Field flatness of refractor

[david_taurus83]

9 hours ago, 1DegreeN said:

Hi David

Are you sure that you have the backfocus correct? The Flat6AII's backfocus of 54.8mm is from the wide rear flange where a T-mount would be up against, but you are connecting via the internal M48 thread and by my measurement the flange for that threaded connection  sticks out by 5mm so the backfocus becomes 49.8mm. Also, you have to add (not subtract) 1/3rd of the filter width so the backfocus you want to achieve is then  around 50.8mm. The components you list come to 53.8mm so you could be 3mm over.

I've got the Flat6A II on my Zenithstar 103 and am very happy with the results. I haven't used CCD Inspector but visually the images are good with just very slight coma remaining at the corners of my APC-C size frame and PixInsight's FWHMEccentricity script confirms this.

cheers

John

Hi John, not quite sure I understand your measurements? The ZWO comes with an M48 to M42 16.5mm spacer and this goes onto the external M48 thread, see attached. Behind that I have a 10mm spacer, 20mm filter wheel, 0.8mm delrin spacers, 1mm assumed for filters and protective sensor glass and the 6.5mm from camera body to sensor. Hence the 54.8mm! Am I missing something obvious?

[david_taurus83]

3 hours ago, geordie85 said:

Where did you get the Flat6AII from as they're not in stock at FLO. Thinking of getting one for my flt98 

I really wanted a Star 71 but they were like hens teeth! I spoke to Ian King and he recommended a Z73 with flattener as it would be more versatile. I really wanted a triplet as a minimum so I went for the GT71. Only RVO stocked these when I looked so I ordered from them. They advertised the older Flat 6 reducer but the box turned up packed with the new Flat6A II.

[geordie85]

8 minutes ago, david_taurus83 said:

I really wanted a Star 71 but they were like hens teeth! I spoke to Ian King and he recommended a Z73 with flattener as it would be more versatile. I really wanted a triplet as a minimum so I went for the GT71. Only RVO stocked these when I looked so I ordered from them. They advertised the older Flat 6 reducer but the box turned up packed with the new Flat6A II.

I love my star 71, even more so since it returned from FLO after a visit to Es Reid. 

I wanted an original flat 6 too but I think they're discontinued now. I may have to send a few emails out to see if FLO are getting any in any time soon, and if RVO actually have them in stock. 

[david_taurus83]

So here are a few screenshots of last nights attempts. Excuse the gradients as i only calibrated with flats and done an Auto background extraction. Not bad at all in the corners with the bottom right being the worst. It was the top right when I used my DSLR so im guessing there is slight tilt in the flattener itself. It would appear my indoor experiment was a success as I got a similar result. Not perfect but ok in three corners and one corner worse than the rest.

[geordie85]

I have the same problem with my top left corner. Might try and solve it tonight since it'll only be  clear for a few hours plus the moon will be bright