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alternative mounting idea; etx 90 ota

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#1 moynihan

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Posted 23 March 2013 - 10:11 AM

I have one of those optically superb old 90mm OTA (textbook star test, alignment, no-image shift, smooth focus) from a long discarded RA mount. Over the years i have used various things to adapt it to other mounting (altered three point rings via scope stuff, L bracket to fit the synta/vixen shoes, etc etc).

Last night i had it out on Jupiter, and having been using a "textbook" sample of the newest version of the C90 (which is very good optically), I had forgotten just how good my etx 90 tube was (yes compared off and on to the brand that shall not be named, that was no better, optically :o ).

So i decided to get it its own little pair of Crawmach custom rings. Then i remembered that years ago, i got one of these.

Went and dug it out of a box of astro-what-not and attached it to the 90. The reason i had not done so already, was the thought, well on my alt-azm mount, the eyepiece will be at 90 degrees, i.e. on the side. This time i thought, well, does not bother me with a newt.
So now my etx 90 rides on my AT Voyager like a little backwards newt, and balances well and sturdy. :grin:

Yes, still interested in a hypothetical version of a Wegat metal back for the little guy (assuming that attachment would not alter my collimation), that would fre the tube for easy ringed standard mounting and balancing, but that is an "if/when"...this may do for now.

#2 Don Taylor

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Posted 25 March 2013 - 09:30 PM

I've done exactly the same with my etx90 and Porta II. I've also rigged up a mount to attach a GSO 8 x 50 RACI finder to the dovetail.

It works ok but I actually have a set of Crawmach rings on order. Should be here in a few days.

I'm also thinking of making a new visual back that attaches to the mirror end of the ota and replaces the plastic with a conventional 1.25 port so I can use a regular dielectric diagonal. It would mount to the ota using the 3 original screws but would have access to the Collimation screws.

I want to do this because my etx was slightly out of Collimation for years and I recently got around to improving it. But what a pain to iteratively adjust and have to remove the back every time. I figure if I had easy access to the Collimation screws I'd never need to adjust them (Murphy's Law).

#3 Don Taylor

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Posted 25 March 2013 - 10:18 PM

here's a few photos.
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I guess the visual back I've described is similar to the Wegat except for conventional 1.25" accessories (not SCT thread) and adding holes to access the collimation screws. Same concept otherwise.

#4 moynihan

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Posted 26 March 2013 - 06:47 AM

Very cool :)
What is the piece that says ADM on it?

#5 Don Taylor

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Posted 26 March 2013 - 07:36 PM

It's the saddle of the (modified) Porta mount. I have the same type on my cg5 equatorial mount. It doesn't chew up the dovetail bars like the ones that just use a rounded bolt do. There's also an adm piece that I attached a synta type finder shoe to so the finder can also attach to the dovetail bar.

I'll attach a few more photos later tonight.

#6 Don Taylor

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Posted 26 March 2013 - 09:18 PM

Here's a photo of the ADM Saddle on the Porta II head. That's the component in the photos of the ETX that you asked about.

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And here's the mount I made for the GSO 8 x 50 finder. I use a 7" long dovetail bar on the ETX and this mount attaches directly to it. That way I can use a large finder with the ETX, or a laser finder or, red dot etc. And I can use it with several other OTAs on the same mount (80mm refractor etc.). I couldn't find another easy way to mount a large finder to the ETX and the original straight through, and optional Meade right angle finders leave a lot to be desired. BTW, the whole set up of ETX and finder can also be used on my Celestron ASGT (CG5GT) equitorial mount too.

It's made from another ADM part (a dovetail adapter) and a Scopestuff finder shoe intended for GSO Crayford Focusers (it's drilled with two holes front to back instead of side to side). Agena sells a similar shoe for Crayford focusers too.

I laid out and drilled the holes in the dovetail adapter using a drill press and small milling table.

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#7 moynihan

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Posted 26 March 2013 - 09:47 PM

Very cool!

#8 Don Taylor

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Posted 26 March 2013 - 10:06 PM

Thanks.

I'm sure that you've discovered that one advantage of the sideways orientation of the eyepiece is that the height of it hardly changes regardless of where you point - and the angle not at all. ( of course it moves with the scope in azimuth......but can't have everything).






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