Telescope Reviews: Offset saddle position for large SCTs on GEMs

Sometimes the subject of tube flexure comes up for large SCTs including C11s and C14s. Some time ago I had a standard AP mounting plate (also called a saddle) on my AP900 with a classic "T" shape: each side of the mounting plate hung equally over the edge of the mount's declination head.

I noticed that the "focus side" or "down side" of my SCT, whose butt rides very close to the axis of rotation on the mount and is a solid casting, had as much support on the dovetail rail as the whole long extension on the other side.

I thought this wasn't a good idea. At that time, AP offered only a "classic T" mounting option for its mounting plate. So I purchased the longest Cassady plate I could, because he was drilling with holes that allowed for offset mounting. When I had the Cassady saddle plate on the AP900 I acutally had several inches more support on the front end of the scope than with the longer AP saddle plate.

This is really what you want to do with an SCT. First off balance will require that you move the SCT butt very close to the axis of rotation. That done, there's no point in wasting the "stiffening power" of your saddle plate on a massive rear casting that is already stiff and can't flex in any case. Use that saddle plate effectively, and offset it!

I have attached a picture of an offset saddle plate on the AP900.

I posted these comments on ap-ug some time ago and perhaps that, along with comments from imagers with heavy camera loads, helped inspire a redesign. AP now offers the DOVELM162 which is 16" long and can be offset. That is longer than the 14" Cassady which seems to me a bit thicker. I can't say which is better, but either is preferable to a "classic T" mounted saddle plate for a heavy SCT where most of the stress is forward.

Even though the AP900 "standard T design" 16" saddle was pretty darn good, when I switched to the Cassady I noticed that the OTA was more stable in the wind. Therefore, I recommend that C11, C14, and other users of rear-end-heavy scopes such as Cassegrains and Maks consider getting a saddle plate that can be offset to give additional support where it is most needed, in the heavily torqued front end.

regards Greg N

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"Aperture will get you through times of no money better than money will get you through times of no aperture."

featuring selected astrojunk:

bunch o' widefield eyepieces
bunch o' narrowfield eyepieces
couple o' Barlows
couple o' scopes
couple o' mounts
couple o' tripods
and a pier 'n' stuff