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Returning astronomer, and a technical question! :)


Welrod50
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Hello all.

As a lifelong stargazer and previous participant in the great halls of SGL, I have returned to the fold and very quickly encountered an 'issue' I'm hoping to get some kind advice on please??  So firstly...hello again everyone.  It's nice to be back :)

I took a couple of years away from this brilliant hobby fully intending to get back in again when the time was right.  Well, in January I was involved in an accident and had a serious leg fracture. I treated myself to a fairly cheap, but enjoyable Celestron LCM 80 to do a bit of visual from the confines of the back garden whilst mincing about on crutches and one thing led to another...

I have since added an ST80, all manner of EP's and bought a cheap but strangely enjoyable Bresser Skylux 70/700.  As a previous practicer of the dark arts, I ended up adding a camera into the mix and have taken some half decent 30 sec subs & got some pleasant enough shots of a few DSO's.  

Now, my problem that I'm hoping to get some tips on is this...

I've decided to cut the Bresser down a bit. I have an Orion flip mirror (mirror box is 3 inches long) and when rigged up, all is within the weight limit of the LCM but I just don't quite have the inward focuser travel needed to bring the EP/camera to focus. I reckon a bit of careful cutting of the optical tube will do the trick, but my query is this. If I'm about 1" shy of focus with the flip mirror, can I take off 4 inches without ruining the scope?? I'm thinking the length of the mirror box will cancel out the shortening of the OTA and the extra inch will allow the whole thing to focus with minimal outward travel - keeping it stable. I've already cleaned & blackened the lenses and modified the rack & pinion to make it stiffer, more stable and generally better to use. I know there is maths involved but rather than give myself a headache, I thought I'd come to you guys again.

And before anyone laughs about using the LCM for AP, having been the proud (and sometimes frustrated) owner/user of an autoguided EQ rig before I know what the LCM mount will and wont do, and its surprisingly good fun getting some 30 sec subs with a rig that takes me about 3 minutes to set up and take apart.

All suggestions welcome.  And it's good to be back.

Clear skies :)

Scott

Cloudy Stoke on Trent

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I can't immediately see why what you propose wouldn't work, but possibly I'd be more tempted to take off, say, 2", fit it all back together (perhaps wedge the focuser in with some bits of folded paper as shims or something) and check where focus is, and then cut a bit more off if it all looks to have gone to plan.

James

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Cheers James.

I have thought of that already - basically taking off a bit at a time so I don't go too far. I guess I'm just looking for a solid start point as it will need to be a precision job and the more times I have to cut, the more times I can [removed word] it up :)

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2 minutes ago, Welrod50 said:

Cheers James.

I have thought of that already - basically taking off a bit at a time so I don't go too far. I guess I'm just looking for a solid start point as it will need to be a precision job and the more times I have to cut, the more times I can [removed word] it up :)

Yes, fair point :)

Another possibility to confirm the setup might be to make a little jig up to hold the OTA (with the focuser removed) and the flip mirror in place so you can get focus properly on some distant object by moving the OTA back and forth.  Once you've done that you could just lay the focuser alongside with the drawtube extended as far as you would prefer and then mark off the cut at the position of the focuser flange.  Or just measure the focuser from the flange to the end of the drawtube and mark that distance from the end of the flip mirror where it would fit into the drawtube.

James

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