"Early returns" :-) :-)
OK, the WO Bahtinov mask is truly trash. So, I didn't use it. I think, howvere, that I can remove the clear plastic mask from the lens cap. That leaves a 86mm or so hole. I can then glue a 92-82mm stepdown ring (in reverse, remember that trick?) and the 82mm thread can be used to take the 82mm Kase Bahtinov mask. Will try that tomorrow if I can find a stepdown ring in my stash.
OK, so I did the usual subjective backfocus adjustment: i.e., change backfocal distance, mount Bahtinov mask, come back indoors, focus the center star, go outdoors to remove the Bahtinov mask, come back indoors to look for a corner star to see if it is better or worse. Rinse and repeat a few dozen times.
I started with the new M54 spacer (presumably needing nominal 55mm backfocus), the 18mm backfocus adjuster set to 18.3mm, a ZWO 54mm filter drawer (20mm), a Chroma L filter (3mm glass), ASI2600MC.
The mechanical distance is 18.3 + 20 + 17.5 = 55.8mm. WO wanted 55mm + 1mm (chroma filter glass) = 56mm.
No cigar! Coma at the corners of the APS-C frame.
I started adding to the backfocus in steps of 0.2mm untill I reached +1.7mm. No joy.
I then step downwards. At -0.5mm, it started looking better. -0.7mm appeared too much (this is why I really dislike this way to bckfocus adjustment, there is no objective way to tell how much and what direction one is off.
So, I dialed in -0.6mm.
OK, at this point, I am 0.9mm away if the backfocus is really 55mm. I.e., my mechanical backfocus is now 18-0.7+20+17.5 = 54.9mm. But that is with a very thick (3mm) filter!
Interestingly, I would be more correct had I used the original M54 adapter! One wonders if they had two set of people collimating the first run of scopes, and one group actually collimated to 55mm.
Anyhow, with this subjective backfocus adjustment, I see this:
and almost more importantly, this:
Not that shabby, really, especially considering this is f/3.8, and this is a single frame, not a stack. I see a halo from the very bright central star, but that is not the concern of today's experiment :-).
The corner stars are pretty round. I should be able to dial in (objectively) better once I have a usable Bahtinov mask to get reliable EAF ∆ measurements. And stacking will give even more symmetrical star shapes.
The full image is here:
http://www.w7ay.net/site/Images/-0.6mm.JPG
Don't think this OTA will go to the trash. Donno why it was trashed on Cloudy Nights. Looks fine to me, if you know how to adjust backfocus.
One day, I will go adjust the tilt at the bottom left/right -- shows up on Siril's tilt trapezoid above, and also on the bottom right panel of the aberration reporter. Usually that amount of tilt is no big deal, but at f/3.8, I may have to waste a night adjusting it. But the Siril tool should make it not too much work. The tilt adjuster in the Pleiades is actually a 4-point adjustment. This should make it relatively easy to remove my tilt once I align the camera's angle to the OTA's angle.
Notice that the average of the star size at top two trwpezoid nodes is 3.29, and the avge of the bottom 2 is also 3.29. So, it is just a left-right tilt of the bottom edge.
So far, it easily wipes the floor with something from Askar, both in speed, and in APS-C coverage. I am retiring the FRA300 -- perhaps retrieve some parts, like the EAF from that set up.
Chen
