Collimation help needed - 11" Edge HD

[x6gas]

Could anyone give me some advice on collimation of my Edge HD?

I have been fiddling with collimation and - in average seeing - the pictures below are basically the best that I could do tonight...

Anyway, seems pretty clear that there is pretty bad over (under?) correction on the scope...

Here is my question: when removing the secondary the other day to fit my collimation thumb screws, I noticed that the slot in the secondary holder was pointing at 9 o'clock whereas everything I've read has said it should be at 3 o'clock (when viewed from the front). I am going to try rotating the secondary by 180 degrees but just wondering if this is likely to have an effect on the spherical aberation?

Comments also welcome on how much better I should try to get the collimation.

Thanks, Ian

colli in.tif

collin out.tif

[ollypenrice]

Could you post the images in JPEG format? You'd get responses if you did so. I can't view them on the machine I'm using at the moment.

Olly

[perfrej]

A quick note about the slot... On my EdgeHD 9.25, as I attempted to unscrew the secondary holder ring, the whole can came loose from the glass! Luckily, I have fairly small hands, so I could get one down the tube and hold the bottom part of the assembly while tightening the top. That, of course, put my slot in an arbitrary position calling for re-collimation.

I am now very careful not to over-tighten the ring for the secondary.

As for collimation, I alway use an eyepiece (about the only situation where one is fitted). It is very easy to see how collimation is, and it is very easy to use your finger, as per the manual, to figure out which screw to turn.

My suggestions are three:

1. Careful with the ring - do not over tighten

2. Do not worry about slot orientation as long as it stays put

3. Collimate with an eye-piece

All the best,

Per

[Klesk14]

*Studies and takes notes*

I may have to do this for my C11 one day.

Do you only need to collimate if you take the secondary mirror off? I plan in the future to try the f/2 imaging out and although in the book it says no collimation is needed when the mirror is put back in but just a wonder?

Should I only collimate if I see the objects in the EP off shape?

[x6gas]

Could you post the images in JPEG format? You'd get responses if you did so. I can't view them on the machine I'm using at the moment.

Olly

Whoops sorry Olly - jpegs below!

Actually, I did manage to do slightly better; the outer focus made it easier to see that collimation needed a tweak to the right and up a bit... but I didn't capture the result...

[Tim]

Thats quite a bit out

Do you have a good 4 or 5 mm eyepiece? If so you can use that to really fine tune the collimation on a star like Polaris. Make sure the star is perfectly centred before you start, and then re-centre after each adjustment.

Cheers

Tim

[x6gas]

Thats quite a bit out

Do you have a good 4 or 5 mm eyepiece? If so you can use that to really fine tune the collimation on a star like Polaris. Make sure the star is perfectly centred before you start, and then re-centre after each adjustment.

Cheers

Tim

Thanks Tim - this is at f/20 (2x ultima barlow), captured on a one minute avi using my SPC880 - the images aren't cropped so are pretty close to the centre of the field of view, I think.

I used Hamal for the star test as that was the closest bright star to Jupiter that I was going to image...

The touble is, with seeing not being great, I find it really difficult to judge how concentric the rings are in real time. I was using the Collimation 3 reticule overlay to help, the mount was aligned and tracking well so the star was staying near the centre of the field of view and I was re-centring after every adjustment. Capturing an avi and stacking the results make it much clearer - but do people do that in the field?

I really appreciate peoples thoughts and advice. I know collimation is a common topic, but I'm new to SCTs and want to get as good at this as I can; that's why I'm using moonlit nights with average seeing to try to get better!

Also, is this level of spherical aberation acceptable (difference either side of focus)? Does it matter and can anything be done about it?

[x6gas]

*Studies and takes notes*

I may have to do this for my C11 one day.

Do you only need to collimate if you take the secondary mirror off? I plan in the future to try the f/2 imaging out and although in the book it says no collimation is needed when the mirror is put back in but just a wonder?

Should I only collimate if I see the objects in the EP off shape?

No, you don't only need to collimate if you remove the secondary. In fact, these scopes are supposed to hold collimation really well after removing and replacing the secondary which I think is a bit incredible to be honest!

Anyway, I have found that I thought my scope was pretty well collimated, but the fact is it wasn't and I am still working on it. If you don't notice it, then don't worry, but for planetary imaging - which is what I have been doing - then you need to be spot on. I think that even for planetary visual your scope needs to be well collimated. I've been reading up and the bottom line is that to get the most out of your instrument, your collimation has to be spot on. Just a little out, and the performance drops off quite a lot.

As you can see from the images above - these are at f/20 with an SPC880 so quite a bit of magnifcation - and the comment is that collimation is still quite a bit off!

[RobH]

There's a way of doing it using a 3 hole Hartmann mask and a webcam or CCD that's dead accurate....I used it on my old 14 inch Meade and, although I was sure that collimation was spot on, it showed to be a tiny fraction out....getting it exact made a remarkable difference.

Problem is, that article's in an old Practical Astronomer mag in my cellar, and I'm in Oslo working and don't get home until the weekend.

I'll look it up, do a synopsis and post it as a sticky.

Cheers

Rob

[Psychobilly]

It was around Feb 2008 if that helps find it...

[RobH]

That narrows it down nicely Peter....the guy who produced PA did run off with a years subscription from everyone who subscribed and never appeared again, and nor did the magazine, so in this case I can't see him complaining .

If he does, we can all ask for our money back.

[Psychobilly]

There was a thread about it on here in Feb 2008.... talking about the article in this months PA mag...

[x6gas]

There's a way of doing it using a 3 hole Hartmann mask and a webcam or CCD that's dead accurate....I used it on my old 14 inch Meade and, although I was sure that collimation was spot on, it showed to be a tiny fraction out....getting it exact made a remarkable difference.

Problem is, that article's in an old Practical Astronomer mag in my cellar, and I'm in Oslo working and don't get home until the weekend.

I'll look it up, do a synopsis and post it as a sticky.

Cheers

Rob

That would be great Rob . Are you referring to the "Duncan Mask" mentioned in this thread?

It looked like being worth a try, but I was put off by the comment at the end saying that the results were disappointing. Was planning on trying to make one from a polystrene pizza base next time we have one!

[RobH]

That would be great Rob . Are you referring to the "Duncan Mask" mentioned in this thread?

It looked like being worth a try, but I was put off by the comment at the end saying that the results were disappointing. Was planning on trying to make one from a polystrene pizza base next time we have one!

No, definately a 3 hole Hartmann mask....I made one for this task from cardboard.

[Psychobilly]

I'll be interested in this as well I have a nice laser cut for my 8" here somewhere...

Peter...

[x6gas]

Okay, so here is my latest attempt at collimating my Edge SCT. I think I'm getting better! This is again at f/20 with an Ultima barlow and SPC880.

I find that I have to capture and stack some video in order to improve my efforts. I've therefore been capturing 30 seconds of avi and stacking the best 15% of frames to get a better indication of collimation, adjusting, then capturing again. Anyone know of a better / quicker way of doing this?

[syd_malicious]

How about Metaguide. I'd love to try it but I don't have a suitable camera so can't comment on how good it is but it looks very interesting.

Paul

[x6gas]

How about Metaguide. I'd love to try it but I don't have a suitable camera so can't comment on how good it is but it looks very interesting.

Paul

Thanks for the link Paul. I guide with my synguider (but I'm hoping that Santa might be bringing a guidecam) so I'm not very familiar with the guiding software packages. This looks intriguing though so I'll give it a go...