[night-vision]

Hi ngcles,

Originally Posted by ngcles

Hi Night-vision,

Congrats on that first night, sounds like it was a blast.

It was

I am no great Schmidt Cassegrain expert but I do know a bit about telescopes generally and I use several different Meade LX200 Schmidt Cassegrainians a lot at Sydney Observatory. The advice offered by others earlier above is good advice and I agree with it. The spreading of the greasy-goo by racking the focuser in and out is particularly good advice and might help quite a bit (not always though!)

I will definitely do this and see if/how much difference it makes.

Mirror flop (to some degree) is part-and-parcel of S/C ownership. Mirror flop while using the 'scope visually is a bit of a pain -- coping with it while imaging with a CCD has driven many a good man to drink. Commercial S/Cs are focused by moving the primary mirror up and down the tube. The Crayford option eliminates this by moving the eyepiece instead, but does introduce other problems like the mount clearance as noted by Mark above. You do your coarse focusing as normal, fine focus with the crayford.
One thing I think no-one else has noted is the _degree_ of the mirror-flop problem you are describing. You describe Jupiter flopping right out of the FOV of a 15mm ep.

Let's hope I get the maths right here but:

2000mm fl 'scope / 15mm fl ep = 133x magnification

Assuming a 50 deg AFOV standard plossl is used:

50 / x133 = 0.37 degree field of view, equating to 22 arc-minutes diameter true field.

Half of that 22 arc-mins that is 11 arc-minutes (or more) -- this is a pretty big flop (assuming is went all the way from centre to out of the FOV). Seems to me at least to be somewhat worse than average.

I did'nt think it was normal, it seemed stange that what ever I was trying to focus on at times went completely out of FOV.

This mirror flop _could_ be a significant contributor to the alignment problem you described. If you use two stars far apart (as you should) for the align, a flop can (and in my experience often does) occur while you are slewing from one star to the other instantly adding a significant error to the alignment. There may be (and likely are) other contributors to the accuracy of your align, but that degree of mirror flop will not help it at all.

At the moment I'm using Hadar and Canopus/Rigel for my two star align untill I'm better acquainted with the night sky. Then I'll use Hadar and something further to the North, any suggestions on which two stars to use? As you mention I dont think with so much mirror flop it's helping with the alignment.

We don't know where you bought the 'scope (or even whether it is new or second-hand) but after I'd tried the fixes suggested above, I'd be checking this degree of flop with the service agents of the manufacturer, or the vendor who sold you the 'scope to see if they can help.

The scope is second hand and around 5 years old, I did take it to Bintel before first light for a check over and a collimation, I can see myself back there after I've tried all of the above.

Don't panic -- it can be rectified or greatly minimised with a few simple adjustments.

That is great news, I was a little worried.
Thanks for a great post Les.

night-vision