How to interpret field curvature? [ED72 + OVL]

[Stub Mandrel]

3 hours ago, Skipper Billy said:

Hi Neil,

What ID are the spacers you need - I have loads of Delrin spacers from 0.5mm, 0.75mm and 1mm - PM me your address if you would like them. (FOC).

Thanks, a generous offer!

42mm - but to be honest I  have a well-equipped workshop so once I have the right spacing I can make my own permanent one. Save your spares for someone not able to do so.

This is my latest -  still off-centre, but stretched in all four corners.

P.S. - I've just checked a Ha sub of the veil and it appears that the corner most affected in previous images is OK with a printed 1mm spacer, but of course the camera is not in the same orientation, so we will see...

Edited September 23, 2018 by Stub Mandrel

[Stub Mandrel]

Looks like a spacer has made a big difference.

Before:

After:

Although the difference looks modest the difference in the appearance of stars is huge with the 1mm spacer. I will try adding another 0.5mm next time.

[FaB-Bo-Peep]

On 13/09/2018 at 13:41, billyharris72 said:

Hi all:

I've just had a first go with my new scope (Skywatcher ED72 with OVL flattener) and early signs are good - think it's going to be a keeper. However, I'm struggling with what I assume is field curvature (never imaged with a frac before so this is new for me). The stars in and around the centre are great but at the edges of the field they are both out of focus and elongated (pointing towards and away from the centre of the image). I just had a couple of questions that I was hoping someone more experienced could weigh in on:

1) Does that sound like field curvature? (90% sure it is but worth asking)?

2) The effect is not equal in all corners (some are actually not too bad). Now I think the setup is square - could this be a distance issue, or do I need to work more on getting the setup square (and if so, any ideas on how / what helps)?

3) On the off chance someone has already dealt with this, does anyone out there know the correct sensor distance for this scope and flattener combo? Initial messing about suggests I might need more than 55mm, but any pointers would be really useful.

4) If not, is it possible to infer anything about the spacing (whether I need more or less) from looking at the curvature?

Sorry ... that's not a couple of questions after all.

Cheers,

Billy

Did you ever resolve this Billy? I've just taken delivery of an ED72 and OVL field flattener and am experiencing exactly the same issue

[billyharris72]

Afraid not. I eventually went for the Skywatcher flattened, which is okay across maybe the central 2/3 of my ASI1600 field, but bad beyond that. Had horrible problems with chromatic abberation at the blue end (not fixable by refocusing, but okay with a DSLR and no filters). Resolved the CA issue with a more aggressive cut off filter between focus wheel and OTA, but the tube has so little in travel on the focuser that it would not then come to focus with the correct tubes between the focus wheel and the cameras.

In the end I sacked it and went back to the 130 P-DS. I think the more aggressive Astronomik "DSO" filters would fix the issue without adding to the optical train, and thus allow focus, but I've spent twice what the scope cost on trying to get it working and am not minded to spend more. 

Hopefully you'll have better luck than me with yours! Quality control does seem to be quite variable

Brst of luck. 

Billy