9 replies to this topic

[okeania]

Hello everyone,

Previously, I was using a Sky-Watcher Quattro 200/800 f/4 setup with the following imaging train:

Sharpstar 0.95x Reducer → Askar M54 OAG (19.5mm) → ZWO EFW (20mm) → ZWO 294MC Pro (6.5mm)
Total back focus: ~55mm
This setup worked without any issues — stars were round both at the center and edges.

Now, I've switched to the following configuration while maintaining the same 55mm back focus:

Nexus 0.74x Reducer → Askar M54 OAG (19.5mm) → ZWO EFW (20mm) → ZWO 2600MM Pro (12.5mm)

However, even with good autofocus and accurate guiding, I'm seeing deformed stars — neither the center stars are round nor the edge stars acceptable (see attached images).

What could be the cause?

Is the back focus requirement different for the Nexus reducer?
Could it be tilt or sensor spacing issue with the 2600MM?
Any advice or experience with a similar setup would be greatly appreciated.
Thanks in advance!

 

Attached Thumbnails

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Edited by okeania, 11 June 2025 - 05:25 PM.

[PirateMike]

Back focus is always a "suggestion". It is not uncommon at all to have to make small adjustment to the specified distance. A read through this short thread may help you... https://www.cloudyni...e-or-not-to-be/

 

Also, not all reducers are made to work with all scopes. It is always (usually) best to use the reducer designed for that particular scope. Purchase a different one and things just might never work well no matter what you do.

 

Is the Nexus a reducer and a coma corrector?

 

 

Pirate Mike

Edited by PirateMike, 11 June 2025 - 05:48 PM.

[mayhem13]

Where to start………

 

you’re at F3 now so axial collimation has to be incredibly precise…….are you confident that you’re well collimated?….looking at your photo, you have the stock focuser which has faaar too much tilt to properly register axial collimation

 

the Nexus is prescribed at 55mm from the flange to the sensor…..I don’t think your imaging train is 55mm……even +/- 1mm is going to wreak havoc on image quality at F3.

 

filters……some filters are incompatible with F3 optics……you’ll have to ask around.

 

With a 2” focuser, you’ll never have a flat field out to more than a 4:3 sensor…….18mm round fov is about the best you can hope for 

[cuzimthedad]

Moving to Beginning Deep Sky Imaging...

[CowTipton]

Askar M54 OAG (19.5mm) → ZWO EFW (20mm) → ZWO 2600MM Pro (12.5mm)

 

= 52mm

 

Add a 3mm spacer?

[michael8554]

Particularly if the centre stars are deformed, you need to examine a short exposure, stretched if necessary.

 

To avoid the PA, PE and guide errors on a 120sec exposure.

[jml79]

First of all, both the 294 and the 2600 have 17.5mm backspacing in their normal configuration. The 12.5mm and 6.5mm numbers are without the required adapters to attach the camera and filter wheel. I also see the thread adapter between your OAG and EFW which adds another 0.5-1mm. I would estimate your actual back spacing as;

 

Askar OAG 19.5mm, EFW-OAG thread adapter 1mm, EFW 20mm, ASI 2600 17.5mm for a total of 57.5-58mm.

 

Try removing the OAG and using the 16.5mm ZWO adapter that each camera should have come with. If that improves things then replace your Askar OAG with a ZWO OAG-L which is thinner (16.5mm) and designed to work with the rest of the ZWO stuff. It will also bolt on to the filter wheel making a nice solid fit with everything in the proper orientation.

[okeania]

Back focus is always a "suggestion". It is not uncommon at all to have to make small adjustment to the specified distance. A read through this short thread may help you... https://www.cloudyni...e-or-not-to-be/

 

Also, not all reducers are made to work with all scopes. It is always (usually) best to use the reducer designed for that particular scope. Purchase a different one and things just might never work well no matter what you do.

 

Is the Nexus a reducer and a coma corrector?

 

 

Pirate Mike

Thanks Pirate

 

Nexus reducer and coma corrector together

[okeania]

= 52mm

 

Add a 3mm spacer?

Image in NINA with 3mm since I don't have 1mm spacer. But things are working much better now by adding black rings. 

[okeania]

First of all, both the 294 and the 2600 have 17.5mm backspacing in their normal configuration. The 12.5mm and 6.5mm numbers are without the required adapters to attach the camera and filter wheel. I also see the thread adapter between your OAG and EFW which adds another 0.5-1mm. I would estimate your actual back spacing as;

 

Askar OAG 19.5mm, EFW-OAG thread adapter 1mm, EFW 20mm, ASI 2600 17.5mm for a total of 57.5-58mm.

 

Try removing the OAG and using the 16.5mm ZWO adapter that each camera should have come with. If that improves things then replace your Askar OAG with a ZWO OAG-L which is thinner (16.5mm) and designed to work with the rest of the ZWO stuff. It will also bolt on to the filter wheel making a nice solid fit with everything in the proper orientation.

Thanks a lot for your reply.
Actually, I rushed to buy the ZWO OAG M68-L when my brother was coming from the US, just to avoid the Trump-era tariffs, and didn’t pay much attention at the time.
Turns out it doesn’t fit my filter wheel — so now I’m stuck with it. That’s the root of the problem.
From what I’ve learned on the forum, it seems that the newer filter wheels are now coming with M68-L compatibility. A friend of mine is expecting one of those soon.
I’ll try to sort this out by doing a gear swap with him — fingers crossed that will fix it.