[tim.stephens (Tim)]

I assume you're using the the GSO FR/flattener? It should be good for apsc sized sensors - I've not tried mine with anything larger TBH.

I'd suggest taking the FR/flattener out for collimation as it will tend to remove some of the field curvature inherent in the uncorrected RC design and make it harder to balance the stars across the frame.

Bahtinov mask is very helpful when dialing in collimation - focus on a bright star in the centre and then check that star at each of the corners to measure the field curvature. Make sure the star at each corner is off focus by the same amount. Do this after you've got the central star collimation right and the airy rings/central obstruction perfectly centred (usually by adjusting the primary).

Here is a short version of what I do:

Do you have a cheshire to check your collimation in the daytime? I found that I can get almost perfect collimation with this tool but the trick is to remove the primary baffle by reaching in and unscrewing it (let it rest on the outer baffles) and keep the telescope horizontal. You can then see all the internal reflections and secondary baffle easily - adjust the primary as necessary to centralise all the reflections and baffles and adjust the secondary so the black center mark is... well... center to the cheshire! You have to be extremely particular when centralising the reflection rings, have a play around with it to be comfortable with shifting both mirrors. When you think it's close, check the collimation from the front with the 'hall-of-mirrors' technique - go right up close to the front on a bright day and actually count the number of internal reflections of the black secondary baffle at all points around the mirror. You should see exactly the same number of reflections, if not, then something isn't quite right and you have to go back to the cheshire to diagnose.

Now here's the thing - these scopes are poorly designed in several ways. When you remove the primary baffle for collimating in this way, you can accidentally unscrew the primary mirror retaining ring because it is one unit. I found out the hard way - all of these scopes have this design flaw and the result is shifting collimation when the scope slews as the primary becomes unscrewed. To diagnose - with the baffle in place, pick yours up from the spider end and hold it vertically. Shake it gently from side to side and see if you here a 'thunk' in the OTA. If so, then your mirror is loose and any collimation you do inside is pointless.

This might seem a little extreme but what I suggest people do is pull off the back mirror cell (black metal bit) to access the primary. Unscrew the primary baffle, then the retaining ring, then pull the primary mirror from the central holder. You might notice that there is a LOT of lateral play in the primary and this is the cause of many difficulties in collimating these scopes.

Pull the white plastic teflon off the holder and add a thin piece of tape around where the mirror sits. I used black electricians tape as it was the perfect thickness to increase the circumference enough to hold the mirror in with a little more force. reassemble everything including the teflon ring, making sure your primary isn't too tight and is seated properly against the cell holder. Then collimate again. I did mine last year and it hasn't moved at all.

Hope that helps a little...

As with the focuser, they are supposed to be collimated but mine was not. I probably should have sent it back but it took over a month to get here one way, the postage was really expensive and I fixed the problem easily with the GSO collimation ring. Check yours with a laser - rotate the focuser and see if the laser makes a circle on a far wall or one the secondary. I should have bought the 2.5 inch...

Tim

Edit: Just had a look at your astrobin picture. Your collimation does look a little off... How long was your exposure? Looks to be the primary needs to be moved at the 1-2 and 7-8 o'clock positions

Also, don't listen to people on the forums when they say 'don't adjust your primary'. You have to when collimating these scopes properly :-)

Originally Posted by Andy01

Thanks for your thoughts Rick & Tim.

I should have mentioned that I'm using a .8 flattener/reducer here as well,

Here's another starfield I shot same night that may be more useful, it's
60secs LUM only.

https://www.astrobin.com/337564/

Rick I don't have a Bhatinov mask - but I'll be ordering a laser etched one soon.

Tim, I'll try your suggestion with a defocused star field with a bright central star tonight (if it's clear at the star party at Snake Valley!) - I have the 2.5 inch focuser and the tilt ring.
I'm reluctant to fiddle with the focuser as it's meant to be collimated at the factory?

What's the primary baffle tape-fix to stop the primary mirror moving please?