ETX TECHNICAL TIPS
From: Roger W. Lange rwlange@PioneerPlanet.infi.net
I purchased a used ETX about six months ago and only recently discovered your page. I'm sure it's helped a lot of budding amateurs (and some not so budding) to better enjoy their instruments.
I noticed a couple of questions about difficulties in Astrophotography and daylight photography with the ETX. I also suggested the cap on the eyepiece and/or opening in an earlier note.
Here's a suggestion on photography that has worked for me: When a camera is attached to the back of an ETX using the short adapter and a "T" ring on your camera body, the focus is extended by 48mm. In other words a 48mm long extension tube placed in the eyepiece holder will allow you to focus through your eyepiece(s) instead of the camera. Lower the mirror, cover the eyepiece, and fire away. Any eyepiece that has the focal plane at the shoulder where the 1 1/4" tube meets the top section will work. I have a dozen that do just that. Unfortunately, the Meade 26mm does not have the focal plane at this position. Also, a 26mm fl eyepiece with a 52 degree field will have a field diameter the same as the short side of the 35mm negative (24mm). So you will see at least the approximate area you will be photographing and it will be a lot brighter. Higher power eyepieces will allow one to make more critical adjustments in focusing. If you have a favorite eyepiece you want to use, focus your camera at a bright land object, determine the length of the tube necessary to bring that eyepiece into focus, and you have your own custom adapter.
And speaking of firing away, since the ETX has a photographic focal ratio of about f/16 (I said about) the shutter speed for a land scene on a sunny day or for a full moon will be the reciprocal of the ISO value of the film, you are using. ISO 400 = 1/400 second. Astrophoto books give all kinds of formulae for exposure times for various objects. All of which show the necessity for guided or at least tracked exposures. Without tracking, exposures through the scope at 1/4 to 1/15 second will be slightly blurred just from earths rotation. Longer exposure times would obviously be worse yet. Opening the shutter and uncovering the lens with the old "hat trick" can't be beat either. Also, viewing through the eyepiece for a few miuntes prior to taking any photos will tell you whether your scope is tracking fairly well.
Well, I guess I've rattled on long enough. Hope this info helps others. Thanks for a very interesting and informative site. Roger
To clarify a question that has been raised, the extension required is 48mm. The tube itself will have to be 24mm longer or 72mm in total length to compensate for the eyepiece holder depth.
Go back to the Tech Tips page.
Go back to the ETX Home Page.