Setting the right distance with an adjustable flattener - what should I look for in image?

[Richard_]

My experience thus far with astrophotography has been with the WO Redcat51. It is a petzval design scope which doesn't require an external flattener, so I've not had to think too much about flattness up until now.

I've recently taken receipt of the WO FLT120mm telescope which I've coupled with the WO Flat68III adjustable flattener (1.0x focal distance, does not reduce). I am capturing images using my ASI533 camera which has a small sensor (~11mm x 11mm) so it has a small imaging circle compared to other cameras. According to the guidance from WO on their website, I should adjust the distance of the flattener to 10.7mm for use with my telescope. I've set the distance and measured with calipers. However, the guidance also shows a back focus of 65.2mm but my system is currently using 56mm of back focus so I've not added extension tubes (I noticed this after my first imaging session).

I've attached a stacked & unlinked-stretched image of Melotte 15, inside the Heart Nebula, (no other processing has been performed) using the L-eXtreme filter and I was looking for feedback from some of the veterans as to whether the flattness of the image is OK or not and whether I need to add in the extra 10mm of back focus. My understanding is that without a flattener, stars will appear stretched at the corners/edges of the images. Looking corner to corner, I do not see any stretched stars, they all look nice and round and in focus so perhaps I don't need the extra back focus either?

I've attached the stack as a JPEG and a FIT should you wish to examine the image. Any comments or feedback are welcome!

Melotte_15_stack_stretch.fit

[happy-kat]

A single frame as JPEG might be easier to see as the stacked image may have rounded the stars, though to my limited awareness the corners aren't obviously off for that camera.

[Richard_]

I've attached two files:

1. Raw FIT file straight from camera
2. The same file which has been calibrated, debayered and registered (XISF)

I'm away from computer so I don't have access to PixInsight to convert, I just have these raw files. 

Session_3_Light_SH2-190_180.0s_Bin1_533_2_gain100_20211217-203003_-10.0C_0001.fit Session_3_Light_SH2-190_180.0s_Bin1_533_2_gain100_20211217-203003_-10.0C_0001_c_d_r.xisf

[Laurin Dave]

Pixinsight reports an FWHM of between 4.45 and 4.6 (that's pixels as I didn't set the image scale - which I've just checked is 0.99) and eccentricity between 0.42 and 0.45.. so it's pretty flat across the frame.  I'd thought with the WO adjustable flatteners that you just unscrewed the back (ie Camera end) to give the required spacing then tightened the locking ring. 

HTH 

[fifeskies]

I found this star test helps to set the correct backfocus

Edited December 18, 2021 by fifeskies

[Richard_]

28 minutes ago, Laurin Dave said:

Pixinsight reports an FWHM of between 4.45 and 4.6 (that's pixels as I didn't set the image scale - which I've just checked is 0.99) and eccentricity between 0.42 and 0.45.. so it's pretty flat across the frame.  I'd thought with the WO adjustable flatteners that you just unscrewed the back (ie Camera end) to give the required spacing then tightened the locking ring. 

HTH 

Thanks for checking! Regarding the flattener, that's correct. You rotate the back end and there is a scale on the body like a focuser. Use this as a guide to get to the required distance then lock with the locking ring. Then measure with calipers for more precision and adjust further if needed. My calipers measured 10.7mm plus or minus 0.1mm. 

It's strange that there are listed back focus distances for each scope. If you look at the other scopes listed on the WO flattener website I linked, if you take the adjusted distance from the back focus distance you get about 55mm except for the FLT91 and FLT120 scopes which is slightly. I found this a bit odd, but it doesn't seem to affect my star shapes like the image "fifeskies" posted. 

20 minutes ago, fifeskies said:

I found this star test helps to set the correct backfocus

Thanks for sharing this. I remember seeing this image before and I definitely don't see these types of star shapes in the corner.