Telescope Reviews: Who are these people...

I am inclined to agree with you. Much of the tweaking I have done in the past I felt was related to the fact that the mirror can move a tiny amount. Remember, we are talking about a few arc-minutes of movement for collimation and this can easily be attributed to the mirror being in an inconsistent spot.

In my C14, I find that if I move to a point in the sky that is much below about 45 degrees from the horizion, my scope's collimatin shifts.

If I am going to be observing a planet low in the sky (as I did with Jupiter), I do indeed collimate at a lower elevation. Then, when I go back to observing higher, I can see the collimation change.

To minimize this, I always approach focus from one direction when collimating and when viewing... Counter- Clockwise. This has the effect of "Jacking" the mirror into position. You are lifting the mirror on one side, causing it to "*BLEEP*" in the same manner. I will generally run the focuser in and out a bit to settle the mirror, then approach by the final focus by turning the focuser counter-clockwise. Running the focuser in and out a half turn before the final appoach seems to allow gravity to settle the non-focuser side so that when you approach final focus, you are alwasy "Lifting" the focuser side of the mirror into position. You can get it into VERY consistent final observing position. To be fair, I only do this when observing at high powers too, because at lower powers, well, it dosen't matter THAT much.

Using this method, I find my focused position is consistently more in collimation. Not alwasy PERFECT, but usually quite good.

I have not had to collimate my C14 but a couple of times in the last year, and most of that was due to the fact that for Jupiter, I was forced to recollimate when the mirror movement was simply too much because of the lower elevations that Jupiter was being viewed at. Now that it is pretty high, I have re-collimated at a higher lattitude and can now use the focus approach method.

Again, using this method means that my scope is often not in "Perfect" collimation, but for most viewing, it is more than good enough (Less than 3 arc minutes error).

So, I agree with you. Mirror movement and inconsistent settling is more than enough to cause an SCT to loose ultra-fine collimation.

Where we differer perhaps is in the amount of miss-collimation that we think is acceptable. Amounts less than 3 arc minutes have little real effect for most viewing, Only for the most demanding viewing will I bother to re-collimate. Otherwise, my SCT holds its collimation for a year or more at a time (using the focus method mentioned above to compenaste for slight mirror settling differences) and I simply don't fret over it.

I have also considered a "Reverse" solution to the problem. I have considered using spring tension on the mirror assembly to ensure that, along with the counter-clockwise approach, the other two points would always be pulled into postiion. I was thinking of using a threaded rod to go into the lock down holes, with a spring on the shaft on the outside of the tube working against a nut. This would allow the mirror to move, but always have some pre-load.

Of course this would only work on a scope with lock down screw holes, but I have seen where someone did something similar and I consider it a very promising idea.

Anyway, my advice is to simply not stress over collimation unless you are going to do planetary observing.

And try my method of working the focuser back and forth a turn to settle the mirror, then pushing the mirror into final position both for collimation and for day to day observing.. Do this and you may find that you don't feel as compelled to collimate every time you use your scope. I can SEE that if I approach from different directions when collimating vs normal focusing, or just in normal focusing, that with ever time I focus, critical collimation varies a bit. Using my method, it varies less. Again, I usually get fine focus that is within my own 3 arc minute range of acceptability and for the vast majority of observing, I think this is more than good enough.

Good luck.

--------------------
Celestron C14, CGE (Big Al)
Astro-Physics 6" f/8 (Buffy)
Televue 101 (No name, but I call it my Widescreen HD Space TV)

The night sky is my mistress. She seduces me away from all other lovers.