H-alpha telescope with Omega Optical filters

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H-alpha telescope with Omega Optical filters

Post by GreatAttractor »

This thread will hopefully end by showing some pictures from a working H-alpha scope :]

After long deliberations, reading SolarChat and David Groski's website I've decided to go with my Bresser 102/1000 mm achromat and the Omega Optical H-alpha filter pair. My primary goal is prominence imaging with good resolution, in medium to small FOV. Visual use is not important (it seems that the filter combination described below should be safe, but I'd rather not risk my retinas; besides, I've looked a few times through a double-stacked SM90 and until I can afford or built something with similar full disk image quality, the visual can wait...).

To keep the overall length down I've opted for a negative/collimating + positive/imaging lens system:



There will be a 90 or 110 mm Baader D-ERF at the front, and between L2 and L3 a KG3 UV/IR cut from Beloptik + one (for proms only) or both OO H-alpha filters.

H-alpha filters are still in transit, but I've already received the lenses from Macony Design, Germany ("md-optics" on Ebay) - a negative meniscus and a planoconvex one. Today I built a (very) makeshift holder out of some PVC pipes and tested the image quality (using Chameleon mono + UV/IR cut + Solar Continuum). The test assembly is wobbly and poorly collimated, but the results weren't too bad.



Objective lens (L1) only:



L1+L2+L3, first attempt:



A serious miscollimation (note the halos above objects). I was expecting that I'd just have to make blind adjustments and trials, but luckily it turned out that when I look down the tube from the positive lens's side, I can see the reflection of "my" tube opening (and my face) in the concave side of the meniscus. This reflection's size is about half the tube's diameter in size and here it was clearly off-center. Carefully turning the nylon screws (which simply press on the meniscus edge) this way and that was enough to bring this reflection to the center.
Second attempt:



Not bad, considering that the pipe itself sags in the drawtube (it's 50 mm OD, not 2") and both lenses are rather not adjusted with L1. Anyway, eventually everything will be mounted in threaded rings machined from a nice 60x5 mm piece of aluminium pipe. I will keep you posted.


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Re: H-alpha telescope with Omega Optical filters

Post by hunterknox »

Loving this thread already. I'll watch with interest...


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Re: H-alpha telescope with Omega Optical filters

Post by marktownley »

Cool stuff! I'm thinking of building a collimating system to house my cak filters...


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Re: H-alpha telescope with Omega Optical filters

Post by GreatAttractor »

Finally, 26 days after shipping (grr... customs processing), I received the Omega filters:



The thick assembly is described in attached documentation as:

"One of the two has secondary elements to control the spectral region from the UV to about 1 micron (...)"

When held against the Sun, it gives a dark red image, somewhat darker than ND 3.8 film.
The second filter is just an etalon; due to leaking other wavelengths, it is bright cyan when looking through it.

Meanwhile the 90 mm Baader D-ERF has arrived too:



A respectable chunk of glass. The arrow indicates the Sun-facing side (I think; will have to re-read the attached info). When held against the Sun, the filter is very bright (brighter than ND 3.8 film).

Long IR blocker on KG3 from Oliver is on its way, so the last thing remaining is drawing and ordering the adapters.


My software:
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Re: H-alpha telescope with Omega Optical filters

Post by markthais »

Well, the filter with the ring is your (Trim filter) or ITF. The red is for the short side blocking and the other coating inside is the long side blocking to at least 1000nm. The other filter is your bandpass. The arrow is either showing you which side is the cover glass or if there is an AR then it is showing where the AR is. Either way it doesn't matter which way you face it. Now if that is a hard coated bandpass then it is showing the coated surface. If it was me I would have the bandpass first then the trim so that you are looking into the red side of the filters.
Oh by the way, 26 days for a custom filter is not bad. A normal time is 8 to 10 weeks. Did they send the bandpass scan with the filter?
Mark W.


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Re: H-alpha telescope with Omega Optical filters

Post by GreatAttractor »

Mark, let me clarify - these are "old stock" filters (http://www.ebay.com/itm/Optical-Interfe ... 232538a706), they were sent immediately after I made the payment. I'm guessing they were delayed by customs on my side (the price is high enough that I had to pay import tax+VAT; however it was still cheaper in total than a used PST). A fellow solar imager here in Poland ordered some wider, cheaper filters from the same seller and received them in a week.
Transmission graphs are included (only for H-alpha vicinity).


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Re: H-alpha telescope with Omega Optical filters

Post by GreatAttractor »

I have all the filters now, so I've spent the last few hours assembling it (PVC pipes for now) and managed to actually point the whole contraption at the Sun!

Here's how I've mounted the D-ERF:



It's held in place directly above the objective lens by thick rubber bands from a pipe holder. Very tight and secure.

This time I secured the negative and positive lenses with pairs of rubber seals - stronger and easier to collimate than the screws. I put Oliver's KG3 filter and one of the OO etalons (1.5 Å) inside a spare eyepiece tube and also squeezed it in place with more seals.

I checked the collimation on a ground target (without etalon), then added the filters and pointed at the Sun at last. It was already past 7 pm, the Sun was moving through some thin clouds, the pipes were sagging, my only mean of focusing was to slide the camera nosepiece inside a pipe... but I forgot all that when I was rewarded by chromospheric details appearing on the screen, oh, it was worth it :woohoo:

Single frames:







And even some surface detail:



I was trying out various gamma and shutter combinations and I can see this will take some learning.


My software:
Stackistry — an open-source cross-platform image stacker
ImPPG — stack post-processing and animation alignment
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SW Mak-Cass 127, ATM Hα scopes (90 mm, 200 mm), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
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Re: H-alpha telescope with Omega Optical filters

Post by swisswalter »

Hi Filip

great move forward. Keep on the good work. I hope you are more sucessful then I was


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Re: H-alpha telescope with Omega Optical filters

Post by marktownley »

Impressive! better results than I got with the Omega filters...


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Re: H-alpha telescope with Omega Optical filters

Post by GreatAttractor »

Second light: quick single frames between clouds.






My software:
Stackistry — an open-source cross-platform image stacker
ImPPG — stack post-processing and animation alignment
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Re: H-alpha telescope with Omega Optical filters

Post by swisswalter »

Hi Filip

another fine result in the right direction. Is that what you can get in terms of sharpness or is there more possible ?


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Re: H-alpha telescope with Omega Optical filters

Post by GreatAttractor »

I'm sure it will be better in the end. For the second light I think I got the adjustment of lenses and the etalon worse (got H-alpha detail only along one edge of camera's FOV), and later I also saw some light leaking around the etalon due to a skewed seal. Plus this time the Sun was higher and the pipes were sagging even more... The next time I post pictures I'll have it all mounted properly.

Oh, and it was through an open window on a hot day, all turbulent like hell ;)


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Re: H-alpha telescope with Omega Optical filters

Post by swisswalter »

Hi Filip

good luck on the third light. You are right, shooting out of a window doesen't really help ;)


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Re: H-alpha telescope with Omega Optical filters

Post by GreatAttractor »

Today was the third light, with the adapters I've ordered:



The first holds the negative lens, the second the etalons, and the third the positive lens. The lenses are currently not adjustable, but the threaded holes for collimating screws are there. Everything attaches to the focuser drawtube, so no OTA modifications were needed:



I haven't decided on the focuser yet (unfortunately nobody makes tiny, light Crayfords). A non-rotating helical would be nice, or I'll just borrow Baader Steeltrack I use with my big Mak. For now it's just a sliding pipe like in previous pictures.

My tilting etalon mount is inspired by Sky-Watcher finderscope mount: the element is held by flexible ring at one end, and at the other it's tilted by two screws pressing against a spring-loaded pin. Currently it's realized by a 50/32 mm pipe reduction and a spring extracted from a clothespin. The etalons (and the KG3 IR cut) are mounted in short eyepiece tubes:





This is the image I get with a single etalon (horizontal dark 1-pixel bands are Chameleon's artifact due to very short shutter):



I suspect that this shape of on-band area is caused by the negative lens's focus not exactly overlapping with objective's focus and/or by their axes being not parallel. Will test and try.

I tried to disassemble the second etalon "combo" (as pictured in previous post); I carefully peeled of the black metal ring, but it turned out the filters are additionally glued together (and, as I wrote, internal reflections give multiplied image of the Sun).

Anyway, so far I was unable to get any H-alpha detail at all when using both etalons together. This will need some more experimenting. Single etalon however already gave me a lot of fun; images in the main forum area.


My software:
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ImPPG — stack post-processing and animation alignment
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Re: H-alpha telescope with Omega Optical filters

Post by swisswalter »

Hi Filip

thank you for the update. I see progress. I wish you good luck geting them running


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Re: H-alpha telescope with Omega Optical filters

Post by colinbm »

Hi Filip
I've got the TravelScope 50 set-up now, & I have an Omega Ha pair, so I will be looking closely at doing this too Thanks :thumbsup:
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Re: H-alpha telescope with Omega Optical filters

Post by GreatAttractor »

Good luck! I'll try this on my TS 50 too at some point (however it won't fit full disk on the sensor, as the effective f.l. will be 421 mm, a bit too much for Chameleon).


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Re: H-alpha telescope with Omega Optical filters

Post by sullij1 »

Execllent work. You and Peter Z are my heros. :bow:


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Re: H-alpha telescope with Omega Optical filters

Post by GreatAttractor »

I'm not sure, perhaps the thick combo could be used this way. On the other hand, the etalon itself could be perhaps used in a double-stacked configuration with a PST or Lunt.


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Re: H-alpha telescope with Omega Optical filters

Post by GreatAttractor »

I've managed to provisionally tune both etalons, but the on-band FOV part was very small and surrounded by blown-up areas. Will have to adjust the negative lens properly and use some more flexible etalon mountings.

Current setup in a combat configuration:




My software:
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ImPPG — stack post-processing and animation alignment
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Re: H-alpha telescope with Omega Optical filters

Post by swisswalter »

Hi Filip

a great combat configuration, allez si!


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Re: H-alpha telescope with Omega Optical filters

Post by p_zetner »

Hi Filip.

Just noticed your thread and wanted to mention that your results look promising. I haven't spent much time with my Omega Optical filter system - I've been busy playing with my Daystar filter and a spectroheliograph! I do plan to get back to this system, especially for studying prominences.

If you're interested in some of my results please check out the gallery of images:

Image Gallery: Omega Filter System

I also have some construction and design information at:

Design: Omega Filter System

One change you might consider in your system is to place the filters in the output of a telecentric lens rather than in the collimated beam. The collimated beam configuration can be more sensitive to optical inhomogeneity of the filters. You can read some of the details of this reasoning here:

Pupils and Images

Cheers.
Peter.


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Re: H-alpha telescope with Omega Optical filters

Post by marktownley »

One change you might consider in your system is to place the filters in the output of a telecentric lens rather than in the collimated beam. The collimated beam configuration can be more sensitive to optical inhomogeneity of the filters.

Peter, How would one start off /go about designing a telecentric system for rear mounted filters like this then? I have no idea where to start. Mark


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Re: H-alpha telescope with Omega Optical filters

Post by p_zetner »

Hi Mark.

I'm attaching pdf notes of my basic understanding of the concept of a telecentric system:
TelecentricLens-ZetnerNotes.pdf

The general concept is based on a 2-lens or 2-lens-group system, the first of which collimates light from the objective, the second acting as a "refocuser". Telecentricity is achieved (in "image space") when the light cones converging on final image points have their principal (or chief) rays travelling parallel to the optic axis (telescope axis). With this property and narrow light cones (>f30), you take full advantage of the narrow bandpass of your filter (which has an angle dependent spectral transmittance). In order to get this condition of parallel chief rays, the exit pupil produced by the collimator lens (ie. the image of the objective lens, itself) must be located at the first focus of the refocuser lens. The 2 lenses (or lens groups) can be both positive or the collimator can be negative. The requirement for collimation is that the 1st focus of the collimator is coincident with the (2nd) focus of the objective. Pictures are worth two million words. Here are some ray-trace diagrams (nothing fancy - done by hand in Paint):

Telecentric Pos-Pos
Telecentric Neg-Pos

Here is the actual design I used for my Omega Opt filter system, constructed with Edmund Optics achromats. The design is based purely on paraxial optics and, if I were to use this one again, I would check it with ray tracing software, particularly to find out if my orientation of the individual lenses was optimal. I use WinLens (free).

Telecentric 3-Element

Here is a later, 4-element design which I use presently with my Daystar filter. This system was originally designed "paraxially" and then optimized using WinLens.

Telecentric 4-Element

WinLens allows you to check the aberrations, etc and I'm pretty pleased with both the theoretical and actual performance of this lens system.

Let me know if anythingis unclear or if you have more questions.

Cheers.
Peter.


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Re: H-alpha telescope with Omega Optical filters

Post by marktownley »

Thanks Peter, I have to hold my hands up here and say I have a lot to get my head around when it comes to telecentric systems, I can get my head around the concept of it, but engineering the maths of it is way beyond me at the moment. I guess my previous question should have asked if it is possible, or if there would be any benefit to replace the front collimating lens of a PST (mod) etalon for a telecentric system. I don't know if this is actually a daft question so apologise in advance if it is.

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Re: H-alpha telescope with Omega Optical filters

Post by p_zetner »

Hi Mark.

I'm not that familiar with the PST optics but, if the front lens is used to collimate, then replacing it with a telecentric will truly mess things up. A collimator, in effect, produces a final image at infinity (parallel beams output). The telecentric is designed for an image plane located at finite distance (converging beams output). Totally different optical requirements.

If you have access to just the PST etalon by itself (no additional focusing optics) then you could consider placing it in the image space of a telecentric. That might be interesting! (Is that what you're proposing?)

Cheers.
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Re: H-alpha telescope with Omega Optical filters

Post by marktownley »

If you have access to just the PST etalon by itself (no additional focusing optics) then you could consider placing it in the image space of a telecentric. That might be interesting! (Is that what you're proposing?)


Yes, this is what I was proposing, would it work, and if so how to do it??? :)


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Re: H-alpha telescope with Omega Optical filters

Post by p_zetner »

Hi Mark.

That's good! The 1st step would be to determine how much back focus (distance from final lens surface in telecentric to image plane - ccd) you want. You'd have to fit the PST etalon within this back focus distance and have room for your camera. If you want to try a single (positive) achromat lens for the refocuser then the back focus would simply be the focal length of this refocuser lens. For lens groups (2 or more) this calculation becomes more complicated and more difficult to explain unless you have some background in paraxial optics.

Once you establish the necessary focal length of the refocuser, the required focal length of the (negative) collimator lens is determined by the magnification you want the telecentric system to achieve. A factor of 2x , for example, requires the collimator to have half the focal length of the refocuser (only negative). If we call the refocuser L3 and the collimator L2 then the magnification is simply: mag = -f3/f2.

Telecentricity of the L2, L3 system requires a specific separation between L2 and L3. The separation between these lenses is determined by f2, f3 and the objective focal length through the formula for d2 in the notes I attached above.

So, you'll have to make some calculations, but relatively simple ones.

Hope this helps.

Peter.


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Re: H-alpha telescope with Omega Optical filters

Post by GreatAttractor »

Thank you, Peter, I might give it a try at some point. Though for now I like how the prominences come out and I'll work to maximize the sweet spot with a single etalon.


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ImPPG — stack post-processing and animation alignment
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Re: H-alpha telescope with Omega Optical filters

Post by GreatAttractor »

Peter,
I'm considering using the telecentric configuration. As you said, the telecentric beam, in which the etalon will be located, should be at least f/30. Since I don't want to have an oversampled image (Chameleon's pixels are well-matched to rather f/13-f/16), I'd need to use a focal reducer after the etalon. Can you offer any advice on that?

I've once tried a cheap, no-name 1,25" 0.5x reducer (screws into camera nosepiece) in Mak 180 (native f/15), quite narrowband (Solar Continuum - 8 nm), and outer 50% of image was distorted (that is, on Chameleon's small 1/3" sensor). Perhaps a longer f.l. achromat or a plano-convex lens, or positive meniscus, like something from Surplus Sched? (This is of course for narrowband only, H-alpha or continuum green light).


My software:
Stackistry — an open-source cross-platform image stacker
ImPPG — stack post-processing and animation alignment
My images

SW Mak-Cass 127, ATM Hα scopes (90 mm, 200 mm), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
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Re: H-alpha telescope with Omega Optical filters

Post by GreatAttractor »

Nevermind, I got a whole load of Surplus Shed lenses (incl. achromats) and will try different combinations.

Here's the picture of the latest (and probably the last) improvement to the old mod:
adapt1b600.jpg
Now it's all coaxial and rigid. B600 diagonal's nose is held in a 1,25"/2" reduction borrowed from a 2" diagonal. There is also a Borg T2 non-rotating focuser and the camera is mounted close to the blocking filter with a custom T2/C-mount rotating adapter.


My software:
Stackistry — an open-source cross-platform image stacker
ImPPG — stack post-processing and animation alignment
My images

SW Mak-Cass 127, ATM Hα scopes (90 mm, 200 mm), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
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Re: H-alpha telescope with Omega Optical filters

Post by swisswalter »

Hi Filip

that looks good, does it produce fine pics ?


Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

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Re: H-alpha telescope with Omega Optical filters

Post by GreatAttractor »

Yup, everything from last week :)


My software:
Stackistry — an open-source cross-platform image stacker
ImPPG — stack post-processing and animation alignment
My images

SW Mak-Cass 127, ATM Hα scopes (90 mm, 200 mm), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
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Re: H-alpha telescope with Omega Optical filters

Post by swisswalter »

Hi Filip

thank you. Oh I wasn't aware of it


Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

from 7 am - 7 pm http://www.nanosys.ch

from 7.01 pm - 6.59 am http://www.wastronomiko.com some times vice versa ;)
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Re: H-alpha telescope with Omega Optical filters

Post by p_zetner »

Hi Filip.

Sorry, I haven't been checking this forum regularly. It seems like you have the optics under control. The prominence animations are great.
Some optics configurations I've been using (telecentric and focal reducers) can be found below.

Original designs for the Omega Optical filters:
http://www.pbase.com/p_zetner/opticsdesign1

Latest designs for the Daystar filter:
http://www.pbase.com/p_zetner/configurations

They're based on Edmund Optics / Surplus Shed achromats (with additional camera lenses for the FR).

Cheers.
Peter.


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Re: H-alpha telescope with Omega Optical filters

Post by swisswalter »

Hi Peter

thanks for the details


Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

from 7 am - 7 pm http://www.nanosys.ch

from 7.01 pm - 6.59 am http://www.wastronomiko.com some times vice versa ;)
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