old orange tube c14 fork-mount tracking error?

Our club has a C14 from the 70's on a fork mount with the AC motor drive. I believe the mount to be totally unacceptable for ccd imaging, but they want a second opinion before passing on purchasing a ccd camera for now. The C14 would be used with a focal reducer. The drive has no autoguider ports and no PEC. Someone has told a member of our club that the mount can take unguided exposures up to 5 minutes. What is a more reasonable exposure time to expect? There is currently no guidescope for manual guiding, so we are talking unguided exposures only. The camera that has been mentioned is the ST237A by SBIG (also not my choice).

Thanks,
--Lonnie

Lonnie,
If I am not mistaken, ccdops, SBIG's software, allows a user to track and accumulate. That is, you can take multiple exposures in whatever duration, in seconds, you set the camera to and the software combines the exposures. This would be like taking one long exposure. I agree with you, a 5 minute unguided exposure is probably wishful thinking however, most mounts can track for for at least 30 seconds without a problem. If you perform a drift polar alignment, I'm sure you could track for longer. The ST237A is a great introductory camera. I have seen many excellent images taken with it using a color filter wheel and LRGB processing. I suggest you contact SBIG. Their tech support guys are great. They can answer all you questions with authority. Good luck
Alex

If it is at all possible get it polar aligned and put a guiding reticle in it (maybe even barlow it). That should give you a real good idea on wether or not it will be suitable.


Robert W.

The ST-237 is presumably being recommended for use in a Fastar configuration. If I'm not mistaken that scope is too old to be Fastar compatible.

Your best bet is to measure the periodic error of that mount. Any way to borrow a camera from someone to do some testing?

You want REALLY large pixels for use on a C14. The ST-9E and ST1001E cameras are well suited for use on the C14. If you must use smaller pixels, then get the Optec Widefied F/5 reducer. It has a large flat field (though you'll want to do a careful collimation for best results) and brings the focal length down to about 2000mm. That's still too long for an ST-7 type of camera, however, IMO.

Ron Wodaski
author of "The New CCD Astronomy"
http://www.newastro.com