Last updated: 7 April 2004

Subject: Using the ETX-90 spotting scope for Astronomy
Date: 4/5/04, 21:15
From: William Carson (
I wanted to send you a couple photos and a short description of my
ETX-90 spotting scope from which I have derived a great deal of
astronomical pleasure. The satisfaction I have had in using the scope is
due in no small measure to the tips which I have gleaned from your

Spotting Scope Spotting Scope
I purchased a 6 inch Newtonian some time ago which turned out to be too large for me and my son to "grab and go." We needed something lighter and quicker to transport, put together, and just look at things in the sky. After perusing your website I decided to purchase an ETX-90 spotting scope and put it on a photo tripod. After some slight modifications it has worked out splendidly. It is apparent from your website that many people like the GOTO feature of the ETX products. I myself have never felt any particular desire to use a computer mount. I would suppose that I spend more time looking for objects than I would spend fiddling with a computer mount, but on the whole I am happy without it. Of course to each his own. I never installed the supplied finderscope. Instead I purchased a Tasco RF37x20D .22 Rifle scope and attached it with some hose clamps. It is quite easy to look through with 2.5 inch eye relief and 3x magnification. I can see the 3 stars of Orion's belt with room to spare, and the extra eye relief keeps me from bumping into the eyepiece with my face when peering into the "finder." It also zooms to 7x if desired. One obvious drawback of a photo tripod is the difficulty of pointing the OTA accurately. This problem is exacerbated by backlash when tightening the locks. Celestron makes a Slow Motion Control (#93804-A) which helps immensely in this regard. It is fairly straightforward to get an object (more or less) centered in the "finder" using the tripod controls, center it with the slow motion controls, and then observe it in the ETX-90. One oddity is the requirement to use the slow motion controls backwards. They seem to be intended to only be used to point the OTA towards the horizon when mounted front-wards, but if mounted backwards the ETX can be used just about all the way to the zenith. Tracking objects as the earth moves is also fairly easy. (However this setup is not suitable for astrophotography.) A drawback of using a photo tripod is that it is not as solid as an astronomical mount which limits its usefulness at higher magnifications. This is especially the case if the wind blows! Nevertheless I have been quite pleased with views of the planets, etc., at 125x. Besides, I have found many advantages to using a photo tripod. It is easy to set up and carry, and easy to use on the steep hill behind my house. The adjustable height also makes it easier on my back when I look though the eyepiece. Speaking of eyepieces, an advantage of the spotting scope is that I can use 2 different eyepieces at the same time. The spotting scope comes with a 45 degree terrestrial prism which attaches at the back port. I find it very convenient to use a low power eyepiece with the prism (a 40mm Super Plossl) and a higher power eyepiece in the usual position. There is usually a combination of tripod height adjustment, spot on the side of the hill, and choice of eyepiece position that makes viewing comfortable for both my wife and I despite our difference in stature, which is a real plus. The bottom line is that an ETX-90 spotting scope and photo tripod is just the perfect combination of gear for me. I have seen so much more of the sky with them than I would ever see with a larger instrument. Indeed, I am discovering that time and experience are strong components of what makes astronomy such a enjoyable and satisfying hobby, and this combo makes that possible. I have written this to express to you, Mike, my gratitude for all your hard work. It was reading the good stuff on your site that gave me the idea of purchasing my ETX spotting scope and tweaking it to get the most out of it. Thank you so much! Sincerely, Bill Carson

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