[gregbradley]

[QUOTE=pvelez;1045801]Greg

if you don't mind, can you elaborate on the hot spot/vignetting issue you had with the CDK17?

I have the CDK12.5 and STX-16803 and find it difficult to flat field out my images - especially in green for some reason.

Jase,

I agree wholeheartedly re support from PW. They fixed up my focuser issues quick smart.

Pete[/QUOT

Probably best if I post some examples. I'll dig one up. I tend to get an area in the centre that is brighter than the surrounds plus heavy vignetting. Flats don't often correct the vignetting. Baffles helped with the hot spot. I think the hot spot is less now since I fitted the primary mirror baffle PW sent me. Perhaps there is one available for the 12.5. My only concern with it is that it is relatively wide so its reducing the valuable aperture somewhat but I am unsure if its totally successful as I fitted it around the time I started using this small chipped SX camera so its no test. The SX camera shows almost no vignetting nor hot spot.

I find 6 flats, done with a white sheet over the aperture taken either at dusk or during the day with the roll off roof closed done at about 25,000 ADU. I then average combine these with no dark subtract. I then subtract a bias from the flat when callibrating the image. Your darks also are critical. If they are slightly off you will get surprisngly bad callibration results. So I make sure my darks are good, at the same imaging temp used and at least 6 subs for the darks, more is better. Take fresh flats everytime you move the camera. I also clean the filters and camera more often as cleanliness is far preferable to flats removing dust donuts and sometimes failing to do so.

As far as worse in green that may be due to light pollution or sky glow which is usually green. Light pollution seems to often be worst in the green channel.

I also remember a post by Rick that the CDK design and also other
RC type designs have conflicting variables. You can make it faster at the expense of more vignetting. You can reduce vignetting but at the expense of making the scope slower. So it comes down to what the market would prefer as a combination of qualities. It seems the 14 inch is a new compromise with long backfocus, less vignetting but slower F ratio. The new reducer sounds good with decent backfocus. The current backfocus is unworkable and not enough room for even an OAG.


Greg.