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PST mod on a C14

Posted: Sun Nov 03, 2019 7:33 pm
by oli14
Hello,

I'm trying to make the PST mod on my C14 with an off-axis 160 ERF-D filter and a BF1800 Lunt. I have already made this modification on a tecnosky telescope of 152.
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During my first attempt I did not succes to do the focus. I did several tests with long rings.
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Valery in this old post :
viewtopic.php?t=17989

You wrote :
Large ERF (say, for a C14) has another option. If the seeing does not cooperate, one can just stop the aperture with 6" off axis mask.
Do you think this setup can work ?
The initial F/D goes from 3910/355 = 11 to about 3910/140 = 27.9.The diameter is reduced with the ERF filter. I take as 140 diameter because the erf 160 is a little eclipsed by the support of the secondary mirror.

PST Fabry-Perrot is optimal at F/D = 10. Maybe it's the problem if my F/D is at about 28 in this configuration ?

Thank you.

Olivier

Re: PST mod on a C14

Posted: Wed Nov 06, 2019 3:47 pm
by AndiesHandyHandies
Hi

At F28 you can remove the collimating lens from the front of the PST as its near enough to F-Infinity, or F40 for a air spaced etalon.

The re-focusing lens will just about preserve the back focus from the PST.

On a 127mm F7.5 with 2x ED Barlow element before PST with Collimating lens at the front removed, working at 4X as Barlow 200mm effectively before the focal point (Rather than 100mm), so F30, I got a good result, with the Re-focusing lens in place.

Andrew

Re: PST mod on a C14

Posted: Wed Nov 06, 2019 4:14 pm
by george9
If you use an off-axis mask, you will likely need to tilt the etalon towards the incoming light if the normal tuning range is not wide enough to get to H-alpha with a tilt. E.g., the Quark Combo comes with a tilt adapter for off-axis SCTs.

George

Re: PST mod on a C14

Posted: Wed Nov 06, 2019 8:30 pm
by marktownley
You haven't got -200mm of back focus needed. Without using a larger ERF I would just use the setup on the 152 frac assuming it is f10.

Re: PST mod on a C14

Posted: Wed Nov 06, 2019 8:43 pm
by marktownley
AndiesHandyHandies wrote:
Wed Nov 06, 2019 3:47 pm
Hi

At F28 you can remove the collimating lens from the front of the PST as its near enough to F-Infinity, or F40 for a air spaced etalon.

The re-focusing lens will just about preserve the back focus from the PST.

On a 127mm F7.5 with 2x ED Barlow element before PST with Collimating lens at the front removed, working at 4X as Barlow 200mm effectively before the focal point (Rather than 100mm), so F30, I got a good result, with the Re-focusing lens in place.

Andrew
Hmmm. Not sure I agree with the theory. I tried my rather nice PST etalon without collimating lens in my 8" HaT in an f28 telecentric configuration, and the increase in bandwidth of the etalon in this setup is huge compared to the same etalon in a collimated configuration. Using barlowed as opposed to telecentric configurations is just going to result in a sweet spot.

Re: PST mod on a C14

Posted: Fri Nov 08, 2019 1:13 pm
by christian viladrich
Indeed,in a telecentric system:
- F/30 and above is "good" with mica-spaced etalons:
Image

- F/50 and above is needed for air-spaced etalons :
Image

In a collimated beam, you need to have the same f-ratio on both the collimating lens and the telescope.

The differences between the telescentric and the collimated mounts is:
- the field is uniform with telecentric, but there is a broadening of the FWHM,
- the is no broadening of the FWHM with the collimating beam, but there is a shift of central wavelenght, i.e. a sweet spot.

There is no free lunch ...

Re: PST mod on a C14

Posted: Fri Nov 08, 2019 1:39 pm
by christian viladrich
Here is the sweet spot radius depending on the max CWL allowed and on the focal length of the telescope:
Image

Re: PST mod on a C14

Posted: Fri Nov 08, 2019 9:50 pm
by marktownley
f50 with air spaced etalons needs rather perfect seeing to say the least.

Re: PST mod on a C14

Posted: Sat Nov 09, 2019 7:16 pm
by christian viladrich
It would also need large pixel size and / or a focal reducer after the etalon. Otherwise, the image scale would be way too large.

Re: PST mod on a C14

Posted: Mon Nov 11, 2019 11:23 am
by AndiesHandyHandies
Hi

I appreciate my use of an ED Barlow and at F30 with a PST Etalon was not optimum but I could see as much detail as in Pedros image with a tad more contrast.

I am retired and cannot justify a 180mm+ DERF and a Baader Telecentric Barlow to just observe optimally a few days a year.

People image at F16 with Daystars because of seeing restrictions so observing at F30 with a PST is similar.

Its a hobby for me. And If I can do visually the same as imagers see I would be happy.

I am about to try imaging on the Vixen with my GX80 camera. The Sun just fits in. I could not take a snap because the battery had run down and the date lost I found out.

Andrew.

Re: PST mod on a C14

Posted: Mon Nov 11, 2019 8:36 pm
by marktownley
If you're happy with the results you're getting then all is good Andrew :)

Re: PST mod on a C14

Posted: Fri Nov 15, 2019 5:38 am
by mdwmark
OK George is right. You will need to tilt the etalon 1.8deg to reline the etalon to the off axis. The tilt will need to line up with you ERF. So it the ERF is at 12 o'clock. The etalon will be tilted backwards at 12 o'clock. This is the same that we have been doing from the 80's with mica filters. Also offaxis does not work well below .5Ang.
Now the part where you say people image with an Daystar at F/16. At F/16 an .5 filter will be pushing 1.2Ang HW. But more important if your not using an D-ERF it will degrading the blockers from the heat. I have repaired many filters with an .54" spot melted on the delrin and the blocker failed.(.54" is the solar image size at 4" F/15).
Mark W.