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Mars imaging with 3 scopes - comparison

Started by RolandC, 07/11/2016 12:34PM
Posted 07/11/2016 12:34PM Opening Post
I did some imaging of Mars last Saturday in fair seeing just after sunset with my 175 refractor, 250 Mak and 305 Mak. I started with my 175 refractor which seemed to give the best visual image, probably because the larger instruments had not settled due to the day's heat build-up. It has been quite hot during the day, around 90F inside the observatory, which then drops rapidly to about 70 when the roof is opened at sunset.

After shooting Mars with the refractor, I went to the 305 Mak-Cass, but there were some tube currents. I opened the back of the scope to expose the quartz mirror and blew some air across the surface from an ordinary room fan placed some 4 ft away. This immediately stabilized the image and eliminated the plumes off the top of Mars. This actually worked better than an internal fan and added no vibration to the scope.

After some 15 minutes of imaging I switched the camera over to the 10" F14.6 Mak-Cass (back also open to get rid of the heat), and was surprised to see a very contrasty image of mars with much surface detail. I was able to get some 8 video frame sets of about 1000 frames each before the seeing deteriorated (as it usually does after sunset).

The contrast of the 10" Mak-Cass is due to the very small central obstruction (23%) which makes it perform almost refractor-like. Both the 175 and the 10" were visibly more contrasty than the 12" Mak because this instrument was designed for wide-field imaging and has a 38% central obstruction. You can see it in the comparison images.

The small central obstruction not only improves contrast, but also reduces the effects of seeing quite dramatically. This is quite easily seen when watching the videos taken with each of the 3 scopes.

Just for comparison I added a simulation of Mars from calsky.com, and sure enough the 10" Mak image clearly shows 3 of the volcanos as well as Olympus Mons to the right of middle. The albedos are different in the simulation (almost inverted), but you can clearly see the giant mountain and the trapezoidal area around it. Lost of other features are shown, with Mars being a puny 15 arc seconds in diameter and very low in the sky. I would love to see what this scope could do when Mars is at opposition, twice as large and high in the sky in excellent seeing.

Rolando

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RolandC's attachment for post 60347
Posted 08/04/2016 03:38PM #1