do i need to collimate a mak

[sparkyjon]

I've just picked up a circa 1990 celestron super c8 plus this thing has been used twice since new.

now Im not certain about cats do they need collimating and if so how? As being so old i think it best i do it if it needs doing.

I have a laser collimator, can it be done with one of these..

the secondary mirror has 3 Alan key screws on it is it something to do with them?

[John]

If it's a C8 then it's a schmidt-cassegrain (or SCT) rather than a mak. They do need collimating from time to time and need to be reasonably accurate in this to give their best performance on the planets in particular. Once in collimation they do hold it quite well though. Only the secondary mirror is adjusted and the 3 hex screws are the way this is done. These can be replaced with screws with knurled heads to avoid the need to use hex / allen keys.

Here is some information on how to do it. A laser collimator of the conventional single beam type won't work though because the primary mirror has the hole in the centre:

http://www.sctscopes.net/SCT_Tips/Maintenance/Collimation/collimation.html

I'd do a star test first before adjusting anything. If the dark shadow of the secondary is in the centre of the "donut" of an out of focus star then the collimation is quite accurate.

[rowan46]

Don't forget to let your scope cool first

[JamesF]

If you do need to collimate, I'd try to do as much as possible during the day or indoors if you can. It's quite a faff compared to a dob. If you have lots of space you can buy/cobble together an artificial star to collimate against (it needs to be quite some distance away for an SCT) which should help enormously.

James

[sparkyjon]

Thanks for the info. I didn't realise until now that a Schmidt and mak were different. The learning curve grows.

[rowan46]

this might help i used one on my 5" sct

[PDF]

Easy SCT Collimation - the Duncan method - AstroVox

www.astrovox.gr/forum/download.php?id=17658&sid..

[John]

Thanks for the info. I didn't realise until now that a Schmidt and mak were different. The learning curve grows.

There are different types of schmidt's and mak's too, just to add to the confusion !

I used to have a Celestron Super C8 as it happens. It gave me the best views of Saturn I've ever had on an evening of superb seeing

[earth titan]

I've found star collimation with a webcam to be the easiest and most accurate method.

Typed by me on my fone, using fumms... Excuse eny speling errurs.

[smudgeball]

Hi there,

Ditto what others have said.

Remember to re-centre star when doing it and only use TINY turns on each screw.

It will work, but if you're too heavy handed, you'll just move the error

aound the eyepiece.

And that can be frustrating, apparently, or so I'm told :-)

[earth titan]

I have to say that when I've done my SCT in the past, I've always ummed and argghed about whether to use a diagonal when collimating or to go straight into the visual back.

Two ways of looking at it:

If you go straight into the visual back, you negate any effects of diagonal alignment etc.

If you use a diagonal, you will collimate as you will viewing.

Never sure which way to go. I always go for the former option however as I think if I get the scope right, everything else should follow on.

It can be a pain if you use an eyepiece for this, hence is use a webcam. This also allows me to see adjustments on screen as I make them.

Typed by me on my fone, using fumms... Excuse eny speling errurs.

[sparkyjon]

I've found star collimation with a webcam to be the easiest and most accurate method.

Typed by me on my fone, using fumms... Excuse eny speling errurs.

that was my next question happy days ....

[rowan46]

Never tried with a webcam. so you may be right but one advantage of the hartmann mask is no laptop required. I would also suggest replacing the secondary screws with bobs knobs or similar it's a little daunting messing about at the front with a sharp metal point whilst trying to look in the back.

[smudgeball]

I'm going to try the duncan mask method.

Got myself some black rubber floor tiles and cut one out.

Already have an artificial star so might try it tonight.

It's either that or take advantage of the lovely clear skies we're having at the moment.........

[sparkyjon]

Never tried with a webcam. so you may be right but one advantage of the hartmann mask is no laptop required. I would also suggest replacing the secondary screws with bobs knobs or similar it's a little daunting messing about at the front with a sharp metal point whilst trying to look in the back.

Thats why i wanted to use a webcam so i can have the laptop round buy the front while adjusting then reach around for focus..

[smudgeball]

Two ways of looking at it:

If you go straight into the visual back, you negate any effects of diagonal alignment etc.

I've always set up as per how I would be using the scope i.e. with diagonal in place.

I'm guessing that when the scope is manufactured and goes through quality control it must get collimated before being sent out.

I wonder what method they use ?