What is Your SCT Image Shift?
Posted 16 November 2005 - 03:19 PM
Posted 16 November 2005 - 03:35 PM
I'm a fairly new SCT user. Bought a Nexstar C11 about a year ago and I'm enjoying it immensely. I was able to roughly quantify my image shift while doing some recent webcam imaging of Mars. My image shift is about 1 full Mars diameter or about 20 arc-sec. I was curious as to whether this is bad, good, or average for this type of SCT . I thought a survey might be interesting. The question is "How much is your image shift and what is you make of SCT?"
That's pretty good for a bigger SCT.
I have a mid-1980's vintage C-11 and it's image shift is a bit less.
Around 10 or 15 arc sec.
Under one Jupiter diameter (45-ish arc. sec) is generally considered acceptable for any size of SCT or MCT.
Posted 17 November 2005 - 02:02 AM
Posted 17 November 2005 - 02:06 PM
Just a guess. It also depends on the position of the telescope. I think the higher the angle, the more the shift.
Posted 17 November 2005 - 05:10 PM
1) turn the focus knob clockwise until you hit the stop (the mirror has hit the rear cell of the scope).
2) loosen the setscrews that are on the side of the focus knob which hold the focus knob to the focus shaft.
3) Tighten the focus knob clockwise until it stops or is very difficult to turn further.
4) Back the focus knob off 1/16 turn counter-clockwise and retighten the setscrews on the side of the focus knob.
5) test how hard it is to turn the focus knob. If it's still easy, you may have nearly eliminated the image shift and hit the adjustment on the head. If it's still too much image shift, repeat instructions 1-4, but don't back off the knob as much the second time.
6) If the knob is very tight and hard to turn, tighten clockwise until it stops again, loosen the setscrews and turn counter-clockwise an additional 1/32 of a turn. Retighten the setscrews and check the focus knob motion. It's OK if it's just a little stiff, but it shouldn't be too tight or focusing at high power will be a very frustrating experience.
What this does is essentiall "*Word deleted by the CN gnaughties gnomes*" the outer primary mirror baffle on the inner baffle so that it rides up and down on the inner baffle with a slight (and I mean slight) tilt toward the focus knob.
Recollimation will probably not be necessary, but it wouldn't hurt to check it on Polaris when you are out under the stars again.
I've seen this procedure eliminate image shift in an instrument with a very little to start with, and it reduced it significantly in another scope that had a lot before we worked on it. It can be repeated as many times as necessary to find that just right balance between focus knob tightness and image shift.
Some focus knobs are intrinsically higher quality (like FeatherTouch) and reduce image shift by their very lack of play.
Posted 19 November 2005 - 07:34 PM
Posted 05 December 2005 - 05:54 PM
Posted 05 December 2005 - 08:20 PM
You can eliminate some, but not all of the image shift by doing the following:
Unfortunately, the knob on the C11 is a friction fit; there are no set screws. There are, however, three screws holding the focus mechanism tightly in place. These should be checked to see if they're all tight.
20-30 arc min sounds about right (mine's slightly less).