[ausastronomer (John Bambury)]

Originally Posted by Tandum

If you look at this thread you'll see that I have pmrid's F3.8 10" newt with a view to giving the beast a tune up while he's tramping around Europe. I've had quite a few newts and peter had a set of great collimation tools so I didn't think twice to help him out as he just got his MX mount and has his hands full with that. I've found that this thing is hard to get right, I'm getting it closer and closer but I still have problems and maybe you guys can help.

I'm sure the problem is the secondary alignment. It has a prostar 3" secondary which has the focuser offsets built in. However I found that the 2 short legs of the spider where 2 mm different as well. I spent a lot of time getting the center bolt into the right spot, I have also center spotted the focuser however
Doing a star test shows it is concentric inside focus but shows coma outside focus. This is giving funny star shapes. When aligning the secondary it always seems to have an apparent flat spot on the top left of the mirror. Does anyone have any tips on aligning the secondary? Should I center the focuser and move the secondary to suit instead of moving tilt on the focuser?

I also have reservations about using an mpcc at this F ratio. The info on opt says it's for newts from F4 to F6 with visual users reporting good results down to F3.7. I have a qhy9 with wheel hanging off it.

Here's a set of lum on tuc 5sec subs and 253 120sec subs to see how close it is. Check out the big stars. and I had fast downloads on to avoid the clouds hence the vertical bars on tuc.

Hi Robin,

The F-ratio doesn't change things too much. A lot of people way overplay the difficulty in collimating fast newts. They are no harder to collimate than slow newts. The optical effects of poor collimation are greater, that is all.

I had to alter the focuser tilt to get the secondary in the middle of the focuser sight tube which indicates to me something is screwed.

There are 101 ways to skin a cat but I would have done this the same way. ie. Get the spider in the correct position with regard to the mechanical axis of the tube. Remove the secondary holder from the spider and square the focuser to the axis of the spider and the mechanical axis of the tube. I normally use some thin plastic shims to do this under the respective corners of the focuser, as needed.

Doing a star test shows it is concentric inside focus but shows coma outside focus.

I am pretty sure this is caused by the secondary mirror sitting too high in the tube. You should be able to pick this up in the sight tube.

Any misalignment of the primary "should" be minor as most of the telescope components are made on CNC machines and should be able to be fully compensated by adjusting the primary tilt only.

My focuser does not give any tilt adjust (its a FT so it isn't a crap focuser)

Brendan,

You adjust this by using thin plastic shims under the focuser baseplate adjacent to the mounting screws.

Cheers,
John B