Re: [ATM] ATM Digest, Vol 38, Issue 28

["Ian McKernan" <I.Mckernan@murdoch.edu.au>]

Thanks for the replies people!

Ok, no spider vanes, the secondary is an aluminised spot.  Typical small f15.4 mak, 130mm diameter.  You have probably all seen them out and about.  Got it off EBay and its been posted from the US to Western Australia.  So, its unlikely it will get sent back since postage was 1/3 the cost, and its up to the seller to pay shipping. Sending it back means that at the end of the day i have paid out 2/3 of what its cost to date and have no telescope, so this looks like becoming a salvage project.

Tried star testing again last night, and the stig is definitely in the OTA somewhere. Tried rotating eyepieces and head and the cross stays in the same orientation. Testing with nothing but an eyepiece in the back (no diagonal), at 166X.

Bit the bullet this morning and did that which i swore i'd never do.  Hoping the astigmatism may be due to strain in the meniscus corrector i took the front cell off.  

The corrector sits in a cell , inserted from the rear, and there is a fraction of side play before the retainer goes on the back. So corrector in cell, followed by an O-ring that runs around the circumference.  A metal retaining ring is then attached to the cell by 6 small screws and tightens down onto the o-ring pressing the corrector into its cell.

The retaining ring was bent so that from the factory, when most was in contact with the o-ring there was about a quarter of the circumference that wasn't. I straightened it and refitted with only a very light pressure.  The screws are now just holding it in contact with the o-ring.  Seems to hold thr corrector in firmly enough with no slop.

Set up the scope and tripod in the bathroom (as from here, with doors open i have a clear line of sight down the whole house) and set up a torch covered in alfoil up the other end with a very small pinhole as a quick and dirty artificial star. Distance about 13m.

Just managed to get this to focus. The out of focus images are very different either side. Starts as a normal looking pattern of small point and rings through to best focus where i got a distinct disk and rings, BUT, with the first ring showing with 4 bright sections and 4 less bright. Going through focus a cross again before it seemed to become a bright ring with only the faintest of dots in the center.

Collimation seems to have stayed ok through all this??  Being all spherical surfaces are maks pretty robust in their collimation??  There are no provisions for adjusting the meniscus corrector on this scope, and i am not game to tough the primary collimation bolts without having done some reading.   Any links or info to a good tutorial on Mak collimation would be great if anyone can pass them on.

One thing i did try was rotating the front cell, and there was one spot that seemed to give particularly good results, with no symmetrical brightening of the first ring.  Had to move it back to line up the screw holes though.

So, i'll be testing again tonight if it isn't cloudy and see how it goes.



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