Celestron Omni XLT 120 PST Mod

[andreahaku]

Hi Everyone,

Today I had the chance to test the PST Etalon/BF on the Celestron Omni XLT 120.

It’s not the perfect setup as the Etalon is not placed perfectly at 200mm from the prime focus point. I still need to take it 1/1,5cm forward but I was curious to see if I was able to reach the focus and try the different D-ERF positions.

Differently from the Vixen A80M setup, I had to place the D-ERF inside the Omni 120 tube as the diameter of the D-ERF is only 75mm (70mm considering the mm taken by the cell).

So I had to find a place that didn’t sacrifice too much the Omni 120 aperture but also didn’t cause any internal air turbulence. I tried 2 settings based on two of the 3 internal diaphragms: the ones that can be easily reached from the tube extremes. The first is the one behind the front lens (less turbulence but also less surface) and the one just in front of the focuser (more turbulence but also more surface).

Later in the evening I’ve found a way to put the D-ERF with the cell in the middle of the tube so that I solved the dilemma and have the maximum aperture available.

To solve the focus/etalon distance problem I substituted the original Omni 120 focuser with a new one from TS that is shorter by 3 cm. That is enough to have the etalon “almost” in the right place and reach the focus with the second focuser. So the first focuser is just used as a spacer/adaptor for the real “ex-PST” block (2″ nose + Etalon + SC Crayford Focuser + BF5). Just a note about the BF: the BF5 now it’s a BF6 as I enlarged the holes

Here’s a picture of the setup:



Despite of the very long Omni 120 backfocus I wasn’t able to reach the focus for very very little (with the DMK41). No problem with the Hyperion Zoom in visual. So I replaced the original nose of the DMK41 with the shorter one I bought months ago and also removed the Baader Barlow adapter. With this setup I have a lower magnification but I have room for precise focusing.

I made a few tests and here’s a comparison of the 3 setups I tested (Vixen A80M mod, Omni 120 with Deep ERF and Omni 120 with Front ERF):



Well, there’re a lot more details and you have to consider that with the 100mm setup I have double the surface than with the Vixen (70mm in reality).

The following images shows are some of those taken today. Don’t put much attention to the dirt etc as I simply wanted to test as man setup as possible and I was not in perfect conditions. Anyway I’m already happy with the results.







Tomorrow I’ll do more test… with a clean CCD

Andrea

P.S. One thing I forgot to point out... the Omni XLT 120 is an f/8 so in reality I'm using it like a 100mm to have the f/10 ratio needed by the PST Etalon

[Bill E.]

Looks good

[oldfield]

excellent, very encouraging result.

[swisswalter]

Hi Andrea

that's coming very good. Keep us updated

[Merlin66]

Well done!
Another PST Stage 2 mod....
Using a 70mm ERF close behind the objective is the best solution....
The back focus from the PST etalon positioned at the "majic 200" will be 200mm.
The results are very encouraging from an effective 100mm aperture.
Onwards and Upwards.

[andreahaku]

Thanks. Today I'll do more tests.
Another good thing is that I don't have any Newton rings with this setup!

[andreahaku]

Now a larger BF urges!
Anyone with a spare BF10/12/15 to sell me?
It'd be really appreciated

[marktownley]

Very impressive setup both of you! Look forward to seeing more results!

[andreahaku]

I love your modding... yours is a REAL Frankenskope. Mine is a cheerleader in comparison
I'm not planning to do anything like that but yes there's a lot of room in it. Also it has huge backfocus. So it let you play a lot inside and outside it.

I made some more test today but despite of the sunny day, seeing was very bad.

One question. Where would you put a 75(70)mm D-ERF inside the scope not to lose the possible maximum aperture (that I suppose is 100mm due to PST etalon limits).

Thanks.

Andrea

[andreahaku]

Hi I'm waiting for them to arrive. Probably a week or two more.
I hope that they will be good as BF. I think so too.
In case they don't I'll surely need a larger BF.

[andreahaku]

I made more test images today with the Omni XLT 120 setup. Sunny but really bad seeing. I tried with the Vixen A80M just to see it was the Omni setup or what. And the seeing was bad with the Vixen as well so it must be the seeing. Tomorrow I'll try again hoping to be luckier.

Here are a few images:












Good but not to test the real potential of the new setup

Andrea

[swisswalter]

Hi Andrea

that is getting better and better : Go on please

[Derek Klepp]

Thanks Andrea your results look promising.Good to view all the comparive pics too.

[marktownley]

Hi I'm waiting for them to arrive. Probably a week or two more.
I hope that they will be good as BF. I think so too.
In case they don't I'll surely need a larger BF.

Love the additions Andrea!

My omega blocker needed a bit of tilt to get on band, but, have to say, was riddled with reflections and a bright spot sat off centre in the fov; transmission is poor too. In all fairness I was warned by the designer of a rather famous German manufacturer of etalons that this would likely be the case and it was... If I had never seen / viewed with Ha I would likely have been impressed, however I know what is possible and as a result found the omegas to be a solid second place. I do hope you get better results tho...

[Merlin66]

Andrea,
As you're working in an effective 100mm f10 beam, you could place the 70mm filter up to just less than 400mm behind the objective to minimise vignetting. (The suns's image will be close to 10mm dimeter then the f10 cone...1000-((70-10)x10)=400mm

[andreahaku]

Thanks a lot my friend.
I know my PST mods can rely on you approvals
You should sign my tubes

[Merlin66]

No problem!
bring it with you when you visit me in Belgium! I'll shout you a Belgium beer, there are only 400 choices....

[colinsk]

No problem!
bring it with you when you visit me in Belgium! I'll shout you a Belgium beer, there are only 400 choices....

Send me some Cantillon. I'll pay for it and the shipping. :;

[andreahaku]

Hy everyone,

I've been able today to find a way to place the etalon at the right (200mm) distance from the prime focus.

Thanks to the double focuser setup (one before the etalon and one between the etalon and the BF) I've been able to also take some comparison shots between different etalon positions.

First of all I have to say that I wasn't sure if it was the case to process the videos or not as it was cloudy and so even during the recording conditions changed quite often. In any case here are the results:



All the images has been "processed" in the same way.

The longer and shorter exposure recordings has been made after the two with "wrong" etalon position and weather conditions were worst. Despite of that the details are back. Naturally after changing the etalon position I refocused

So the good news is that I'm done with the new Omni 120 (100mm) setup... I think. The bad news is that weather won't allow me to take any "serious" shot before at least a couple of days

Andrea

[marktownley]

A useful comparison! Thanks for that Andrea!

[swisswalter]

Hi Andrea

thank you very much on that job. That is very helpful

[andreahaku]

Hi,
here's a picture of how I shortened the distance between the first focused and the tube:





I simply removed the white ring reducing the tube diameter to 96mm. I used longer screws with spacers. I'm making an aluminium ring to "close the hole" But that's mainly for aesthetic as the setup is very solid and works well this way as well.

I just posted a quick image I took today HERE

[marktownley]

Great setup Andrea!

[vikeljay]

Can you write up everything you did in detail including what parts you used as where you purchased them. I want to achieve the same type of results with a Meade 80mm f/11 scope. Thanks.
Jim