The weak link in the optical train
Posted 13 August 2013 - 07:43 PM
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!
Posted 13 August 2013 - 10:17 PM
Posted 13 August 2013 - 10:28 PM
Posted 15 August 2013 - 06:08 PM
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.
Posted 15 August 2013 - 09:03 PM