Fri, 04/01/2022 - 14:50
I'm in the process of evaluation options for 12" aperture scopes. I'm considering a range of quality and expense options ranging from TS Optics F4 Newtonians to a Planewave CDK 12.5. Working, for now with my current camera, ASI294MM Pro, I've calculated image scales ranging from 0.57"/px for the 'reduced' Planewave to 0.95"/px for a TS Optics F4 Newtonian.
Based on a rule of thumb in the 'Guide' that the target FWHM should spread over 3-5 pixels, it seems that the options in the 0.80-0.95'/px are a bit better than those from 0.57-0.60"/px. Is this a correct way to look at this issue of image scale as it relates to light curve quality?
Thank you for any responses !
I currently run a 12.5 inch IDK scope with the same camera. It seems to function well. I do not use a guide camera and rely on my mount to do the work. I prefer this since the clouds do not screw up the image sequence. Pixel scale when 2x2 bin .901arc sec/pixel and half that bin 1x1. I frequently defocus to keep my ADU's to keep in the linear range (my analysis suggests loss of linearity over about 40000) and have found the FWHM around 3.5 burns out my brighter targets. I have had good luck using a CBB filter
Thank you Bryan
Are you using a reducer with the CDK? I worry about having enough backspace for a reducer and an OAG. I have a Mentor helping me ramp up for exoplanet work who is encouraging me to guide. I have a Paramount mX+ Mount but am waiting for a pier adaptor before I can test it. Depending on the outcome, I may or may not feel a need to guide. Is your concern that the impact of thin clouds could mess up the guiding so it’s worse than no guiding at all?
With my 294MM, I calculated an image scale with a reduced CDK 12.5, of 0.56 which isn’t ideal with our usual seeing conditions. How do you get your 0.901 ?
Thanks for responding !
I am running a…
I am running a paramount MX+ as well. once that mount is aligned and you run T point and supermodel, it has superior pointing and tracking. I have not fully optimized my tracking and have about 100 pixel drift over a 4 hour run. I use exotic to reduce the images and that software does very well tracking the images. AIJ has a lot more trouble tracking that amount of star movement so I usually have to manually stack images for AIJ to work. I would expect if I fine tuned my mount that my drift would greatly improve but have not gotten motivated to do that so far.
If you bin 2x2 you double the pixel size essentially so that is how I get to .901 arcsec/pixel Recently started experimenting with 1x1 binning for very clear nights and that had my scale at .495 I think..
I like guiding with the mount alone . . .if clouds show up then I toss those images but I do not loose tracking. I just go to sleep and let er run
no reducer being used. I have a AG Optics IDK 12.5 . . .very happy with it. On a backyard pier
sent you a message on slack exoplanet channel . . . I found you on there too. The slack channel gets a lot more traffic.
Image Scale /question..
I can't quite follow the image scale mentioned in your notes. See if you can straighten me out on my (miss)understandings below on the ASI294MM camera. The info below is based on Maximdl and NINA where I ran variations in Binning input settings to determine the Fits Header outputs for the images captured with those input settings.
Bin 1= 2.3u pixels and is the 'unlocked' version with low full well depth
Bin 2 is the 'default mode' and runs with h-ware binning to get 4.63u pixels (I think)
Bin 3 is 6.95u pixels
Bin 4 is 9.26u pixels
If your IDK has a FL= 2128 mm (from website), then I get image scales at:
Bin 1 = 0.223" image scale
Bin 2 = 0.448" image scale
Bin 3 = 0.663" image scale
Bin 4 = 0.909" image scale
I use Sharpcap quite a bit for straight EAA observing but it has a unique way for selecting Image Modes and Bin settings so I'm dodging that one in this question.
With my current setup of an 203mm F4 Newtonian and iOptron CEM70G, I've not had any image alignment issues so AIJ is fast and flexible. Yesterday, I reduced and analyzed 195 transit images captured by our club's CDK 17" (unguided) and ended up having to plate solve every image to align them adequately for the photometry to run. The CDK 17" I sometimes use at 'itelescope' in NM, is also run unguided but I've never needed to align or plate solve anything but the first image and that's just for confirming target and comp stars.
Anyway, sorry for the long note.
ps: I've never used slack but am heading over there to check it out. Thanks for mentioning it.