Edge HD 8 - Spacing, tilt or something else?

[SyedT]

Hi guys,

After finally getting some clear skies, I've managed to get collimation as good as it will be for now. The issue now is that I'm either having tilt or spacing issues. I have an Edge HD 8 with 0.7x reducer, camera is the QHY268M (APS-C chip). I have an external Primaluce Lab Esatto 2" microfocuser to reduce the effects of mirror flop by using the mirror locks. Stars are reasonably round in the centre, but getting significant distortion at the edges (see below). The vignetting is easily sorted with flats so that's not an issue. Initially I thought it may be a spacing issue as the stars seem to radiate outwards i.e. I need more spacing. Not convinced though as the pattern doesn't appear to be consistent all the way around (could be tilt mixed in here causing differing distortion?), and more importantly my spacing is already beyond the optimal (106 mm is the recommended spacing including 1 mm for to compensate for filter thickness, and I'm currently at approx. 108-110 mm). The other option is tilt. I've run ASTAP's tool for checking tilt which seems to indicate very minimal tilt if any. All connections are screw-on or threaded except for the focuser which is held on by 3 grub screws (they're pretty solid and the adapter allows for a self-centering mechanism so it sits in the same position regardless of how you've put it on). The mirror locks are engaged at all times except when I've needed to adjust collimation.

I tried the method of back-calculating focal length and comparing the theoretical operating focal length and seeing whether I actually do need more spacing regardless of the manufacturer's recommendation. My current calculated focal length based on the image scale for 2x2 binning is (7.52 * 206.3) / 1.06237 = 1460 mm. The theoretical operating focal length according to the white paper https://s3.amazonaws.com/celestron-site-support-files/support_files/edgehd_whitepaper_final.pdf should be 2125 * 0.7 = 1488 mm. This according to a rule of thumb I found somewhere would equate to an extra spacing of 28/3 = ~9 mm. Looking at this, do I need more spacing? Or is it really not as simple as that considering the mirror moves during focusing, and I have an external focuser?

Would appreciate any help!

A picture of the rig for motivational purposes  

Edited June 16, 2021 by SyedT

[newbie alert]

Looks like spacing issue..  I thought the spacing on the hd was about 130mm?

My standard 8 is at 105mm and with the reducer should have 1280mm fl, but I have 1306mm

[knobby]

Looks like spacing as the elongation is equally pointing away from the centre (imho) not far off though.

[SyedT]

14 hours ago, newbie alert said:

Looks like spacing issue..  I thought the spacing on the hd was about 130mm?

My standard 8 is at 105mm and with the reducer should have 1280mm fl, but I have 1306mm

 

11 hours ago, knobby said:

Looks like spacing as the elongation is equally pointing away from the centre (imho) not far off though.

Cheers guys. My thoughts were about spacing too, but the issue I had was that with the 0.7x reducer, the recommended spacing is reduced to 105 mm (vs 133 mm without it). The only thing I can think of is that this is inaccurate as it may be based on the advertised focal length of 2032 mm, whereas the actual focal length in the white paper is 2125 mm. This would also fit in with the extra focal length that I seem to need according to the back calculation (if it really is as simple as that). I'll try out extra spacing at the next session and see how it turns out.

[newbie alert]

On 16/06/2021 at 17:39, SyedT said:

 

Judging on this, send it to me for a thorough testing😁😁

[Ross gould]

Following this thread as my set up is celestron hd 800,0.7 reducer,oag,filter draw,zwo asi533.

from sensor to reducer 105mm, plus 0.7 for filter.

I think it needs more rings to increase back focus.