200mm Observatory Scope

[tilbrook@rbe.ne (Justin Tilbrook)]

Hi,
Here are some images I’ve taken over the years. The CCD used is a Sac-7 , with a Sony progressive scan HAD ICX098 4.5 diagonal, pixel layout 640 x 480 pixel lay out 5.6 x 5.6 um.
The camera itself is very basic and fan cooled, so there’s quite a bit of noise, I always get the best results on cold nights. When I got the camera and first used it , the star images across the field were distorted, I thought it was my scope. This surprised me, because me because I have parks mirrors.
It turned out to be the chip, which was glued in crooked, I had to pull the CCD apart and align it properly, tricky operation!
The 400mm scope.
I’m a little confused by your question on drive rates needed for tracking objects in the sky. I know the planets and moon have different sidereal rates , but unless you are tracking them for hours at a time it’s no problem. As far as deep sky tracking goes, it’s similar to Keplers second law ( equal areas in equal time ). The angle between Dec 0 and Dec 90 is no different, so you should only have to drive at a constant rate.
As far as the A/C synchronise motor, it is a constant rate but I have managed to cheat a little. I use a 12 volt inverter to run the drive and have used an in line resistor on the 12 volt side to vary the rate a little.
The 200mm observatory scope.
The 200mm scope use a 12v DC industrial timer motor, I purchased this from a second hand shop for $10.00. It’s still a crouzet motor with a one revolution per hour reduction box. The rate of this motor can be varied, on the motor itself and by a remote switch in the observing hut. The Dec axis is also a 12v DC motor, from a video recorder. I then fitted a 10 to 1 reduction box on it, which in turn drives a threaded rod for further reduction, ( see observatory scope image ).
Tracking.
I’ve setup the scope so it has dual controls, one set in the observatory and the other in the observing hut. The programme I use to control the CCD has two windows, captured images and guiding window.
When I’ve centered the object manually in the observatory, I go back to the hut and check the video window, if there’s bright stars in the field, video mode is fine for guiding, and if there’s no bright stars, I use short exposures to guide on.
Tracking times.
While the scope will keep the image centred for anything up to an hour , the longest exposure is 90 seconds, I might take 30 images of which perhaps 10 are ok. This gets around any drive errors, bad seeing or any other gremlins that will ruin an image. I just stack images.
Dec drive clutch, here’s two images, the 200mm and 400mm. The 400 is better design improved version of the 200. Both images are self explanatory , I hope.

Cheers,

Justin.

[alistairsam]

Thanks for the pics and detail Justin.
Pretty ingenious setup.

As for the tracking rate, I initially thought that the rate was constant as well, but learnt that there is a lot more to it and a few factors like refraction vary the tracking rate between objects near horizon and near zenith. Also King rate.
details in this thread, post 12,15 and 17
http://www.iceinspace.com.au/forum/s...+rate+tracking
autoguiding would compensate for changes.

I hadn't thought of keplers law though. will look it up.
what is the focal ratio for your scope?
what video camera do you use for your guiding scope?

[tilbrook@rbe.ne (Justin Tilbrook)]

Hi,
Thanks for the thread on sidereal rates, as you I had no idea that there as so many variations. Reading the threads it seems the nominal rate is the way to go, with a bit of guiding. I guess it all depends on exposure times you want.
My observatory scope is F/6.
I actually use the CCD itself for guiding, with the R.A. and dec controls right next to the computer I adjust as needed, when the next image comes up in the video window. Of course I discard the adjustment frame, thats no problem because I take a minimum of ten frames for any image. I try to stick to short exposures.
My setup has limitations, both in tracking and the CCD I use. So stick to what I can do and don't push the limits. One day I'll get a better CCD then I'll want to upgrade the scope and tracking.
Viscious circle!
Cheers,
Justin.