Go Back   IceInSpace > Beginners Start Here > Beginners Equipment Discussions

Reply
 
Thread Tools Rate Thread
  #1  
Old 07-11-2018, 11:57 AM
Startrek (Martin)
Registered User

Startrek is offline
 
Join Date: Dec 2017
Location: Sydney
Posts: 1,120
Collimation ( advice for newbies like me )

I was one of those newbies who thought collimating my visual dob and astrophotography reflector was to just a simple case of using a laser collimator to adjust the secondary and primary and your done.However after more than a year getting away with just using a laser collimator I found out the hard way whilst imaging the lagoon nebula at my dark site taking 5 minute subs that something was wrong. Seeing conditions were perfect, no moon, no wind and no mozzies , but the images were showing stars with one end blurred and irregular shaped.I first though itís my primary mirror as I moved it 4mm forward the previous week with cork spacers to give me more back focus on my DSLR.So I checked collimation with my laser collimator and it seemed ok.This had me stumped !! It wasnít until I posted my problem on IIS that a few members advised to buy a Cheshire and adjust the secondary through the focuser sight tube. I found out it was out of alignment quite a bit even though the laser looked ok.Once adjusted and aligned I went out on the next clear night to perform some test images on bright and dim stars and they were perfect , tac sharp and nice sharp diffraction spikes. Problem solved !!
So the moral to the story is donít just rely on your laser collimator , learn how to use a Cheshire and use your laser as a cross check that both secondary and primary are aligned.
Experienced IIS members were a huge help in sorting my collimation problem
Buy a Cheshire and learn how to use it ( itís worth the $60 investment )
Cheers
Clear Skies
Reply With Quote
  #2  
Old 07-11-2018, 12:27 PM
mental4astro's Avatar
mental4astro (Alexander)
kids+wife+scopes=happyman

mental4astro is offline
 
Join Date: Jun 2008
Location: sydney, australia
Posts: 4,542
Excellent to hear, Martin!

Not so crazy, are we...

Remember, always START with checking the secondary mirror with the Cheshire, and only after that (and if any tweaking) do you use the laser! A laser will not fix collimation issues of the secondary. First, secondary mirror. Second, primary mirror.

Many people confuse the issue because the terms "primary" and "secondary", and assume that this somehow also relates to their order of importance, or they make the mistake that a laser is a "fix all". Great to see that you noticed a continued problem and that you asked for help. Then your thread becomes an invaluable resource for others with the same problem!

Alex.

PS: Marc's advice about auto-collimators is also excellent! I have all three, Cheshire, laser and auto-collimator, and will use all three if I find I cannot solve a collimation issue, or not "repeatable".

Last edited by mental4astro; 07-11-2018 at 12:39 PM.
Reply With Quote
  #3  
Old 07-11-2018, 12:33 PM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Hinchinbrook
Posts: 17,875
For a newt the catseye kit with cheshire and auto collimator I find is the best thing. Lasers have their place too. If you apply the laser and it's not square in the focuser then you will miscollimate your scope. Some newt you can't collimate with a cheshire unfortunately. So there's a variety of tools to do the job. In the end the hardest thing is about using them consistently the right way so you can have repeatability in your measurements. I also highly recommend pattern lasers such as howie glatters attachements.
Reply With Quote
  #4  
Old 07-11-2018, 02:00 PM
JeniSkunk's Avatar
JeniSkunk (Jenifur)
Registered User

JeniSkunk is offline
 
Join Date: Jul 2018
Location: Brisbane
Posts: 179
Quote:
Originally Posted by multiweb View Post
For a newt the catseye kit with cheshire and auto collimator
Marc, who makes that kit, and who sells it here in Australia?
Cheshires and laser collimators I'd learned of, when I was figuring out what telescope to buy, but I've never heard of an auto collimator till this thread.
Reply With Quote
  #5  
Old 07-11-2018, 02:15 PM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Hinchinbrook
Posts: 17,875
Quote:
Originally Posted by JeniSkunk View Post
Marc, who makes that kit, and who sells it here in Australia?
Cheshires and laser collimators I'd learned of, when I was figuring out what telescope to buy, but I've never heard of an auto collimator till this thread.
https://www.catseyecollimation.com/

Not sure if any local retailer stocks it but I can't see why not.
An auto collimator involves some kind of mirror so the light path is folded a couple of times. The idea is to overlap and match all the corresponding reflections on each pass to increase accuracy.
Reply With Quote
  #6  
Old 07-11-2018, 04:54 PM
Jason D's Avatar
Jason D (Jason)
Registered User

Jason D is offline
 
Join Date: Aug 2009
Location: California USA
Posts: 116
Quote:
Originally Posted by Startrek View Post
... but the images were showing stars with one end blurred and irregular shaped.... It wasnít until I posted my problem on IIS that a few members advised to buy a Cheshire and adjust the secondary through the focuser sight tube. I found out it was out of alignment quite a bit even though the laser looked ok... .Once adjusted and aligned I went out on the next clear night to perform some test images on bright and dim stars and they were perfect , tac sharp and nice sharp diffraction spikes. Problem solved !!
So the moral to the story is donít just rely on your laser collimator , learn how to use a Cheshire and use your laser as a cross check that both secondary and primary are aligned.
Clear Skies
You stated that when you first used the cheshire, the secondary mirror was way out of alignment even though the laser collimator indicated alignment.
Can you elaborate? I presume the secondary mirror was not centered under the focuser. How far off was it?

Jason
Reply With Quote
  #7  
Old 07-11-2018, 05:40 PM
ChrisV's Avatar
ChrisV (Chris)
Registered User

ChrisV is offline
 
Join Date: Aug 2015
Location: Sydney
Posts: 1,103
Trouble is an auto collimator costs way more than my newt is worth. Must get one some day though.

+1 about the Cheshire. What a wonderful device. But it would be easier to use if my eyesight was better
Reply With Quote
  #8  
Old 07-11-2018, 05:57 PM
Startrek (Martin)
Registered User

Startrek is offline
 
Join Date: Dec 2017
Location: Sydney
Posts: 1,120
Jason
Yes the secondary was not centred down the focuser. I never mentioned it was way out. I unscrewed it from the holder, checked the base where the 3 tilt adjustment screws push against the surface to ensure the surface was not to pitted. Then I re installed the secondary, first softly tightening the central mounting screw holding the secondary and looking down the Cheshire at the same time to get close alignment with the focuser tube ( bit of a juggling act ) then carefully tightening/adjusting the tilt screws, but not too tight.Once lined up concentrically I tightened up and then adjusted the primary.After that I checked both secondary and primary with my Orion deluxe laser collimator and found alignment was very close but not perfect.
When I purchased the Orion deluxe collimator over a year ago I fashioned a V block out of 3mm aluminium angle with hardwood base to check the accuracy of collimation of the collimator ( saw this method on a YouTube clip ) Located 4 metres away from my living room wall it proved very accurate rotating it +360deg ( red dot appeared quite stationary)
Martin
Reply With Quote
  #9  
Old 07-11-2018, 07:38 PM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Hinchinbrook
Posts: 17,875
Use a draw tube to center your secondary, not a cheshire. It's to align the primary. Then you use an auto collimator or laser to tilt the secondary, then realign the primary with the cheshire and iterate until you get closer.

Use two discs cut out of a plastic milk container and place them between the secondary collimation screws and the secondary holder. This way it won't bite into the holder and will also be easier to tweak.
Reply With Quote
  #10  
Old 07-11-2018, 10:12 PM
JeniSkunk's Avatar
JeniSkunk (Jenifur)
Registered User

JeniSkunk is offline
 
Join Date: Jul 2018
Location: Brisbane
Posts: 179
Quote:
Originally Posted by multiweb View Post
https://www.catseyecollimation.com/

Not sure if any local retailer stocks it but I can't see why not.
An auto collimator involves some kind of mirror so the light path is folded a couple of times. The idea is to overlap and match all the corresponding reflections on each pass to increase accuracy.
Thanks for the info on this Marc, but it won't be much use to me since my telescope has a 1.25in eyepiece barrel.
The Catseye auto collimator is for 2in eyepiece barrels only.

From the site, Which CATSEYE Tools Do You Need? - https://www.catseyecollimation.com/needs.html

edit: fix typo

Last edited by JeniSkunk; 07-11-2018 at 10:13 PM. Reason: fix typo
Reply With Quote
  #11  
Old 08-11-2018, 04:38 AM
Jason D's Avatar
Jason D (Jason)
Registered User

Jason D is offline
 
Join Date: Aug 2009
Location: California USA
Posts: 116
Quote:
Originally Posted by Startrek View Post
Jason
but the images were showing stars with one end blurred and irregular shaped.

Yes the secondary was not centred down the focuser. I never mentioned it was way out.
Martin, both statements are inconsistent. I am not questioning your observation but rather the conclusion. There are three collimation alignments:

1- Optimizing field illumination:
-- Using a sight-tube/cheshire tool, this alignment is done by ensuring the secondary mirror is centered/rounded under the focuser.
-- This alignment can't be done using a regular laser collimator unless a special holographic attachment is used.

2- Eliminating focal plane tilt:
-- Using a sight-tube/cheshire tool, this alignment is done by adjusting the secondary mirror to stack the cross-hairs with the primary mirror center spot
-- Using a laser collimator tool, this alignment is done by adjusting the secondary mirror to redirect the laser beam to the primary mirror center spot

3- Eliminating coma:
-- Using a sight-tube/cheshire tool, this alignment is done by adjusting the primary mirror to stack the tool's pupil reflection with the primary mirror center spot
-- Using a laser collimator tool, this alignment is done by adjusting the primary mirror to redirect the returned laser beam back to its source.


The observed issue you have reported is more consistent with misalignments that relate to the 2nd/3rd descriptions above which are within the laser collimator capabilities. A misalignment that relates to the 1st description is manifested as an uneven/off-center illumination but should not impact diffraction, sharpness and/or shape of stars -- unless it is way off.

It is possible that there could have been other factors that led to your observation.

Jason
Reply With Quote
  #12  
Old 08-11-2018, 07:58 AM
Startrek (Martin)
Registered User

Startrek is offline
 
Join Date: Dec 2017
Location: Sydney
Posts: 1,120
Jason
Thanks for your comprehensive explanations regarding all facets of collimation alignment.
Yes my problem could have been related to item 2 and 3
Cheers
Reply With Quote
  #13  
Old 09-11-2018, 06:01 AM
TareqPhoto (Tareq)
Registered User

TareqPhoto is offline
 
Join Date: Oct 2017
Location: Ajman - UAE
Posts: 189
I bought a Newtonian cheap and i wanted to buy a very expensive laser-based collimator [Howie] but i couldn't afford, so i bought a nice quality Cheshire, and just only yesterday i started to collimate the scope after nearly 8 months sleeping in the box, and i have to say it was a BIG nightmare for me to collimate it regardless it is F5 Newt, the main or only problem was the secondary mirror actually, not the primary, the only problem with primary is i couldn't reach all knobs while looking through eyepiece collimator, but it did it and i don't know if it collimator is proper now or not until i test it for stars, i just made a quick test using a regular eyepiece 25mm looking at LED light on my house wall that is nearly 20-25meters away from my door, it was sharp view really, too bad i didn't tried it without or before collimation to see the difference.


I think i will need a laser collimator only if i need to adjust primary more often.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 09:04 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Advertisement
Meade Australia
Advertisement
Celestron Australia
Advertisement
NexDome Observatories
Advertisement
Lunatico Astronomical
Advertisement
SkyWatcher Australia
Advertisement
OzScopes Authorised Dealer
Advertisement
Bintel
Advertisement
Astronomy and Electronics Centre
Advertisement