Focal Reducer Offset to Camera

[Rossco72]

Howdy wise and knowledgable imagers,

I have never been entirely happy with my stars, they aren't awful but they also aren't very good. I have a William Optics GT81-IV telescope with a William Optics Flat 6A mk3 reducer/flattener. These are supposed to be able to produce a 44mm flat imaging circle. I use a ZWO ASI533MM Pro camera which only has a 24mm or so diagonal and so I was hoping for pretty much excellent stars and flatness across the entire image.

 took a series of shots and used ASTAP and Pixinsight to try and assess the quality of the stars, the eccentricity and FWHM etc. but I could do with some help understanding where I am going.

Each shot is taken, assessed and then the adjustable field flattener rotated one turn of the very fine threads which is probably less than a mm. However I am now considerable further away than the recommended 62.1mm for the GT81 telescope. Am I going too far or do I need to keep going further?

Below are the series of FIT images followed by the ASTAP analysis images and then the Pixinsight analysis images. Each set is 1 turn of the reducer further away than the set before.

1_30.0s_Bin1_L_20230514-225148.fit

2_30.0s_Bin1_L_20230514-231103.fit

3_30.0s_Bin1_L_20230514-232039.fit

4_30.0s_Bin1_L_20230514-233632.fit

6_120.0s_Bin1_H_20230515-225159.fit

7_30.0s_Bin1_L_20230515-230931.fit

8_120.0s_Bin1_H_20230515-225817.fit

9_30.0s_Bin1_L_20230515-232831.fit

 

Anyway, apologies for the long post, but I would appreciate input from anyone with experience in setting the correct offset and whether I should continue to move further away to dial this in.

Cheers

 

 

5_30.0s_Bin1_H_20230515-224627.fit

[Stuart1971]

What it looks like to me, is that the flattener is introducing tilt the more you move it out, which is skewing the figures somewhat…the tilt diagram is altering quite a bit over the images, when the stars all elongate towards the middle it means you need to move the flattener further away from the sensor…but if you have tilt this can really confuse things, so maybe the threaded flattener, the more you screw it out is sagging and causing tilt..

[Rossco72]

@Stuart1971 I understand tilt may make it impossible to determine, but in your opinion do I still need to move the camera further back? The last set of images still appear to show stars radiating from centre.

[Stuart1971]

19 minutes ago, Rossco72 said:

@Stuart1971 I understand tilt may make it impossible to determine, but in your opinion do I still need to move the camera further back? The last set of images still appear to show stars radiating from centre.

Well, there is absolutely no harm in trying…and yes it does look that way, that is needs more backspace…👍🏻

[michael8554]

[Rossco72]

5 hours ago, michael8554 said:

Thanks. i have that, this was my starting point but i am way over 70mm now