I'm looking for some help. I'm working with a college who has some grant money to put up a robotic observatory whose prime purpose would be exoplanets. The dome would be an Astrohaven 7 foot. Not certain of the setting as far as light pollution, it could be suburban to rural.
What telescope / camera combination would you recommend that could fit in that size dome?
Thanks so much for your advice!
Some of the things I picked up so far.
- The FWHM of the star should be spread across 3-5 pixels minimum. For okay seeing conditions, that would be roughly 0.67 arc-second per pixel just to be sure without defocusing though you could get away with 1.3 most of the time.
- So take your scope's focal length (assuming you already have one lined up), multiple its length in millimeters by the desired pixel resolution in arc-seconds and divide it by 206.
- If you don't have one lined up, buy a SCT (Schmidt-Cassegrain telescope) and it should be on a fork mount with its base connected to an equatorial wedge and on a permament pier in a water-proof observatory.
- The fork mount is for avoiding interruption caused by a meridian flip.
- SCT is because that's the only comercially-available telescope on a fork mount with the necessary aperture.
- Equatorial wedge for polar alignment
- Permanent pier because a fork mount telescope on a wedge is not as stable as a scope on a German-equatorial mount and could tip over. So stick with a GEM if you can't install a permanent pier
- A waterproof structure (which you already have) to keep you from having to lug out a heavy SCT and set it on an equatorial wedge mounted to a permanent pier for every session. Again, if you can't install a permanent pier, then set up a GEM+telescope that can fit inside your observation dome.
A camera will be harder to pick up. Currently CCD cameras are falling by the wayside with production on commercially-available CCD chips being stopped, but CMOS cameras with a bit depth higher than 12-bit being few and far between. ZWO and QHY just coming out with a 16-bit, full-frame monochrome camera starting at $4000 before the obligatory 2" electronic filter wheel. All other astronomy monochrome CMOS cameras, with the exception to the ones with the IMX178 chip (which ZWO has recently discontinued and only QHY has) are 12-bit.
The factors to look at for exoplanet cameras are
- FWC (Full well capacity). More is better for avoiding saturating your pixels.
- RN (Read noise). Less is better.
- Dynamic range. This is measured by taking the FWC and dividing it by the camera read noise. To convert it to stops, use the equation below
- dynamic range stop = log(FWC ÷ RN) ÷ log(2) or
- dynamic range stop = ln(FWC ÷ RN) ÷ ln(2)
- Both will get you the same answer.
- Bit depth. High dynamic range will not make much of a difference if the bit depth is too low. Bith depth should be higher than the dynamic range stop of the camera. I've even heard bit depth should be 2-bits higher than the dynamic range stop of the camera.
CMOS cameras have high dynamic range because they have lower RN but most of them have low bit depth (12-bits) while CCD cameras have high bit depth (all of them are 16-bit except for the video CCD which are only 12-bit) but unless you bin them, their dynamic range will be way too low. Also, commercial CCD cameras are expensive and, in the long term, being phased out (though that will be another decade or two). For a CMOS camera, I would suggest going with the new 16-bit ZWO ASI6200MM or, if you go CCD, go with one that doesn't have an antiblooming gate (so long as you're careful not to let the image get saturated and have electrons bleeding into the adjacent pixels) because they are more sensitive and have a larger FWC. So that would be any CCD with the KAF-0400, KAF-1001, KAF-3200 or KAF-1603 chip. SBIG, Moravian, and Apogee sells CCD camera with those chips in them. Also, those chips are a better fit a SCT since their pixels range in size from 6.8 µm to 24 µm while the ASI6200 pixel is only 3.8 µm which would strain the tracking accuracy of your mount unless you bin to no less than 2x2, though preferably 3x3 or even 4x4 for those longer focal lengths.
I hope this helps.
Does anyone have any feedback on the FLI Kepler cameras such as the KL400? Here is a link to the stats: https://www.flicamera.com/spec_sheets/KL400BI.pdf
Sensor TypeBack Illuminated CMOS
Active Pixels2048 x 2048
Pixel Size (microns)11 x 11 µm
Imaging Area (Diagonal)22.5 X 22.5 mm (31.8 mm)
Full Well Capacity90000 electrons
Typical Readout Noise1.6 e-
Dynamic Range94.6 dB
Frame Rate24 fps (Rolling HDR)
Cooling MethodAir and Liquid (see Note 1)
Max. Cooling (Air)45°C below ambient
Dark Current (typical)0.015 eps at -20C
InterfaceUSB 3.0 (Optional QSFP; see Note 2)
Data Bit Depth16 bit (see Note 3)
Optional MountsNikon or Canon
External Trigger In/OutStandard
SDK / SoftwareKepler SDK / FLI Pilot
Weight3 lbs (1.3 kg)