Darryl

Thanks all for the comments. So it seems people do make it work with larger instruments. I'm entirely imaging so the added complication of access to an eyepiece is not an issue. With my previous set-up the pier was fairly long, so the OTA was high enough to allow access pretty much down to the horizon. To clear the dome at zenith with the new OTA i'm going to have to shorten the pier which will limit access to the lower part of the sky. But as i'm at a UK sea-level location its barely ever worth going down there anyway. Looking forward to getting the house move done now so I can get things back up and running when Autumn darkness returns! Cheers, Darryl

I recently upgraded to a 14" OTA (my forever scope) to go with my recent 'forever mount' purchase. Unfortunately I think I might have 'over scoped' for the dome.. Despite measuring up in advance and coming to the conclusion that it would all fit in my 2.2m pulsar dome, in reality it all seems a bit tight when it's all together. Everythng clears but there's not much wriggle room. I'd rather not have to change the dome too, but it so happens that in a month or so we are supposed to be moving house, which would provide an opportunity to go to a larger dome if needed. So, does anyone on here make a 14" work in a 2.2m pulsar or has 'apperture fever' got the better of me this time? Darryl

You could try taking evening (or dawn) sky flats first on one side of the pier, then after a flip and then back on the other side of the pier. Differencing the resulting flats would give you an indication of their repeatability. I've had similar problems with flat calibration in the past on the Issac Newton Telescope (in a previous life when I had access to big scopes), we spent a lot of time trying to track down flat field errors and in the end put it down to scattered light upsetting the flats. Flat fields can be a right pain, best to avoid the meridian flip if at all possible. The only other though that I had is whether the star PSF varies significantly as a function of field position. If the photometry apperture is relatively small then there could be a significant difference in the amount of light outside of the apperture depending on where the star is in the field. An 'apperture correction' is often used to overcome this problem, particularly with systems with strong coma or field curvature. Cheers, Darryl