It is currently Sat Feb 24, 2018 6:27 am

All times are UTC




Post new topic Reply to topic  [ 8 posts ] 
Author Message
 Post subject: Collimating a Newtonian
PostPosted: Sat Mar 24, 2012 11:06 pm 
Offline
Member

Joined: Tue Feb 28, 2012 6:48 pm
Posts: 75
Location: Bolton
DRatledge wrote:
you just might need to do some collimation ......


I had another go tonight armed with a bit more information and I think you've correctly diagnosed one of my problems. Poor collimation does seem to be my problem with the eyepieces and my focusing issues. The 25mm eyepiece seemed easier to focus but the images are small so its very hard to tell. When I add the barlow or switch to the 10mm focusing never quite seems to get there.

I found this information on Astro Baby's Guide to Collimation, where she shows this image as an example of bad collimation

Image

This is very similar to what I'm seeing, although I think mine is more like this image below,

Image.

The bottom edge of the star does appear to be in focus and shows a rounded bottom edge with just the top section looking like its being stretched out to three or four points.

{edit}
although this was an image I took of Jupiter the other night using the dSLR attached to the telescope and it shows no signs of the focusing problem

Image

here's a blown up image of a star taken at the same time which shows what I'm seeing.

Image

_________________
Phil
Skywatcher Explorer 200P & EQ5


Top
 Profile  
 
 Post subject: Re: collimating a Newtonian
PostPosted: Sun Mar 25, 2012 6:56 am 
Offline
Member
User avatar

Joined: Mon Oct 12, 2009 6:47 pm
Posts: 1249
Location: Bolton
I've got our "Cheshire eyepiece" collimation tool at the moment:
http://www.boltonastro.org.uk/forums/viewtopic.php?f=17&t=112
I'll fetch it along to Sharples on Tuesday evening if you'd like to borrow it.

SkyWatcher's own 2-page guide to collimation is quite good:
http://telescopesandbinoculars.co.uk/acatalog/COLLIMATINGANEWTONIANNEW.pdf
and you'll probably find that your primary mirror has a black target-ring in its centre, to make setting up the secondary alignment (figs. d & e) even easier - just get this on the cross-hairs in the Cheshire eyepiece and then proceed to setting up the primary.


Top
 Profile  
 
 Post subject: Re: Collimating a Newtonian
PostPosted: Sun Mar 25, 2012 8:44 am 
Offline
Member

Joined: Tue Feb 28, 2012 6:48 pm
Posts: 75
Location: Bolton
Thanks Ross, I am going to buy one but it will probably be in a weeks time when I'm off work for Easter so borrowing the Societies Cheshire would be great.

I have been looking at the Antares Laser Collimator, what are your thoughts on these laser versions over cheshires?

Also if collimation was my issue wouldn't the image of Jupiter have also shown the same effect?

I did test the collimation using a 35mm film canister and I could see the black ring on the primary so I'd assume the scope was set up ok. However I did struggle to get the canister into the eye piece holder so I wasn't completely confident that I had it aligned correctly in the eye piece holder.

_________________
Phil
Skywatcher Explorer 200P & EQ5


Top
 Profile  
 
 Post subject: Re: Collimating a Newtonian
PostPosted: Sun Mar 25, 2012 1:19 pm 
Offline
Member
User avatar

Joined: Mon Oct 12, 2009 8:27 pm
Posts: 370
Location: Adlington
Seeing the circle in the centre of the main mirror does not mean you are collimated. Judging by the stars you are way off. You need to see your eye or the hole of the film canister INSIDE the circle on the mirror.
The society has a laser collimator but unless you really really really know what you are doing will probably make things even worse! You have to start with these by adjusting the focuser to get it dead-on square so the laser dot will hit the centre of the circle. You may not be able to adjuct the focuser and skipping this stage is a recipe for disaster. I've given up using lasers - much too complicated - stick with the cheshire or film canister.


Top
 Profile  
 
 Post subject: Re: Collimating a Newtonian
PostPosted: Sun Mar 25, 2012 4:11 pm 
Offline
Member
User avatar

Joined: Mon Oct 12, 2009 6:47 pm
Posts: 1249
Location: Bolton
poconnell wrote:
I have been looking at the Antares Laser Collimator, what are your thoughts on these laser versions over cheshires?

We also have one of these: http://www.boltonastro.org.uk/forums/viewtopic.php?f=17&t=113
but see David's comments - I agree that a Cheshire will get you to the right result more easily.


Top
 Profile  
 
 Post subject: Re: Collimating a Newtonian
PostPosted: Sun Mar 25, 2012 6:49 pm 
Offline
Member

Joined: Tue Feb 28, 2012 6:48 pm
Posts: 75
Location: Bolton
Thanks for the info, I'll have a go with the Cheshire first then and see how that goes.

A week or so ago I found a film canister in a drawer at home and having just read up on collimation I decided to have a go. I drilled the hole in the cap and cut off the base only to find the canister was a little too big to fit inside the eye piece holder on the focuser. Today I had a look through my old black and white developing equipment and in the boxes were some other canisters to try. Only one fitted and unfortunately it was a clear white one :( Thankfully even after wrapping it in black insulation tape it still fit into the eye piece holder.

So I've just finished my first attempt at collimating the scope. It was a lot harder than I thought it would be and I hope I haven't made it worse. Initially I looked at the secondary mirror and noticed that I could only see one of the mirror clips. After measuring the spider position to check it was central I was surprised to find that it was quite a bit out. After adjusting that I still couldn't see all of the mirror clips. So I tried adjusting the secondary mirror.

This was the most difficult bit and the adjustments just didn't seem intuitive as the adjustments didn't always do as I expected. I've definitely got it better than it was when I started but I'm sure its far from perfect. The mirror does seem round when viewed from the canister and I can now just see the edges of the primary mirror clips.

I only had to make a very small adjustment on the primary in order to centre the mirror shadow but I found these adjustments much more intuitive and I was happy to move it way out of collimation as I played around with the adjustments knowing I could easily return the mirror shadow back to the centre.

I'll have a go at focusing and defocusing on a star later and see if there is any improvement, but I'd still appreciate it if someone could have a look on Tuesday evening and show me how to do it properly.

_________________
Phil
Skywatcher Explorer 200P & EQ5


Top
 Profile  
 
 Post subject: Re: Collimating a Newtonian
PostPosted: Sun Mar 25, 2012 8:46 pm 
Offline
Member
User avatar

Joined: Mon Oct 12, 2009 8:27 pm
Posts: 370
Location: Adlington
Phil,
Remember its the reflection of your eye, or the hole in the film canister, that you need to see inside the ring on the main mirror. That means everything is on axis. The secondary, being flat, only really effects vignetting.
We'll sort it Tuesday.


Top
 Profile  
 
 Post subject: Re: Collimating a (quasi-)Newtonian
PostPosted: Tue Sep 25, 2012 6:39 am 
Offline
Member
User avatar

Joined: Mon Oct 12, 2009 6:47 pm
Posts: 1249
Location: Bolton
I've just encountered a strange hybrid telescope - it looks like a short-tube Newtonian, but has a sort of Barlow-lens fitted in the bottom of the draw-tube to extend its focal length.
With this type, it's impossible to collimate it by looking down the empty draw-tube unless the lens is removed; fortunately it just unscrews:
Attachment:
File comment: Unscrewing the lens from the bottom of the draw-tube.
But first tilt the tube so that if it slips out of your fingers it won't drop onto the primary or secondary mirrors!

newtcolli1.jpg
newtcolli1.jpg [ 143.95 KiB | Viewed 6836 times ]
Attachment:
File comment: Put the lens to one side whilst you complete the collimation.
newtcolli2.jpg
newtcolli2.jpg [ 165.61 KiB | Viewed 6836 times ]

The collimation adjustments are summarised in this two-page guide from SkyWatcher:
http://ca.skywatcher.com/upfiles/en_download_caty01316546623.pdf
but this tutorial from AstroBaby gives full details of how to set everything up from scratch:
http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 8 posts ] 

All times are UTC


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group