Telescope Reviews: Some rambling, feedback solicited

Ok, work has been hectic for a month, and I've had little / no time to sit here and read on things. Took a break this morning, did a little reading, hunted a few details down, and specifically went back and read older threads on measuring transits etc.

Short history, chris and I started down the road of taking 'pretty pictures' using the telescopes and dslr cameras 3 years ago. We've slowly upgraded equipment, and have reached the point where we have everything we need to do some great stuff with the 'pretty picture' dlsr photography. Chris has always been an avid photographer, so, she's really enjoying this newfound photography application. So, imagine my surprise over dinner the other night when she pipes up 'I think I'd like to try doing exoplanet work'.

In all the threads, and re-reading everything here on the subject, as well as a few other places, the most important lesson I've taken from it, first learn the limits of existing equipment, then, if and only if, those limits prevent you from doing what you want to do, start looking at new equipment. To do photometry for measuring an exoplanet, we need to be able to measure magnitude changes at the 0.01mag level. Here's the list of cameras we have, and, the reasons _I_ believe they are unsuitable.

dsi-pro - To many hot pixels. My particular copy of this camera has an inordinate number of hot pixels at 30 second and longer exposures. At 2 minutes it's got a huge background gradient, probably some kind of thermal issue with the electronics. I can do 0.1 mag measurements with it fairly strait forward, but, getting accuracy to 0.01 mag just isn't going to happen. The noise / gradients etc are going to drown out any measurements at that level. In the dead of winter, when ambient temps at the telescope are well below freezing, this camera is pretty good, most of it's problems seem to go away, but that's only 2 months of the year. That suggests to me, a similar camera with 20 or more degrees of cooling can give much better results.

dslr - We have 3 different dslr varieties here, but, they've all got 12 bit a/d converters. I cant get the precision needed using 12 bit a/d, and the bayer matrix is a bit of a hinderance.

qhy5 - normally used as a guide camera, but, can be used as a 10 bit monochrome imaging camera. It's pretty noisy on longer exposures, and 10 bit a/d is not going to cut it. I haven't tried a real time stack and add process with this one. It'll take a fair amount of time/effort to write a program for that, and I dont have the time in the next few months, work wont allow me to take that kind of time.

My conclusion, if we want to seriously measure some of these things, then, a different camera is going to be required. At present, I've kind of narrowed it down to two options, and, am interested in feedback from folks on my train of thought for each.

Option one - This is the cost effective option. Starshoot II which is somewhat better resolution than the dsi-pro, and it's cooled, so, should be useable for a much longer part of the year. For us, middle of the summer is irrelavent anyways, we get a month centered on the solstice where there is really no night to speak of. It does get dusk, but, at these lattitudes it doesn't get dark. The drawback to this camera is, unregulated cooling, so, we'll have to build up libraries of darks at various temperatures, which means initially, taking darks with every session. With this camera, we'll still need an autoguider, but, both of our kits already have the Kwiqguide and it works well. If we go this route, the starshoot camera would be used primarily in the 8 inch SCT with a 0.5 focal reducer inline. I'll need to add a filter as well for this one.

Option two - Not so cost effective, but, substantially more capable. I've been looking at the used market for an SBIG ST-2000XM, and they can be had complete with filter wheel and filters for a price that is about 8 times that of the starshoot, but, a reachable number. Regulated cooling and a much larger sensor. Most of the used ones come complete with the filter wheel and a set of filters, so, it's also useable as a 'pretty pictures' camera. For us, this would mean we can build a dark library for specific temperature points, and not have to shoot darks at the time we do the real measurements. it also opens up some other potentials, and, these are the specific things I'm wondering about, and hoping folks with more experience in this subject can flesh out for me. We'd probably use this one with the 0.63 reducer on the C8.

- Since the transit stuff is a lot of frames over a long period, I'm thinking that I could use the filter wheel, shoot successive frames behind different filters, and end up with light curve plots for different parts of the spectrum, all from one evening of shooting. The idea is, once I've established the exposure time required behind each filter, change filters between every shot. Is this a valid concept ?

- And here comes the real 'off the wall' idea we had, and I'm really curious about your thoughts on this one. Again, considering that transits are slow events, so there's plenty of time to do various types of images in each phase, we were contemplating this. In the 5th slot of the filter wheel, most folks seem to put an Ha filter, we were thinking of putting a diffraction grating in. So, the idea is to take two spectrum shots before the transit, two more during the transit, and a couple more after. Assuming I can solve the issues of spacing between the filter wheel and the ccd, as well as deal with the re-focussing required (all of our telescopes have motofocus on them, which we can control from the computer), would there be any chance of 'useful' spectrum data taken this way during an exoplanet transit ? I'm thinking specifically of something like the star analyzer sitting in the 5th slot on the filter wheel.

Ok, any and all feedback is interesting. We are going to spend some money now, just haven't decided if we are going to 'go small' or 'go the gusto'. If our thoughts on putting the grating into the filter wheel actually have merit, that may well be the deciding factor for us....