Talk about frustration ! I have been battling with the SCT and the EAF which uses the focus knob to carry out the focusing. This i knew was going to be not the best, but i didn’t realise it would be hopeless !!
After many hours trying to get a decent focus curve, i gave up. So how to fix this ?
As you can imagine, when you focus there will be a sea-saw affect with the primary mirror, for visual astronomy this is no problem, but for astrophotography , this “Slop” causes a not so nice focus curve ! So how do we fix this pretty epic issue !
We put a focuses in the image train ! There are many you can get, i decided on this, I have had to put in couple of spaces to make up the back focus. The 9.25 and greater SCT HD’s have locking knobs for the mirror, this prevents it from moving. The idea is get the telescope in focus with the knob, lock the mirror in place and then just use the Esatto do the fine focusing for the filters.
I’m still waiting on a clear night to test this, but prelim testing would say YES, but there is one issue I’m not sure how I’m going to tackle, and this is the varying of the back focus ! I think because I’m using an APS sized CCD i should be fine with a little bit of back focus variance.
Lets wait and see !