Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_imaging_challenge_banner_comet_46p.thumb.jpg.9baae12eeb853c863abc6d2cf3df5968.jpg

brickman7

Help! I made a mistake and I am Desperately In Need!

Recommended Posts

I am new to a telescope and my scope was off at least half an in two directions. So, I loosened the center screw of the spider and the whole dang thing just dropped down, loose!!! It actually rotated down also. So, I reached in and held to mirror back up and tightened the screw slightly tight. Now, I look through the focuser and the spider is off center. The question I need answering ( that I can not find the answer on any video ) is when I look through the focuser without an eyepiece in, what should I see?  I'm thinking that I should see is the cross of the spider, my eyeball, and the ring of the primary center, all centered. Am I correct? Do you understand what I'm saying?  No matter what I try, all I can get centered is my eyeball and the primary ring, centered! I am lost and confuses as crap.  Please help!!

Edited by brickman7
  • Sad 1

Share this post


Link to post
Share on other sites

I suggest you look up any reflector manual and study the collimation routine. It takes a little getting used to, but once you get the hang of it, it's easy enough.

Share this post


Link to post
Share on other sites

Hope your   main mirror didn't suffer any damage, or the secondary.
First lesson learned hopefully. Don't undo screws until you are familiar with
what their function is.
Collimation of a Newtonian can be a frustrating task, until one becomes very familiar
with the process, so practice the laid down routines until it becomes second nature to you,
and remember that once mastered, you can be sure that the telescope will deliver it's  best
performance, all other influences, such as good seeing, adequate cooling of the optics, and 
sturdy mount. The latter especially, if you are intending Imaging the Sky's wonders.
Good Luck to you.
 

 

Edited by barkis

Share this post


Link to post
Share on other sites

Top tip: keep the scope horizontal when tweaking the secondary mirror, but I'll sure this lesson is now learned. 

First thing is to make sure the secondary look a round and not oval when looking through the focus tube.

Share this post


Link to post
Share on other sites

The optical-system consists of only three components...

1. The primary mirror, and at the bottom of the optical-tube

2. The secondary mirror, and near the front of the tube

3. The focusser, specifically the drawtube which racks in and out

Both mirrors are adjustable, particularly the secondary-mirror assembly which includes the "spider"...

342374555_secondaryassembly5a.jpg.8eb089697d10f2fa5ea55a5510cbc0b5.jpg

Note the threaded ends of the spider.  That makes them adjustable, with nuts on the outside of the tube, and to center the secondary-hub within the opening of the optical-tube.  Your telescope may or may not be so equipped.  Also note that the secondary-mirror is oval in shape, however it is tilted at a 45° angle.  Therefore the primary-mirror and the focusser "see" it as being round instead; a circle.

The secondary-mirror can both tilt and rotate, and via those screws on the right within that image.  It is the secondary-assembly that gives folks the most trouble.  It can tilt and rotate in most every direction, but it must come to rest in only one position, and aimed at the center of the primary-mirror and the center of the focusser, simultaneously.  The secondary-mirror simply transfers the image produced by the primary-mirror to the observer's eye...

newt_scope2

The light from the object in the sky enters the tube.  It then strikes the primary-mirror at the bottom.  Since the primary has a curved surface, either a spherical or a parabolic, it takes that light and forms it into a cone.  The goal is to get the tip of that cone to the eyepiece, and that is accomplished by the necessary and unfortunate intervention of the secondary-mirror.  It's a wonky, yet effective, design.  It's been around, basically unchanged, since 1668.  And if that wasn't enough, the last thing on Newton's mind was in making his telescope easy to use. 

Incidentally, there's really no collimation to be done with another type of telescope, normally: a refractor.  The primary-lenses are already aligned with the focusser(we hope); a straight shot from one end to the other...

refractor - bare

However, the vast majority of amateurs who use refractors also use a star-diagonal, and for improved comfort whilst observing.  There's no optical advantage whatsoever in the use of one, however.  As a matter of fact, its inclusion into the light-path can degrade an image, and just as a Newtonian's "diagonal", the secondary-mirror, degrades an image, too.  The refractor is even older, and the very first telescope: 1608.

By the way, what is the make and model of your telescope?

Edited by Alan64
  • Like 1

Share this post


Link to post
Share on other sites

Thanks for all the information from all. I knew you guys would come thru for me.  Again THANKS! 

Share this post


Link to post
Share on other sites

My scope is a Celestron Astromaster 130 EQ.  I had to center the primary myself and I handled that fairly easy.  That made me feel confident in aligning the secondary mirror but the center screw and the tilting screws were not very deep in the fitting because I didn't turn the center screw but less than two revolutions. That thought makes me uneasy. I'm worried that my scope might be a dude from the get-go! See, I have never seen the sky through my scope. Since I got it, our weather in south Louisiana has been raining or cloudy. Also, we have rampant West Nile virus all over the area from the damn mosquitos and it's killing people.  I'm fairly patient due to being an old fart so I have just been waiting. This next week starts my viewing windows due to the cold fronts moving down finally! I have to gets my mess straightened out soon! 

Again, thanks all!

Share this post


Link to post
Share on other sites

You can make adjusting the tilt of the secondary a bit easier.  I replaced those three set-screws, of the hub, with longer screws that have a thumbscrew-like head on them.  You have more control over the adjustments that way, using your fingers rather than fiddling with a hex-key.  You would need to take one of the three screws and have it sized.  It's going to be metric; an M3 or an M4...

1587995080_secondaryfix3.jpg.cee932deea3b752b7409bfeb1cc4844e.jpg

I also inserted a nylon washer in between the other side of the hub and the mirror's stalk(arrow), and to prevent the screws from digging into the back of the stalk.  That way, no permanent depressions can be made where the tips of the screws can get stuck.  You want to be able to make fine adjustments without that problem.  I got my screws from my local hardware, not Home Depot or Lowe's, although I did get the washer from Lowe's, but I'm pretty sure a local hardware would carry those, too.  You would have to take the secondary assembly apart to install the washer.  I glued mine on to the back of the stalk with epoxy.

Edited by Alan64

Share this post


Link to post
Share on other sites

Alan64, that info is great and I will make a point to replace those screws! But right now, if I can replace those and get this lined up, I may wait on the nylon washer and save that for a future upgrade. I just don't feel I can tackle removing the spider right now.

Thank You!

Edited by brickman7

Share this post


Link to post
Share on other sites

To align the spider, just get a piece of paper, once have spider secured

Then tighten each spider mounting screw until the spider is central

Just measure each way and mark on piece of printer paper

Once that done, then can align secondary mirror

The primary mirror should not be out of focus

 

  

Share this post


Link to post
Share on other sites

Download Astrobaby's guide linked to above and follow it to the letter with a Cheshire collimator - do absolutely everything that is suggested even if it sounds 'odd' the things with the bit of paper are inspired.  There was no-one that was more worried or prevaricated more that I when it came to undoing screws on my telescope, but in the finish I followed the guide to the letter and things worked OK.  Online videos of the procedure are helpful in providing confidence to unscrew things, make them really loose and put them back together again, but nothing is as comprehensive as Astrobaby's written guide when it comes to doing the job.  The only thing I will add is sometimes the final turn of screws just alters things fractionally at the last moment so sometimes you need have things positioned to allow for final movement when you make it.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Recently Browsing   0 members

    No registered users viewing this page.

×

Important Information

By using this site, you agree to our Terms of Use.