Meade SCT collimation woes

[lukebl]

Hi folks,

Not another 'How to collimate thread?' Can anyone help me here?

A couple of years ago I bought a second-hand 12" Meade LX90 SCT, as a planet-killer, or so I thought. At the time I became a bit disillusioned, as it didn't seem anywhere near as crisp and sharp as my humble 10" Newtonian, despite all my efforts to collimate it. I got a bit fed up with comments online about how much 'easier' SCTs were to collimate compared to Newts. Nonsense. Everyone talks about doing a star test for fine-tuning the collimation, with neat pictures of perfect airy disks. How is that possible when the star is wobbling around like jelly?

Anyway, I gave up on it and it's been in storage. However, having moved house and starting afresh I thought it was rather an expensive doorstop, so had a go at collimating it again. I're tried everything again, and have made a Duncan mask to help. What I really want to know is do these photos look as though it's now collimated? Sadly, planetary views still seem inferior to my 10" Newtonian.

Here's a view with focus in. It looks fairly concentric, allowing for atmospheric turbulence:

And here's a view with focus out:

Here's a view with the Duncan Mask after adjusting the collimation screws. The idea is to get a nice Y-shape, and this is about as good as I could get:

And finally, here's an in-focus view of Arcturus. All wobbly and rubbish. Visually, the stars aren't round and seem to have coma off to one side.

So, the basic question is, does that look reasonably well collimated? I've never collimated an SCT before, and don't know how far out this appears to be. To me, it's certainly NOT easier than collimating a Newtonian!

 

 

[YKSE]

I agree with you, many SCT collimation guides are written for people who has already known how to collimate a SCT, these guiedes were plainly useless for someone who has never done it yet.

When I got my C8, I tried many guides, as shown in this old thread:

The only one I felt useful for first timer is this one

http://www.astromart.com/articles/article.asp?article_id=548

Read the guide a couple of times, follow the steps there, you'll get your SCT show you what it capable of.

PS. Let your scope cool at least 2 hours before collimation, your defocused pictures look like there're quite some heat current.

[Peter Drew]

Anyone who can collimate a Newtonian secondary should be able to collimate a SCT secondary, still only three screws to adjust and no worries about it facing the focuser, offset or too far up or down the tube. The big difference is that there is a large optical leverage tilting a SCT secondary so very small increments of adjustment are essential. Thermal equalibrium and good conditions are necessary for either type of telescope.

[Davey-T]

Big SCTs can be a bit of a problem due to tube currents, doesn't take much to upset the perfect Airey disc which is why folks resort to drilling holes and fitting fans and insulating the outside of the tube.

My 10" is no problem but that extra couple of inches seems to cause havoc.

Dave

[Highburymark]

The shadow in your images - which is basically centred - suggest that you've achieved rough collimation. Next you need to do it with a higher mag eyepiece. As others have said, it is essential that the scope is cooled/at ambient temperature, and seeing isn't too turbulent, otherwise it's impossible to get a crisp image. Trouble with current humid weather where I am in London is that crispness is hard to achieve. Jupiter looks woolly even with perfect collimation. That is why artificial stars can be useful - and you don't need to track the star across the sky. 

[Gazabone]

The only thing I can add to the above is to make sure you place the star every time after you adjust the collimation screws. 

I have 6" and 8" cats and rather like them both, feeling slightly envious of your 12" though!

hope you get it sorted, happy viewing. ??

[Riemann]

I use the Duncan method and then check that I get an Airy disk at the end.

Looking at your pictures I would say you have it very close up but not quite right.   The top right hand spoke is a little too far in.   That final bit does make quite a lot of difference.  

I found the process much easier with an artificial star.   I got a cheap one from Hubble optics and it works really well.

By the way, before I started using the Duncan method, I tried the traditional way and I found it impossible to judge whether the out of focus rings were exactly concentric and which way they needed adjusting.   People with better eyesight than me might be able to do it but it was quite beyond me and I never managed to get an Airy disk that way, i.e. never got the collimation spot on.   On the other hand, I got an Airy disk first time using the Duncan method with an artificial star, both with the artificial star and with real stars straight after. 

[lukebl]

Thanks for all the info, folks. The scope is stored on a permanent pier outside now, so tube currents shouldn't be a problem. 

I've never managed to see a nice Airy disk, probably due to turbulence, so maybe I'll invest in an artificial star. Otherwise, like I said, it's a rather expensive doorstop! 

[Davey-T]

One problem with using an artificial star and a big SCT horizontally is mirror shift when then aiming it skywards.

And the corrector plate is only centred with cork or similar spacers which can crush after a while.

Dave

[Riemann]

10 minutes ago, Davey-T said:

One problem with using an artificial star and a big SCT horizontally is mirror shift when then aiming it skywards.

And the corrector plate is only centred with cork or similar spacers which can crush after a while.

Dave

That is something I forgot to mention and it did worry me.   I am lucky in this regards as I have a long split level garden and put the telescope on the lowest level and the artificial star on top of a step ladder on the highest level.   The split was 100ft horizontally and probably about 10ft vertically from the top of the mount to the artificial star.   Still quite shallow but as it turned out enough slope to avoid the problem as the first thing I did after collimating was to check the result with a star near the zenith.   Also I did all focusing / defocusing on  the artificial star very gently to avoid the chance of mirror shift.

With a typical garden you might need to enlist the help of a neighbour - e.g. use their upstairs window ledge to place the artificial star.

Something else I should have mentioned in my original post is that it is important to do any refocusing with the final adjustment in the direction that drives the mirror upwards, as pointed out by someone in another thread on collimating SCTs. This is important to minimise the effects of  mirror shift.  in a Celestron SCT with the standard focuser that means an anti-clockwise twist of the focuser as a final adjustment.   Not sure which direction is correct in a Meade. 

 

[lukebl]

Going back to doing a 'star test', I'm still unsure what you do if poor seeing prevents you seeing an Airy disk? One thread I read suggested that with a larger SCT like mine, it's rare to be able to see an airy disk at all. What then? Newts are easy by comparison. A Cheshire collimator does the job for you, no stars required.

[michael8554]

It's a dilemma with a 12" scope.

Final check at 500x is often suggested, but with your scope and typical uk seeing you get your jelly image.

An artificial star as high up as possible, maybe a ball bearing in a tree down-sun of your setup, will mostly cut out the seeing problem and allow find tuning.

Michael

[Davey-T]

You could knock up something like this or buy one if you're feeling flush.

Dave

http://www.hubbleoptics.com/artificial-stars.html

[Grotemobile]

This is what I did on my C8 HD. Don't know if this will help U. What I did was use a star.

[Peter Drew]

I've only seen near perfect Airy discs and diffraction rings (not to be confused with extrafocal rings) produced by telescopes over 8" aperture on an optical collimation bench under controlled conditions.

[Davey-T]

You could have a day out and take it here.

Dave

http://sctelescopes.com/

[2STAR]

On 6/10/2016 at 22:07, Grotemobile said:

This is what I did on my C8 HD. Don't know if this will help U. What I did was use a star.

Not sure I like his technique so close to the corrector with that big screwdriver !!!!

[Dave In Vermont]

SCT's are 'MADE' for installing Bob's Knobs on - one at a time, never pull all at once - and being certain to wait for the thing to reach ambient inside and outside the OTA. Also store any Cassegrain with the nose either level, or a bit inclined so the front is higher than the rear. The lube used to grease the primary-mirror on it's sled could be in excess and drip. You don't want that 'drip' to land on the inside of your corrector-plate.

Nudge, nudge - know what I mean?

Wink,

Dave

[Mr Spock]

Bob's Knobs are a great investment. My C9.25 has never been easier to collimate.

None of that faffing around in the dark with a small crosshead screwdriver...