Flat field not flat

[TrickySpot]

Hi Guys,

Been away from the hobby for a while, but over the last few months rebuilt an imaging rig. I have the WO GT-102 with its dedicated 0.8x reducer and am using an Atik One 9.0 as my camera. 

If you look at the attached photo, you'll see the stars in the corners are still stretched...am I doing something daft, or is the GT-102 0.8x reducer just not that great?

I've screwed the reducer directly onto the camera, then straight into the scope, any ideas/thoughts would be most appreciated!

Clear Skies!

Martin

ET2.tiff

[Oddsocks]

Martin,

Your flattener distance to the camera CCD is incorrect, this will cause over or under correction and coma will be evident in the image.

Assuming you have the William Optics flattener type 6a, the correct distance from the shoulder of the M42 thread on the flattener to the camera CCD chip is 60.95mm, measurement "A" as described in this WO document: http://www.williamoptics.com/accessories/images/Flat6A-for-Canon-EOS-T-mount.jpg

For the Atik One - Nine, the distance from the face of the camera's M42 thread to the CCD chip is 28mm as described in the Atik dimensional document here:

http://www.atik-cameras.com/wp-content/uploads/2014/08/One_mech.pdf

Subtracting the two known distances, 60.95mm - 28.00mm leaves you with a distance to fill of 32.95mm.

Since the flattener and camera both use M42x0.75 "T" threads then you can use standard M42 male-female "T" spacers to fill the gap and put the flattener at the correct distance from the CCD chip.

You can make up this distance using individual mix and match M42x0.75 "T" spacer tubes and spacing rings or use a single adjustable spacer like the Baader Varilock 46 from FLO which adjusts from 29mm to 46mm in length.

http://www.firstlightoptics.com/adapters/baader-varilock-46-lockable-t-2-extension-tube.html

Once you have the flattener to CCD distance right then the coma will be corrected.

Note: if you have the newer William Optics flattener type 68-II then the rear flattener thread is M48 and the distance to the CCD is only 51.5mm and would require a shorter Baader Varilock model plus M48 to M42 adaptor but you said you have screwed your flattener direct to the camera so I'm guessing it is the type 6a flattener you have, just in case, the WO document for the type 68-II is here and you would need to recalculate the distances and spacer components required accordingly:

http://www.williamoptics.com/accessories/images/P-FLAT68II-Usage-Chart.PDF

As usual, the information given above is not guaranteed to be accurate, double check everything stated before purchasing anything and verify with the retailer by phone or email that the parts you are buying will be the right ones for the job.

William.

[TrickySpot]

Thanks a million William for your detailed reply. I did make a massive assumption that as all the CCD cameras I've had over the years have had their CCD's pretty much the same depth from the housing, that the flattener would be designed to be in the correct range from the CCD, but I can see clearly now that I was just being daft!

I have a few spacers and one is in the region of 30mm, I will test as soon as we get another clear night!

Kind Regards,

Martin