Back focus required with Sky-Watcher .85x Reducer/Flattener for ED80?

[Ouroboros]

Does anyone here use a ZWO ASI2600 in combination with a Sky-Watcher .85x Reducer/Flattener for ED80?  If so what spacers have you ended up using?

The back focus for the  Sky-Watcher .85x Reducer/Flattener for ED80 Is stated as 55mm.  Nominally this should be obtained with the 16.5 and 21mm extenders, provided with the ASI2600, plus the 17.5mm  in the camera itself.

However, on close inspection the  stars in the corners of my images are noticeable eggy radiating from the centre - indicative I understand that the camera is too close.

I decided to get some thin spacers and last night I spent a couple of hours adding 1, 2 and then 3mm spacers with, frankly, very little effect. The stars are just as eggy in the corners. 

Has anyone else experienced this and what spacing did you end up using? 

Edited September 17, 2022 by Ouroboros

[Clarkey]

I haven't used a 2600 but I have used the Rising Cam IMX571 equivalent using the the standard spacing and it was fine. I think I may have had to add an extra 0.5mm but it was close. Have you used the reducer with any other cameras?

[Ouroboros]

10 hours ago, Clarkey said:

I haven't used a 2600 but I have used the Rising Cam IMX571 equivalent using the the standard spacing and it was fine. I think I may have had to add an extra 0.5mm but it was close. Have you used the reducer with any other cameras?

Yes, and this is one reason I’m somewhat surprised by what I’m seeing.   For several years I used a Canon 450D and Canon M48 T-Ring Adaptor, the combination supposedly providing the necessary 55mm.  The corner stars, though not perfect, look acceptably round in images taken with my Canon. That being said, the 2600 pixels are 25% smaller than in the 450. So I guess any distortion might be more noticeable now.

Having said this, the distortion is really only obvious when zooming in. When the image is displayed full screen it is barely noticeable. So part of me thinks I shouldn’t worry about it. I am surprised though that adding up to 3 mm made little difference. In fact if anything the extra 3mm made the star distortion worse. Worse in the sense of eggy-ness radiating from the centre.  There didn’t seem any point in adding more spacers so I stopped at 3mm.

[newbie alert]

Unfortunately specs given only work on products with the same level of manufacturing tolerances, any deviation has a effect, so specs should be a guide and not set in stone

I'd keep adding spacers until you get to the point that you're happy with..the bigger the sensor the more accurate spacing has to be

[Clarkey]

1 hour ago, Ouroboros said:

Yes, and this is one reason I’m somewhat surprised by what I’m seeing.

I think we can assume the corrector is OK (I had one from a bad batch and it was very poor. After being optically checked along with the telescope it was changed by FLO). I might be worth putting one or more of your subs online to have a look at. Even allowing for manufacturing tolerances I think and extra 3mm should be more than enough. Also are you certain you are using the right spacers? (I'm not trying to be insulting - but just to rule it out).

[Ouroboros]

On 18/09/2022 at 10:43, Clarkey said:

I think we can assume the corrector is OK (I had one from a bad batch and it was very poor. After being optically checked along with the telescope it was changed by FLO). I might be worth putting one or more of your subs online to have a look at. Even allowing for manufacturing tolerances I think and extra 3mm should be more than enough. Also are you certain you are using the right spacers? (I'm not trying to be insulting - but just to rule it out).

Fair question. We can all make daft mistakes. I’m using the black Astrodymium spacers from FLO. They certainly look like 1mm. 

Yes, I’ll try to get some pics together.

For convenience reasons I’m also wondering whether there’s a way to check back focus during the day.  

Edited September 30, 2022 by Ouroboros

[Ouroboros]

On 18/09/2022 at 10:43, Clarkey said:

I think we can assume the corrector is OK (I had one from a bad batch and it was very poor. After being optically checked along with the telescope it was changed by FLO). I might be worth putting one or more of your subs online to have a look at. Even allowing for manufacturing tolerances I think and extra 3mm should be more than enough. Also are you certain you are using the right spacers? (I'm not trying to be insulting - but just to rule it out).

I finally got around to getting some more back focus measurements done, and subs into a presentable form. The object by the way is Caroline's Rose Cluster. 21-09-22

I started out at the nominal 55mm position. So that's with the 16.5mm and 21mm extenders, provided with the ASI2600, plus the 17.5mm  in the camera itself.  All attached to the ED80 0.85 reducer/flattener.  

Adding spacers I tried 56mm and 57mm. Deciding it was not getting any better I plumped for 55.6mm.  I didn't adjust any the tilt. I feel I want to get the distance about right before doing that.  I'm not sure what to conclude from this lot.  Or where to take it from here. Any thoughts or suggestions? 

 

[Clarkey]

I think the 55.6 looks best but not perfect. What does it look like in colour? I am just wondering whether there is some minor CA effect. If you could try it at a single frequency with a filter (or show a colour image) it might give a better idea.

Edit: looking back at my ED80 images, this is not much worse than mine with a slightly smaller sensor. Also the 'smear' was mainly the blue halo so caused by CA. Maybe the best option is not to look too closely😁

Edited September 30, 2022 by Clarkey

[Ouroboros]

@Clarkey Yes, that might be what I end up doing. Looked at a full size image the elongation in the corner is barely if at all visible.   
 

Picking up on your suggestion though I’ll do some debayering and see how they look.