attachments
P1400331 rsz 400.JPG
P1400331 rsz 400.JPG (47.93 KiB) Viewed 7750 times
P1400315 rsz 400.JPG
P1400315 rsz 400.JPG (32.9 KiB) Viewed 7750 times
P1400322 rsz 400.JPG
P1400322 rsz 400.JPG (25.46 KiB) Viewed 7750 times
P1400317 rsz 400.JPG
P1400317 rsz 400.JPG (28.48 KiB) Viewed 7750 times
P1400311 rsz 400.JPG
P1400311 rsz 400.JPG (46.1 KiB) Viewed 7750 times
P1400308 rsz 400.JPG
P1400308 rsz 400.JPG (40.56 KiB) Viewed 7750 times
P1400307 rsz 400.JPG
P1400307 rsz 400.JPG (30.25 KiB) Viewed 7750 times
P1400306 rsz 400.JPG
P1400306 rsz 400.JPG (44.51 KiB) Viewed 7750 times


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Merlin66 »

Rusted,
Look forward to seeing the results of your drive unit.


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

Merlin66 wrote: Sat Nov 23, 2019 7:20 am Rusted,
Look forward to seeing the results of your drive unit.
Thanks.

I have just tried 1.2mm Ø plastic coated, copper stranded, electrical flex in place of the o-rings.
There is still a little bit of slop in the etalon outer shell but nicely low, friction levels.
It hardly matters but the free play just seems like an unnecessary compromise.

On Monday I shall buy some 1.6mm grass strimmer/trimmer monofilament nylon.
Cheap as chips in 15m [50'] lengths and globally available I would imagine for commercial grass trimmers.
This thickness of nylon should take up the remaining slack and still be easy to fit into the o-ring grooves during assembly.
While providing a nicely slippery [low friction] surface between the inner and outer, etalon shells.

I think we are fast approaching a low skill, low risk, compromise-free, cheap, universal, remote control, motor drive for the PST etalon.
I just need to confirm the correct belt length now. I'm expecting 250mm x 6mm wide, GT2 2mm pitch belts to be ideal.
With one small, 16T timing pulley/sprocket needed to fit on the 5mm motor shaft.

This is using the Skywatcher [clone] motor and its supplied bracket.
To fit the motor straight onto the AOK Swiss rear etalon adapter.
Using 63-64mm stainless steel, screw driven hose clamps. [aka. Jubilee clips or worm drive, hose clips.]

Total cost around £65GBP or $85US depending on source and delivery charges.
These 9-12V DC gearbox motors are widely sold with different names and prices.
I am going by UK/European pricing and assuming you already own the AOK etalon adapters.
Which would obviously be extra.

All this assumes that the etalon has already been freed from the Meade-Coronado PST.

https://fullerscopes.blogspot.com/2018/ ... -form.html


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

A quick update to save people wasting time and money on lower friction packing materials.
To replace the PST O-rings [50x2mm] for remote, etalon motor drive.

1.6mm monofilament, for a grass trimmer, is slightly too thick.
The friction is very close to the original PST O-rings.
Really not worth the effort.IMO.

I found some thinner 1.3mm monofilament for a lighter grass trimmer.
This is slightly better than the plastic covered, 1.3mm multicore copper flex but not noticeably.
The outer etalon shell is still slightly floppy for my tastes. Though perfectly usable, with very low friction.
I wouldn't t waste time or money on this material if something better is available locally.

1.5mm monofilament fishing line [200 lbs?] is available but not locally enough for me.

After searching online I have finally ordered some 50mm x 1.5mm nitrile O-rings.
Ridiculously cheap for a packet of 10 [£1.50] but delivery time from the UK is unknown.
I'm hoping for EU letter post and only a few days to arrive.
Then I will report back if I have found a valid alternative to the original PST O-rings [50x2mm] for motor drive.

Still waiting for the eBay[UK] sourced 250mm 120T GT2 timing belts.
The local stockist was out of stock of this one size.


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Merlin66 »

Rusted,
The original PST O rings were 47 mm x 1.7 mm.


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

Merlin66 wrote: Tue Nov 26, 2019 4:05 pm Rusted,
The original PST O rings were 47 mm x 1.7 mm.
Really? Are these figures from actually measuring the O-rings? Can there have been changes over the lifetime of the PST?

I measured the grooves first and then the thickness of the O-rings with several Mitutoyo vernier calipers.
It being difficult to measure a loose O-ring's major diameter without a conical gauge I chose to measure the bottom of the groove instead.
Now I have O-rings on order which will not be stretched but should still offer some extra shell clearance on thickness alone.
If I re-order and aim for 47mm, for some stretch, I will have to allow for shrinkage in thickness with a wild guess.
Do O-rings shrink in thickness with stretching like a normal rubber band?


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

It get worse!!

I turned a mandrel for my O-rings to just slip over without stretching.

I now make the internal diameter 42mm!

After measuring several times, with different vernier calipers, I still cannot make my O-rings any thinner than 2mm.

I know these rings will be very lightly loaded so wonder if it really matters if the new rings are stretched or not.
I'll have to wait until they arrive and then order more accordingly.


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Merlin66 »

Rusted,
These are actual O ring measurements 47mm x 1.7mm.
I bought a minimum batch (20?) and they have been successfully used around the world to "improve" PST's by reducing the slop caused by worn O rings.
They are stretched a little to firmly sit in the grooves...the width of the grooves we measured was 1.8mm x 1.45mm deep.
viewtopic.php?f=9&t=25461


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

Merlin66 wrote: Wed Nov 27, 2019 10:35 am Rusted,
These are actual O ring measurements 47mm x 1.7mm.
I bought a minimum batch (20?) and they have been successfully used around the world to "improve" PST's by reducing the slop caused by worn O rings.
They are stretched a little to firmly sit in the grooves...the width of the grooves we measured was 1.8mm x 1.45mm deep.
viewtopic.php?f=9&t=25461
That is really weird. I can only report what I find when I repeatedly measure my own o-rings.
There was no sign of my secondhand PST donor having been dismantled.
The drive screw was still covered in the paper tape so the O-rings could not have been touched.

My etalon turning ring has always been difficult to turn. Irritatingly so.
Even after I tried different lubricants it did not improve matters.

I drew a complete blank trying to obtain thinner o-rings locally.
So have had to order blind from the UK.
What should have been a quick mod for remote etalon tuning has turned into a marathon!
I am now at the mercy of the European postal service with potential delivery dates up to Christmas!

Thanks for the link to the other thread on replacement O-rings.
At first guess it is hard to see how outer shell stability affects the inner etalon.
I suppose it is possible that the drive screw could rock from side to side.
Which might apply lateral forces on the etalon.

I haven't tried imaging since I fitted the rather sloppy 1.3mm monofilament in place of my rubber o-rings.
I'm using an extended period of continuous, heavy overcast as an excuse to play with the toys.
No sunshine forecast before Friday at the earliest.

I might place an order with your source for a range of o-rings to try.
The friction must be reduced for the belt drive to work but with minimum slop.


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

Duplicate post.
Last edited by Rusted on Tue Dec 17, 2019 2:44 pm, edited 1 time in total.


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

Finally! After three weeks and three days in the post the new timing belt showed up. 250mm x 6mm 2GTS.

This length of timing belt will allow a 16t pulley [sprocket] to be used without any modification.
Any larger pulley 20T would need a curve filed in the smaller "Skywatcher" motor support bracket.
The bracket is badly angled as supplied and must be bent to a true 90°to ensure a straight belt run.
Two, screw type, hose clips 64mm Ø will clamp the supplied [smaller] motor bracket safely to the AOK etalon tail adapter.
Stainless steel is slimmer and better looking than galvanised screw clips.

P1400423 rsz 600.jpg
P1400423 rsz 600.jpg (78.73 KiB) Viewed 7448 times
It is heavily overcast today so the following video is pretty awful.
Here I am using a HitechAstro DC focuser controller via its manual push buttons.
In practice, the DC controller software will put a useful focuser panel on the video capture computer screen.
The "buttons" on the screen panel will allow pseudo "stepper" mode or continuous motor runs in either direction.
The timing belt will slip safely at each end of the etalon tuning travel.
The PST etalon O-rings have been replaced by a single wrap of 1.3mm monofilament nylon.
You are free to experiment with alternatives and alternative O-rings but etalon band friction must be low.

P1400435 rsz 600.JPG
P1400435 rsz 600.JPG (116.19 KiB) Viewed 7448 times
I tried hard to avoid anything which was not easy to find or could be used without any further work.
The focuser motor kit contains the necessary bracket.
You'll need to find the 250mm x 6mm 2GTS belt, a 16T timing pulley x 5mm bore and two x 64mm stainless steel hose clips.


https://youtu.be/ZFDys4BFYb0

Can't seem to embed the YT video.


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Merlin66 »

Looks promising....
How does it perform in real life?
What about a solar video shows the tuning/ de tuning??


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
User avatar
marktownley
Librarian
Librarian
Posts: 42272
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Has thanked: 20437 times
Been thanked: 10245 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by marktownley »

Good result!


Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

marktownley wrote: Wed Dec 18, 2019 6:22 am Good result!
Thank you Mark.
Sorry it took so long to complete the project after my initial posts.
I really thought it would only be a couple of days for the belts to arrive.
But I was at the mercy of 18th Century European postal speeds!
With regular stops for fresh draw horses for the 2mph narrow boats along the British canal system. :roll:

Anybody who can lay their hands on the simple materials shown in this thread can have a motorised etalon.
Without requiring any machining, special manual skills or expensive parts.
Despite any attached label or increased price, all of these DC focuser motor clones look identical.

The worst part is freeing up the etalon rotation to allow the little motor to easily drive the knurled band.
I am still hoping to find an everyday replacement for the original O-rings somewhere in the 1.4-1.5mm Ø range.
It may end up needing heavy, sea fishing monofilament but the motor drive belt system doesn't care about a bit of "slop."

The "packing" line material really doesn't have to be in the form of closed, rubber rings.
It is only a means of support for the etalon outer shell and its "knurled" rubber band.

A length of solid monofilament line, cut just short of the circumference of the etalon housing grooves, is all it needs.
Again I thought it would be simple to find such a line in common weed trimmers but found only 1.3, 1.6, 1.65 and 2mm Ø.
The 1.3mm is slightly too small and causes a floppy etalon band. All the others are just too large and jam the etalon movement.


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

The cloud was a real teaser today but I managed a few, short trials with the etalon tuning motor.

P1400455 rsz.JPG
P1400455 rsz.JPG (138.33 KiB) Viewed 3545 times


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
User avatar
Rusted
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1738
Joined: Fri Jan 19, 2018 11:00 am
Location: Central Denmark
Has thanked: 8016 times
Been thanked: 1938 times
Contact:

Re: Skywatcher DC Motor drive for PST etalon.

Post by Rusted »

A small update: A slightly shorter 240mm timing belt arrived in the post.
This meant I could have a lower profile on the etalon drive motor than with the 250mm belt.
I had to use the heavier of the two motor support brackets supplied in the focuser motor kit.

The bracket is too long and both the support bracket and motor plate need a curve filed on their ends.
Best to remove the motor plate to avoid getting metal dust in the motor/gearbox from filing.
[Remove the two, tiny, black, cross-head screws.] File and de-burr, then clean the plate carefully before refitting.

P1400488 rsz 600.jpg
P1400488 rsz 600.jpg (88.54 KiB) Viewed 3509 times


http://fullerscopes.blogspot.dk/

H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
Post Reply