Set it up in daytime. Be very careful to avoid the Sun. The Moon can also work. PhD2 is annoyingly sensitive to misfocus.
Be sure to correct your guidecamera's focal length in PhD2.
Focusing an OAG guide camera is very different from focusing the imaging camera. When focusing the OAG camera we are often far out of focus; the two hole Hartmann mask will show two images of the same star that appear reasonably sharp because the holes act to increase the F ratio, and the stars will move apart or come together as you adjust focus. This makes it relatively easy to make the coarse adjustments necessary during the initial setup of the OAG and guide camera:
The above works OK even using when using sliding barrel focus of the guide camera. The ideal is place a parfocalizing ring on the guide camera prior to placing it in the OAG, with parfocalizing ring all the way up on the barrel to allow for maximum focus travel, then focus the imaging camera first (Bahtinov mask), and then place the hartmann mask on the scope and then focus the guide camera. When the guidecamera is focused, move the parfocalizing ring so that it's snug against the OAG guide camera port and tighten the setscrews and leave the parfocalizing ring permanently in place. You never have to adjust OAG focus again: you focus the imaging camera and drop the guide camera (with parfocaling ring in place) into the OAG and you're ready to go.
I switched recently to using an OAG (ZWO). I strongly urge you to do the initial focus matching during the daytime. I tried at night after thinking I had the two close based upon the math. I wasted a couple of nights trying to get them both in focus. Finally I did what others had suggested and tried it out in the middle of the afternoon. I used the neighbor's tile roof down the street. Took about 15 minutes to get both the main camera and the OAG in focus. That night I was able to do my first successful OAG guiding.
Trying to match the focus on a moving target (Moon) just adds an extra complexity to an already touchy situation.
So please excuse the long post - but perhaps this info may be helpful to someone with a similar situation. This turned into a 2 night affair.
To sum up - I've been working on switching from 50mm guidescope with a SSAG over to OAG using the Celestron OAG and a new ASI174mini guide cam. When I finally got things to work - I got some of the best guiding numbers I have ever gotten - BUT - it really looks like unless there is another solution that I have missed - I am going to have to image at F/10 instead of with FR at F/7 as long as I'm using the T5i.
Here's why - (and if anyone can shed any light on this/has found a solution - I am all ears because I'd like to be able to image at F/7 - but can't see right now how to do it)
Back focus requirement using the F/7 reducer for the 8" Edge is 105mm (from the front of the reducer to the imaging sensor)
So.. I set it up this way:
Edge > F/7reducer > Bluefireball adapter > 48mm adapter (skinny one NOT the SCT adapter) > OAG > 48mm adapter > T-ring > T5i
This got me to within a mm or so of the 105mm. (~8 + 29 + 12 + 55 = 104mm)
I set it up this way because using the SCT adapter (which is 25mm) + 55mm of distance for the T5i + the adapters + the width of the OAG - was putting me way over the 105mm. (Switching eventually to a dedicated astro cam with less back focus will eventually solve this problem I hope - but that's another conversation)
So I connected up the above train - and tried to dial in both cameras. Got perfect focus with the T5i. No problem there.
But I could not bring the ASI174 to good focus. Could not get a good image. Tried the Hartmann, the Bahtinov, used the moon, and bright stars - but the image was just bad. So I took things apart - looked down the tube and it really seemed to me that NOT using the SCT adapter and substituting the narrower M48 adapter was what was causing the problem. Despite the large size of the 174 sensor - it was just not grabbing enough of the light path. And I had the prism all the way down. The inner edge of the reducer really appeared to be too close to the prism and was cutting off a lot of the light path. The physical opening at the end of the reducer was (so it appeared) just too close to the prism and seemed to be what was causing problems with the guide cam image. But that was the only way to get the proper spacing.
I have seen some other posts here and on Stargazer's Lounge - where several folks have reported having focus problems when trying to use the F/7 reducer with a DSLR. The back focus dilemma.
So, next night, I switched things out to this (which put me back at F/10):
Edge > SCT adapter > OAG > adapter > T-ring > T5i
Reset all the parameters in PHD2 - did dark library - did a calibration and Voila! - beautiful guide cam image (more guide stars than I ever saw with my SSAG by the way) It appears to me that skinnier adapter was the problem. When I ditched the FR used the SCT adapter instead - the opening was larger and further away from the prism - and the prism was able to utilize the full area of the light path rather than be blocked by the edges of the reducer.
So, it looks like as long as I continue to use the T5i - I'm going to have to image at F/10. Not the end of the world - but I was hoping to image @ F/7.
At least until I can purchase a good imaging camera and ditch the DSLR.
After the switch - the guiding appeared to be much improved. Although I did not track it for an hour or so - it was much better than I ever got with the guidescope and SSAG. Best I could usually do with that set up was about 1 - 1.20 RMS or worse.
After the switch back to the second image train above - I was getting .5 - .6 RMS. Tried it in different parts of the sky - and it stayed pretty consistent. (I still need to run PEC on my mount - just haven't gotten around to it yet)
Anyway - here's a screen shot of the guiding:
I did not guide for a real long time - but I did slew the scope around to different quadrants and it seemed to guide well all over.
I really like this ASI 174 - easy to focus, easy to find stars.
So that's where I'm at - I was very pleased to get good guiding first try.
If there's no work around to this - I guess I'll have to be satisfied with F/10 for now. Any thoughts or insights - much appreciated.
Edited by pgs/sdg, 16 August 2019 - 02:59 PM.