Back Focus Issue ED80?

[Wackyguiding]

Hi all,


I have had my current set up for the past 6 months and have started to try to fix the star shapes in the corners. It normally wouldn't bother me but I'd like to do mosaics down the track.



Current equipment set up:



Skywatcher ED80-Original Focuser-SW 0.85x Reducer/Flattener-16.5mm spacer-21mm spacer-11mm Spacer-ZWO UV/IR Cut filter 1.25"-ASI294MC Pro (6.5mm).


Back focus required is 55mm.


This totals to 55mm spacing from the reducer to sensor. Using the 1/3 rule for the filter, the glass thickness is 1.1m (could only find this from a supplier website). This comes to 55.37mm. However, the star shapes don't seem to change regardless if I put 0.5-2mm spacing.


First two photos are both top left of frame and different targets due to obstacles during the night.



281827
No spacers, 55mm reached with zwo spacers


281828
2mm metal spacers




Next two are top right

281829
No spacers

281830
2mm metal spacers

If anything 2mm looks worse.

Could someone please let me know if it looks to be a spacing problem or a reducer/flattener quality issue or something else entirely? I will try two 0.2mm spacers to get as close as to the .37mm extra required but it may be a while with the weather. I was surprised to see no real change with the different spacing, and would be surprised if it required another 5mm of spacing as that's a fair way out from the specification.



Cheers

[LewisM]

If I’m reading right, you’ve only counted 6.5 mm for the ZWO cam. Unless you removed the black front ring, you need to include the 11mm for that, totalling 17.5mm for the camera alone, leaving 37.5mm to make up in spacers. Or is the 11mm spacer you mention the cameras black front ring?

Or am I misreading?

[Wackyguiding]

Yes I have included it, 16.5mm + 21mm + 17.5mm (made up of the 11mm ring and 6.5mm sensor to flange. Sorry should have made it clearer.

281832

[LewisM]

Is perhaps the 294's 4/3rds sensor too large for the corrected image circle of the ED80 reucer/flattener?

[Wackyguiding]

In my limited knowledge I wouldn't have thought so. I can see the entire sensor with space around the edges when looking through the entire OTA from the front. Although I guess that might not be a factor.



Interestingly for comparison I found a test shot from when I first got the scope, tested with the 0.85x reducer/correcter in question and my 6DII attached with a T ring.



This is the middle right edge of the frame.

281850


It was unguided at the time on a HEQ5 Pro. Looking at the star shapes across the whole sub shows tracking error, but wouldn't cause the tails on the edge stars?

[The_bluester]

Have those images come in in the right orientations, the ones for the top left look like a bottom right, and the top rights look like a bottom left. Ignoring that, if all four corners look like that it does look like more spacing might be required, I reckon the ones with the extra 2mm look slightly better. Have you got more spacers you can insert? Like remove the 2mm and put in 5mm even just to look for a definite change, you would hope if you go far enough to get the star shapes in the corners to at least go from warp speed to fishbowl, then you can say for certain than you have too much spacing and work your way back.

Also, how are you focusing? I have found the best results when chasing spacing or tilt to focus carefully on a single, centered star. If you are using something like SGP and focusing on the average across the field it can make aberrations or field curvature harder to see.

[Wackyguiding]

I unfortunately do not have anymore spacers, I run out of room on the camera flange after 2mm to attach the 11mm ring. Looks like I'll have to get some, probably something I should have done when I started.



All corners are similar, some worse then others. The sensor was angled differently for some but the issue seemed to remain regardless. I don't think tilt is a major contributor but if get close that might show up a bit more.


In a mad rush I was testing out my zwo EAF and letting the autofocus routine run on the ASIAir Pro run which I did each time I changed the spacing. Now you mention it I have no control over what star it selects. I have recently installed NINA on my new laptop but I have only ever used the AAP for control.



I will order some spacers and test, thanks.