To use a coma corrector or not? New OAG forces change of plans.

[Cumbrianwolf]

The kit is currently sitting at TS (GSO) 10" 254 mm f/ 4 1016 mm parabolic imaging Newtonian, TS Coma Corrector for f/4 optics, off axis guider with SVBONY 305 PRO Guide camera and Canon EOS 7D imaging camera. Now with the corrector I end up dropping to f/3.5 at around 900 mm focal length. I collimate before and sometimes after cool down with a calibrated laser collimator, which is often second checked via a Cheshire eyepiece, but in over ten years the laser has never moved out of alignment, unlike the cheap ones I once bought, but this Baader is true quality made. So can I risk not using it as this coma corrector as this one is designed for insertion into an eyepiece holder, of which most OAG do not have? I am awaiting the OAG to arrive, so have not tried the formulation and only surmising what can be done. I know I should wait and take a shot with and without it for 100% confirmation, but I like to plan things out firstly before wasting lots of time outside setting up.

[paul]

I can't see how you can avoid to swap out for the collimator. I tend to get my system to the focus point, take out the camera and CC then put the laser collimator in. My focuser is the low profile Skywatcher type (it has a retractable extension). The combination of camera and CC can give you focuser sag if you're unlucky (the focuser extension sagged for me). I used some parfocal rings on the CC (its very long) to raise it up the focus tube to avoid using the extension. This allowed the focuser to stay mostly retracted where it seems to be most supported (and zero image shift as focus is adjusted). 

 

As you have a nice spider use it to check your collimation - spikes for stars at the edge of the fov and central should be symmetric about the star. (I haven't done the out of focus check  for the diffraction ring symmetry , I've never found my skies stable enough to see them so I doubt it would add anything to the image quality).

[Cumbrianwolf]

1 hour ago, paul said:

I can't see how you can avoid to swap out for the collimator. I tend to get my system to the focus point, take out the camera and CC then put the laser collimator in. My focuser is the low profile Skywatcher type (it has a retractable extension). The combination of camera and CC can give you focuser sag if you're unlucky (the focuser extension sagged for me). I used some parfocal rings on the CC (its very long) to raise it up the focus tube to avoid using the extension. This allowed the focuser to stay mostly retracted where it seems to be most supported (and zero image shift as focus is adjusted). 

 

As you have a nice spider use it to check your collimation - spikes for stars at the edge of the fov and central should be symmetric about the star. (I haven't done the out of focus check  for the diffraction ring symmetry , I've never found my skies stable enough to see them so I doubt it would add anything to the image quality).

This is the focuser https://www.teleskop-express.de/shop/product_info.php/language/en/info/p3947_TS-2--MONORAIL-Dual-Speed-Focuser-for-SC-Telescopes.html which is very solid with a high weight rating. I collimate with the focuser fully retracted then when I set up the focus on the camera I will lock it off at that point and then commence collimation, after that point is reached it is within specification so much so that I can do the diffraction ring test, and it always works out, I just need to make sure of finding a really bright star. Never heard of using the diffraction spikes, so that is a consideration in the future and something new I have learned.

 

As for swapping out, I was in a dilemma in regard to fitting the corrector lens into the OAG system as it was designed to be placed into an eyepiece holder whilst connected to the camera rather than it being threaded on the external surface, and as you can tell I waffled on too much causing confusion.  https://www.firstlightoptics.com/coma-correctors/skywatcher-coma-corrector.html I just need to wait and see what the fittings are with the OAG and work something out.

Edited January 31, 2022 by Cumbrianwolf

[Cumbrianwolf]

Well that was a big failure as the OAG will not focus the camera nor the guide camera. I have shown all extension and converters I have, OAG not in picture.

Edited February 1, 2022 by Cumbrianwolf

[AstronomyUkraine]

1 hour ago, Cumbrianwolf said:

Well that was a big failure as the OAG will not focus the camera nor the guide camera. I have shown all extension and converters I have, OAG not in picture.

Sounds like you need to focus inwards. The only solution to that would be to buy a low profile focuser.

[Cumbrianwolf]

1 hour ago, AstronomyUkraine said:

Sounds like you need to focus inwards. The only solution to that would be to buy a low profile focuser.

Yes, that was the case for both the guide camera and the imaging camera, mind you it may need some amount of inward travel to come close to focus, I may have got the imaging camera to focus with a tweak on the primary mirror screws but dammed if I could get the guider to come close as it would only focus when sitting onto of the periscope with no extension! I never thought it would be a game of expensive experimentation! Oh well, have not decided on whether to keep the OAG or sell it on again.