PST modification from Italy

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LorenzoM
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PST modification from Italy

Post by LorenzoM »

Hello, my name is Lorenzo and i am writing from Rome, Italy!
In my life i am Phd student and my research field is related to surface physics.
My hobby is obviously astronomy :mrgreen: i have several telescope in particular refractors dedicated to deepsky imaging.
I have also a PST and i am here to have some advices about pst modification, several questions are below! :mrgreen:

- First question is very easy, how is possible to find the good etalon position?
My donor telescope is a 102/800 stellarvue, the f ratio it's not really good this is true but worst there is not enough backfocus to put the etalon 20 inside prime focus.
So my idea is to use a telecentric powermate 2X to extract the focus and to reach the good f ratio that will be f/16. An higher f ratio than f/10 i think is always good because ligh cone is smaller than f/10 itself.
The focal lenght is about 1600mmm for the future if you suggest me that this system works i can try a smaller amplification factor!
Is it correct? theoretically can the system work?

- The focus extraction will not change as function of barlow position because is a telecentric system.
Now how can i found the good position for the etalon?
My idea was this one...because i have read that when etalon is well placed put the focus about 21.5 cm away from the rubber knob and this distance is fixed (is it correct?) if i focus with black box and original objective and after put the blackbox on the donor telescope and after adjust etalon position until i find the focus on this way the etalon should be placed in right position.
Is it correct?
I have done some tests and these are the images acquired with qhy5II. I am not really good to process but i am not sure to have obtained good results,
What do you think?
Sole2.jpg
Sole2.jpg (197.04 KiB) Viewed 5238 times
Sole.jpg
Sole.jpg (197.23 KiB) Viewed 5238 times
Which software do you use to process the images?

- Another point is the sweep spot of etalon, in my etalon the effet is really evident and i think there is no way to solve it. Does the problem come from etalon or from BF?
Changing the bf can reduce this effect?

- Finally i have a question about ERF!
reading here http://www.daystarfilters.com/inout_art ... Refractors daystar suggest that a common UV/IR cut filter is enough! What do you think?

I think i will continue in pst modification but i am thinking to buy also the new daystar filter to have smaller bandpass than modified pst!


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Re: PST modification from Italy

Post by Merlin66 »

Lorenzo,
Using a good telecentric really requires the removal of the front collimating lens of the PST assembly....
Yes, the optimum positioning is 200mm inside prime focus. Yes, you can "pre-set" the black box focus then use this to establish the best etalon position.
Registax 5/6 or AutoStakker are the favourite processing software.
The sweet spot is independent of the blocking filter. Really down to the etalon/tuning and a good f ratio.
Re ERF - I personally think you need much more than just a UV-IR cut filter as an ERF.
I always recommend the Baader D-ERF.
Drop me an email for a copy of my PST mod write-up.


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Re: PST modification from Italy

Post by swisswalter »

Hi Lorenz

welcome to that wonderful site. Have fun, sun and mod the hell out of it.


Your first pics are great, so you are on a good way. Following Kens advice will lead you to the goal


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: PST modification from Italy

Post by LorenzoM »

Thanks for the welcome.
So the point to remove the front collimation lens was not really clear! I will try as soon as possible!
Please use my mail massimi.loreATgmail.com to send me your work!
I hope there are informations to clarify this point!
Why a telecentric lens makes collimating lens unuseful?

For now i am using modification in photographic mode and i prefer don't spend money for the ERF, if i will success to obtain better results i think i will buy it to use also in visuale mode.
If something happens to the camera don't really matter!
To by sure i have mounted a termocouple (K type) on one side of the UV/IR filter and i check after several times, the filter has never become hot!


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Re: PST modification from Italy

Post by swisswalter »

Hi Lorenzo

I succesfully melted down a K-Line filter not using an ERF at all and a Beloptik UV/IR cut doing the same :oops:


Doing strictly WL and Ha work I would go with an external Baader D-ERF, doing CaK too I would take an internal triband filter (Beloptik) or an external Beloptik UV/IR cut . The later I have not yet tryed my self but hope to do in the near future


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Re: PST modification from Italy

Post by marktownley »

Hi Lorenzo & welcome to the forum.

Good first results there. Really easy way to find the -200 position; get a metre rule / tape measure and gently insert into the focus draw tube of the scope, push this up until it comes (very lightly) into contact with the back of the objective. The point that reads 600mm is where the collimating lens on the PST needs to be.

The sweet spot is the result of the collimating lens that the PST assembly uses: The etalon see's field angles from the diameter of the sun but not instrument angles and as a result a sweet spot is generated. Using a telecentric in the way you describe won't do anything to improve the sweet spot, if anything it will complicate things. You could try rotating the etalon assembly in the focuser, you will find there is a certain point where the sweetspot is minimised and contrast is enhanced.

I would heed the advice about an ERF, you will end up frying things. Another complication will be excessive thermal loading of the etalon causing a bandpass shift, as you try and compensate this by tuning the etalon you will find banding effects can start to occur. A third consequence of insufficient ERF is that the inside of the tube will heat up and will affect your image in the form of poor seeing. I know this is what Daystar recommends, but they are recommending it for a Daystar filter, a totally different ball game than a PST mod.

Mark


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Re: PST modification from Italy

Post by LorenzoM »

Using a good telecentric really requires the removal of the front collimating lens of the PST assembly
Today i have tried to remove the front lens of the pst, but i was not able to reach focus in any way...


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Re: PST modification from Italy

Post by marktownley »

If it was a 'true' telecentric you would have to remove the front and rear lens assemblies. Televue powermates do not work in the same way with a PST mod that a 'true' telecentric lens system does.

I would just stick to running the whole thing f10 for best results tbh...


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Re: PST modification from Italy

Post by Merlin66 »

Mark, et al,
The rear re-imaging lens of the PST etalon assembly should work OK with a collimated input beam from a telecentric lens......


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Re: PST modification from Italy

Post by marktownley »

I've been doing a lot of investigating and understanding about the whole thing regards the difference between a collimated beam and a telecentric beam, and the way these work with etalons, to the point come this months pay day I shall be buying a telecentric system to use with my PST mod - ditching the collimating / refocusing lenses in the original pst assembly and running solely with a telecentric beam instead - same way as the solar spectrum filters work. This will have the effect of completely removing the sweetspot that is inherent with the original PST imaging system, as a result the neither the bandpass or the bandwidth of the filter will be shifted over the fov, and, as such, the resultant image should be both more uniform and detailed. I'm fairly confident I will be giving the rear mounted daystar filters a good run for their money ;)

It may cost more to do this than a normal PST mod, but, to me, it is the next logical step in improving the performance of the humble PST mod into the same league as the professional filter makers.


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Re: PST modification from Italy

Post by swisswalter »

Hi Mark

good luck on thad mod. Are you also removing the the back lens from the PST or do you even have to remove it :oops:


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Re: PST modification from Italy

Post by marktownley »

Yes, back lens will need to be removed too...

Using the TZ4 will take the EFL up to ~4000mm, however i plan to use the 0.5x reducer on the camera nosepiece to bring me back down to my usual ~2000mm EFL for day to day imaging...


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Re: PST modification from Italy

Post by swisswalter »

Thank you Mark. I'll stick to chemistry, optics are not mine. I have a hard time to understand it. Collimating, telecentric etc. :oops:


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: PST modification from Italy

Post by Carbon60 »

Hi Mark,

I had a similar idea for the same reasons, but probably I used the wrong telecentric arrangement at only f25. I removed the front and rear lenses from the PST etalon assembly and stuck a 2.5X TeleVue on the front and my DMK41 at the rear using custom made adapters. I imagine yours will be a more effective solution.
DSC_0189.JPG
I abandoned this owing to a lack of suitable imaging performance.

Stu.


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Re: PST modification from Italy

Post by marktownley »

Hi Stu.

I remember you doing this project. IMHO the TV powermates are simply not up to the task in the same league as the baader tz or custom designed telecentric when it comes to using for a Ha rear mounted filter. They (IMHO) are principally designed as an imaging amplification system, rather than a Ha specific tool. Yes, Daystar tout them, but I wonder how much better a Daystar would perform with a properly designed telecentric.

Mark


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Re: PST modification from Italy

Post by LorenzoM »

i have read the new post about telecentric and your answers here!
So also the rear collimating lens should be removed when telecentric system is used.
Mark why do you believe powermate is not suitable for halfa work? I agree that dedicated halfa telecentric system like the baader is optimized for this work but i guess also the powermate should work in a good way!
What do you think?

When i will have time i try to remove also the other collimating lens, any suggestion about how to dismount it?

And i have ordered a quark daystar for cromosphere ;) so pst modifcation will be stopped until daystar will arrive! quark + pst etalon what could happen? :D


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Re: PST modification from Italy

Post by LorenzoM »

Yes this is the same configuration i used!
The powermate and both collimating lenses on the etalon!
Could you post some picture of the sun obtained with your modified pst?


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Re: PST modification from Italy

Post by marktownley »

Hi Lorenzo, for a telecentric to work properly they need to be a certain distance from the scopes focal point, and the telecentric itself has a focal 'zone' where a sharp image can be reached. I'm not convinced with the TV that there is much depth to this 'zone' and as such may struggle to get an etalon, blocking filter and eyepiece / camera in there. I may be wrong though... To take off the rear collimating lens in the PST etalon assembly it is just in a cell that unscrews, simples!


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Re: PST modification from Italy

Post by LorenzoM »

Hi Lorenzo, for a telecentric to work properly they need to be a certain distance from the scopes focal point, and the telecentric itself has a focal 'zone' where a sharp image can be reached. I'm not convinced with the TV that there is much depth to this 'zone' and as such may struggle to get an etalon, blocking filter and eyepiece / camera in there. I may be wrong though... To take off the rear collimating lens in the PST etalon assembly it is just in a cell that unscrews, simples!
Ok but one thing it's not really clear. Televue declares that amplification factor for the 2X powermate remains the same up to a focus position of 10cm from the powermate itself.
Ok it's true that the focus position it's about 20cm from the etalon so this means that the distance is really far from data declared by televue, how could this influence on the etalon?
The baader telecentric lens could give a better result? why do you think it?

Neo your images are really amazing! Can you describe the optical path?


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