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The weak link in the optical train

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#1 Cymrych79

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Posted 13 August 2013 - 07:43 PM

I'm just getting into astrophotography (at least, via telescope anyway), and the other night managed to find a few hours before the muggy dew clouds descended to hook my Canon to my 8" LX200GPS for the first time at prime focus.

Had a fair amount of success eventually in the Double Cluster, but noticed that when using the Meade microfocuser the collimation was thrown way off. If I de-focused a star to clearly see the shadow of the secondary, that shadow was all the way down overlapping the bottom of the diffused star light, making a croissant shape rather than a ring. Investigated the optical train, and observed quite a bit of slop where the microfocuser attaches to the grooved ring adapter that screws onto the back of the OTA (well, the end of the f/6.3 focal reducer in this case). In short, I just couldn't get the microfocuser thrumb screws (or the stock set screws) to tighten down enough to truly hold the assembly rigid, at least not with my camera hooked on the end.

So, for the limited time I had left that evening, I just removed the microfocuser and muddled along as best I could with just the course focus knob on the back of the scope. Obviously, for any serious imaging this lack of more precise focus control is a serious detriment. Not to mention time consuming and a total PITA, running back and forth from the computer to the scope trying to zero in, only to look at the knob in a slightly cross manner and have it seemingly defy logic and jump even more out of ideal focus rather than closer to it. A complete case of 1 step forward, 2 back, more often than not!

Did some research and looked at some aftermarket focusers, etc. The Moonlite set-ups look fantastic! Just a little pricey at the moment, at least with the micro-stepper motor.

So while I rub my nickels together in the hope that they spawn baby nickels to invest in a decent aftermarket focuser, does anyone have any short-term suggestions which may eliminate some of my sagging optical train? That goofy grooved ring seems to be the weakest link in the system ... any way to adapt the microfocuser to not have to use it? The only thing I really came up with was to line the groove in the ring with some rubber, just to distribute the force from the contact of the set screws a bit more, and to eliminate some of their room in which to slip.

I'm sure that I'm not the first to run into this issue in Meade SCTs, so please, any and all suggestions are greatly appreciated!

- Jason

#2 nitegeezer

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Posted 13 August 2013 - 10:17 PM

You have the exact same setup as I have, and I have no problem with the microfocuser, the reverse is true I think it is fantastic. I finally got a clear sky so I am about to head out, but will check in later to see what you find. I would suggest taking the focuser off and making sure the screws will go all the way through, I have no other idea and want to run before my sky changes.

#3 nitegeezer

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Posted 13 August 2013 - 10:28 PM

I just took a quick look at my setup, and I did change out the mounting screws to something I could more easily work in the dark. Since I did that, I don't think it has been off the scope and I have no problem. The only thing I can say is make sure the mounting ring is tight to the scope, and then make sure the mounting screws are tight. If you would like any pictures it will have to wait until tomorrow.

#4 Cymrych79

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Posted 15 August 2013 - 06:08 PM

Yeah, I had to hit the beautiful clear skies last night as well!

I've also changed out the standard set screws to thumbscrews, for exactly the same reason, being able to tighten them without fumbling around in the dark with an hex wrench.

The only thing I can think of is that maybe the replacement screws just aren't contacting the ring properly. The ring groove is concave shaped, and although I haven't looked too closely, I'd bet these replacement screws are flat bottomed. I might try filing a bit of dome to the bottom of the screws, see if I can get a more snug fit to the ring.

On the plus side, I'm getting a lot better at fine focusing with just the coarse focus knob on the back of the OTA, and with my 450D mounted at prime without the microfocuser I have no issues pointing vertically, which made going for the Dumbbell at midnight possible.

#5 nitegeezer

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Posted 15 August 2013 - 09:03 PM

Doming them would probably be a good idea, but in addition I would have it off the ring and make sure all the screws go through their holes cleanly. I am wondering if one of the threads has a burr in it so it feels tight before it even touches the ring.






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