ZWO OAG V2 issue with Skywatcher 2" coma corrector for F5 Newts - Forums

Topic starter

Apr 20, 2022 · Edited: Apr 20, 2022

Just wanted to highlight an issue I've experienced having just purchased the ZWO OAG V2. The problem is that the M42 thread on the coma corrector is so long that it hits the mirror stalk on the OAG before it is totally screwed in. As such, all the tightening pressure is against the stalk rather than the flat face of the OAG. It feels nasty and can't be either good for the mirror stalk or for obtaining a truly flat field. That said, I have been using it for a few days like that capturing M81 and M82:

M81 and M82 - Early posting ... need much more acquisition time

My solution to the problem was to buy:

* a pack of M48 spacers
* a pack of M42 spacers
* a new female to female 9mm M42x0.75 extension tube to replace the ZWO 11mm one

My train now looks like:

- 2" coma corrector
- 1mm metal spacer (+1mm) Totally fixes the stalk clash issue.
- OAG V2 (+16.5mm)
- 0.5mm metal spacer (+0.5mm)
- 9mm extension tube (female to female) (+9mm)
- ZWO male-to male M42 adapter (+2mm)
- 0.5mm plastic spacer (+0.5mm)
- ZWO 8x1.25" filter wheel (+20mm)
- ZWO ASI1600mm (+6.5mm)

That's a total backfocus of 56mm (an extra 1mm for the filter glass effect)

I went for the 9mm rather than a 10mm extension tube as it gives me the option to tweak the backfocus inwards if I need to.

Just wanted to share as the Skywatcher/ZWO combo is very common and I had not seen it mentioned before.

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Fredrik Ödling

Apr 24, 2022

Thanks for the information.

I'm also using a f5 (200pds) newt with the 2" skywatcher CC (the 0.9x "short" one) and was looking to get an OAG for next imaging season together with the new ZWO EFW 7x36mm and a 533mm. I didnt pull the trigger on the OAG yet, I'll have to keep this in mind although your solution seems to work fine!

Andy Wray

Topic starter

Apr 24, 2022

Fredrik Ödling:
Thanks for the information.

I'm also using a f5 (200pds) newt with the 2" skywatcher CC (the 0.9x "short" one) and was looking to get an OAG for next imaging season together with the new ZWO EFW 7x36mm and a 533mm. I didnt pull the trigger on the OAG yet, I'll have to keep this in mind although your solution seems to work fine!

I haven't had a chance to try the new spacer setup yet (clouds

), but because I have 3mm either way of travel on the helical focuser I know that adding the 1mm spacer in front of the OAG can't possibly cause me any problems.

Michael Ring

Apr 24, 2022

The same problem exists for Esprit Flatteners, their thread is quite long. My soluion was to put them in my turning machine and to shorten the threads ( a millimeter was enough when I remember correctly..)

The new OAG-L is better, there you have 5mm of threads from the front plate and another 3mm from the prism to the front plate, That's ~3.2 mm more than the old OAG.

Concise

Steven

Apr 25, 2022 · Edited: Apr 25, 2022

I'm having issues with this aswel.. On an Esprit, like Michael said.

But because the setup that I'm going to be using, I might not be able to fit spacers in without ruining the backfocus.
So I might have to attack the threads of the flattener. Don't have a turn table though.. might have to attack it with a Dremel, but that's not quite as "precise" as it might need to be

Either way, I'm also having issues with the AOG+Filterwheel combo..
the AOG doesn't quite line up right with the filter wheel, meaning I can't really get my guide camera in the right position.
(using a 290mm, normal one.. not the mini). And again, can't really ad spacers to that one to fix that.

Any suggestions on that? Ideally without the suggestion of ditching this OAG, getting the OAG-L, as that one isn't compatible with the "non-mini" versions of the camera's... But that might be the "better" solution to the issue.