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View Full Version : Collimation problem on a 10 inch reflector



Brian T
2010-Apr-09, 09:44 PM
So a few weekends ago, some friends and I went to a retro, antique, and scientific swap meet. The first thing I noticed upon walking in the room was a glorious 10 inch reflecting telescope. Obviously, the rest is history and here I am asking for help. I've never owned a telescope before this one, so this has all been really educational. It's an older model Meade with a lot of modifications.

I've read a few articles on collimation but I haven't come across any so far that address the peculiar issue I'm having. When I first brought the telescope home, the collimation was way off. I managed to get the primary and secondary mirrors fairly close to being aligned in concentric circles, although I can't seem to get them perfectly centered by myself. I lack a collimating eyepiece, and the adjustment screws are far away enough from the eyepiece tube that I can't turn them and still look through the tube at the same time.

Now, with my telescope functioning with a "good enough for now" collimation, the views of the Moon are fantastic and utterly breathtaking. Mars is a bit fuzzy but Saturn is very cool. One problem became apparent when gazing at the Moon, though. After a minute with such a wide and bright backdrop, a circular shady spot is apparent in the middle of the image. I assume this is the collimation dot on the center of my primary mirror (circled in red on the attached image).

I get the impression that this isn't normal. I'm sure the collimation dot should only visible during the actual collimation process, not viewing. I'm guessing that while my primary mirror is more or less pointed along the right axis, perhaps it's not the right *distance* from the secondary. With a parabolic mirror, the dot should disappear from viewing once it's at the right distance, I would think.

Also, when I have the moon centered and I back off the eyepiece (i.e. several inches away, too far for actual viewing), the outline of my secondary mirror and supporting vane become apparent. This is what the sketch on the right depicts. It's not a cross-shaped vane spider, just a single piece of metal offset from center, with the secondary attached to the side of that. Obviously, I'm not going to hold my eye six inches from the eyepiece when viewing, I just didn't know if this was normal or indicative of another problem.

I plan on joining the Fort Worth Astronomical Society and going to some star parties. I'm sure that someone would be able to help me out there, but I'd really rather not show up in public not knowing the basic maintenance skills for my own scope.

So, has anyone had experience with this peculiar problem?

Image link: http://imgur.com/hGayc

http://i.imgur.com/hGayc.jpg

ebutts
2010-Apr-12, 09:04 AM
Hi Brian,

How much magnification were you using? The reason I ask is that telescopes with a secondary obstruction often behave like you describe, when using too low magnification resulting in a too large exit pupil. The magnification is calculated as the focal length of your telescope divided by the focal length of your eyepiece, and the exit pupil is the aperture of your telescope divided by the magnification. The trick is to keep the exit pupil smaller than your own pupil or you will see the shadow of the secondary mirror in the focused image. Usually during astronomical observations the image and your surroundings are quite dark and your own pupil will expand to a maximum of 5-7mm allowing the use of low magnification. Conversely, during daylight observations or observations of bright objects (eg. the Moon!), your own pupil will contract and the shadow of the secondary will become visible at low magnification.

Hope this helps.

kris27
2010-Apr-12, 01:51 PM
Hi Brian
Ha ...... I picked up a reflector telescope (Tasko 114/900mm) on the street, we have every 3 month rubbish collection by city council. Some one kicked out the TELESCOPE. It required quite a bit of restoration work, finally I managed to get it in like new shape. Than saga started. Like yours ....
I never had a telescope before, so I did not know anything abt. colimation and stuff I did have no idea what I suppose to see with it ?????
I found numerous posts on the net regarding reflectors. This is what I did:
1. Made colimating tool out of film canister top - tiny hole in it and used piece of tubing to mount on focuser - this allowed to align primary mirror and secondary. You seen on net numerous pictures showing what you suppose to see. On your picture looks like that secondary mirror is not se tup correctly. The vertical line should be crossing the black dot right through the centre.
You have to use alignment screws on the spider to set this up ..... quite pain but at first .... but after few minutes you get it right.
2. Now the nightmare .... it took numerous attempts to adjust the "cell", name for arrangement that holds primary mirror. It was impossible to look at eyepiece and adjust the screws at the bottom at same time. So I tried to turn two screws leaving one untouched (3 screws there) - just turn a bit and look in eye piece, than turn again and see the result. Trying for a few days finally I got almost right, at least when I look at moon it was nice.
Forget about stars ...... I could not focus at all .... so tried the main mirror again ..... finally I gave up.
3. I had a rest from telescope for a few days, went to Astronomical shop bought - The laser colimating tool, it is just tube with lasing diode in it, it cost $AU120. Went home and tucked it on to focuser. Here we go ..... I can see that I am really far out ..... so in matter of minutes I got the "beast" telescope colimated spot on.
4. The big night .... first Mars, well find the thing on the sky with telescope ... it was a ***** job, but ealier I bought finder (it was missing when I picked scope) managed to align finder with the star (Mars) so when I managed to find object in center of finder it was in center of eyepiece (almost). I could focus on any star on the sky from now on.
5. You read about temperature satbilisation of the telescope .... this is another *****. It takes long time, an hour ++++ to settle down, and used laser colimating tool to verify my colimation again. It was out ..... so a minute or so I got thing back in to the shape. Since than when telescope settles down every thing is OK.
ALMOST ..... just to find out that I have still air currents in the tube, windy night .... picture was kind of sparkling with colours, like looking via prism. And the atmosphere is giving troubles as well. To have a nice observing night I have to go out of the city, somewhere in the bush, the best will be top of the hill ....

Have fun ..... Kris

Brian T
2010-Apr-15, 02:35 PM
Hi Brian,

How much magnification were you using? The reason I ask is that telescopes with a secondary obstruction often behave like you describe, when using too low magnification resulting in a too large exit pupil. The magnification is calculated as the focal length of your telescope divided by the focal length of your eyepiece, and the exit pupil is the aperture of your telescope divided by the magnification.

I think you might have just nailed it. I was indeed using a low power eyepiece (alternating between 25mm and 32mm, actually) in order to get a nice wide field image of the Moon. Next time the Moon is a little brighter I'll have to take another look and check if the problem goes away with higher power eyepieces. As for the exact magnification I was using, I'm not sure. The guy that I bought the scope from told me the focal ratio, but now I can't remember it. I've emailed him to ask if he remembers, so we'll see. Thanks for your input, it really was helpful!


On your picture looks like that secondary mirror is not se tup correctly. The vertical line should be crossing the black dot right through the centre. You have to use alignment screws on the spider to set this up ..... quite pain but at first .... but after few minutes you get it right.

Actually, that's how it looks when viewed from the top as well. The supporting vane holding the secondary mirror is offset from the center. I'll probably end up replacing it with a more modern spider vane assembly. As it is, there are no alignment screws on my secondary mirror, only a nut on the vane that can be tightened or loosened to allow the secondary mirror to slide up or down respective to the primary mirror.

glappkaeft
2010-Apr-15, 11:58 PM
I think you might have just nailed it. I was indeed using a low power eyepiece (alternating between 25mm and 32mm, actually) in order to get a nice wide field image of the Moon. Next time the Moon is a little brighter I'll have to take another look and check if the problem goes away with higher power eyepieces. As for the exact magnification I was using, I'm not sure. The guy that I bought the scope from told me the focal ratio, but now I can't remember it. I've emailed him to ask if he remembers, so we'll see. Thanks for your input, it really was helpful!

The focal ratio is just focal length/diameter, i.e. 900mm/114mm = F/7.9 but in this case it's not that relevant since magnification is focal length of telescope/focal length of eyepiece, e.g. 900 mm/32 mm = 28x. Finally exit pupil is scope diameter/magnification, e.g. 114 mm/28x = 4 mm.

shawnhar
2010-May-21, 04:07 PM
Hey is that an Odyssey Red tube 10"??? I just bought one, looks exactly the same as you descrbe and I too see the dot, but only when I use the 27mm "adjustable" eyepiece that came with it, no other eyepieces do it, they person I got it from was the original owner and gave me all the paperwork, it's 1150 FL and an f/4.5 so it's pretty fast. I have concentric circles on a star collimation, but it looks offset through the collimation cap.

Brian T
2010-Jun-07, 02:04 AM
Sorry for the late reply, shawnhar. I don't think it's a brand name telescope. Many components have a distinctively home-made look about them. For that matter, my brother and I built a new Dobson styled base for it and given the whole thing a new paint job. I also have a laser collimation tool on the way. Hopefully that should help with the fine tuning.

shawnhar
2010-Jun-07, 05:54 PM
LOL, sometimes I don't turn my computer on for a month. I am curious about the base you built, what kind of bearings did you use? I am seriously considering a lazy susan type application for mine. By the way, the Coulter's looked completely homemade, red cardboard tube, duct tape and a giant hose clamp wrapped around the mirror with particle board mirror cell. Have you tried for any Deep Space objects like Galaxies or Nebula? Good luck with the laser, I have read that it is only good for aligning the primary mirror, I am still using a film canister with a shiny washer glued inside, seems to work ok.

Brian T
2010-Jun-19, 10:02 PM
Got the laser collimator from Orion. It works great! Getting the scope ready to go is a snap now. Last night I caught some great views of the Moon and Saturn before they disappeared under the trees in my backyard.

I haven't had any deep sky opportunities just yet. Now that I've gotten a much better grasp of how to collimate this thing I'm a lot more confident about the idea of tossing it in my car and driving out to the countryside.

The mount that my brother and I did was made from a type of particle board called MDF. http://en.wikipedia.org/wiki/Medium-density_fibreboard As a material, I don't really recommend it after doing a big project like this. It's dusty as hell and it splits really easily unless you drill pilot holes. If there is ever a next time, I'll either opt for some actual wood, or build a frame from aluminum and/or stainless steel. All surfaces were primed and spray painted. For bearing surfaces, at the moment I'm just using medium duty felt pads. They're just placeholders until I can get some scrap teflon. I can't wait to upgrade the bearing surfaces, because while felt holds the tube nice and still, it's almost impossible to make ultra fine adjustments because it wants to stick in place.

The whole project has been quite a learning experience. I'll have to post some pictures sometime.