Spacing required to link QHY168c to Skywatcher Field Flattener

[old_eyes]

Over the last year or so I have had much more fun out of imaging with my Canon 1000D than I have with the Atik 314L+ mono with filters. Despite what I hear about the improved performance to be obtained from a mono plus filters, I actullay find I do more satisfying imaging with a colour camera. I'm still using the mono for narrowband, but a combination of indolence, time pressure and Welsh weather pushes me towards colour. 

So I thought since an uncooled DSLR worked well, why not get a cooled APS-C sensor set up for astro-imaging with good electronics, and I have just received a QHY 168c.

Trying to think ahead I realised that since the DSLR required a field-flattener, so would the QHY 168c. With the right adaptors I took the T-mount off the Skywatcher Field Flattener and coupled it to the 168c.

Last night was first-light. Pointed it at M31 and got images with quite short exposures (some faffing around with recalcitrant software, but that is par for my course). So far so good, but I had stars streaking away in the corners and obviously had a field curved like a beach ball.

Obviously the distance from the flattener optics to the sensor plane was completely different from the DSLR.

So I think what I need to do is to measure the distance from the lens in the flattener to the back of the Canon T-mount, add the Canon back focus, and make sure that the new configuration has the same total distance from lens to sensor. Does that make sense?

To check it was field curvature I used the trial version of CCDInspector to make sure it wasn't a collimation problem in the sensor. CCDInspector reported some tilt, but the main problem was the wild field curvature. I figure I'll sort that out first and then use the center and tilt ring to fine tune. 

It took me quite a while to figure out how the center and tilt ring is supposed to be used, but I think I've got it now. If anyone knows a written up procedure for using one to achieve collimation, let me know.

The only problem is with the weather we have been having the 30 days trial of CCDInspector will probably expire before I get another image 

[Spikey]

@old_eyes It's the distance from the shoulder of the reducer to the surface of sensor that needs to be 55mm.

[Guest]

Actually, you can also refer to the technical data provided by SW, the working distance for the SW Field Flattener is 55mm, the Back Focal length for your CCD is 18, so you would need 37mm of distance in Adapters.. and you should be done..

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p6515_Skywatcher-Field-Flattener-fuer-Esprit-80ED-triplet.html

Just in case the 30 days trial of CCDInspector should expire before you get another image..

I also have the CCDInspector.. Full version.. a great peice of technology / software..

Good luck

Martin

 

 

[old_eyes]

Thanks Artem. I had spotted that, but the field flattener Ihave is this one https://www.firstlightoptics.com/reducersflatteners/skywatcher-field-flattener.html and there seems to be a lot less information around. But as both you and @Spikey suggest, working on the assumption that it is 55mm with the canon T-mount  in place it should work out OK.