Clear but tiny image after collimating

[Oz Ramos]

I tried following the Astro-Baby and the ATMSite guides to collimating my scope, using a cheshire eyepiece (first, with a disastrous home made version and then a commercial one). If it matters, my scope is a 6" TwinStar and the eyepiece is VueZoom 8mm-24mm, although I tried with the stock lenses as well.

The problem is that the image I'm getting fills up about 1/3 of the actual lense, and is slightly above the center. Before I started trying to collimate, the image filled up the whole lense, but was just a tad bit out of focus.

The image below shows my problem. Left is before (slightly blurry); right is more focused but smaller:

[Astro_Baby]

Collimation wont affect the magnification of a telescope. What object are tou looking at in that drawings above ?

I would hazard a guess and say the image on the left is wildy out of focus - but on the info you have supplied I cant really be more helpful.

The scope appears to be a bird jons design as the specs I found for it suggest its got a 1400mm focal length - the scope is nothing like that long so I imagine its got a barlow built in at the base of the focuser. I cant quite see how this could be causing problems but bird jones type scopes are often awkward to collmate.

Can you deatil what steps you took with collimation and what object you are looking at in the drawings you have given ?

[DMA]

Hello

Done some research on the bird-jones design and found that collimation will be easier if you can take out the corrector lens.

other people seem to have been able to take out theirs on other makes of scopes so hopefully yours can be removed as well.

If you can remove it follow asto-baby's collimation steps then replace.

These scopes seem to be very very hard to collimate because of the corrector lens and the

spherical mirror.

These telescopes are showing up more and more because spherical mirrors are cheaper to make than parabolics and computer controlled lens design makes it easy to design the corrector lens.

Dave

[haitch]

Just did a quick Google and I suspect you have a short tube reflector - Can you have a look and tell us if there is a lens at the bottom of the focuser tube?

[RonVen]

Maybe you put the corrector lens in backwards, causing the smaller image.

Ron

[umadog]

I must say, I'm confused as to what you mean by the "image" filling up only some of the lens. The image isn't formed in the lens, it's formed on your retina. Unless your eye is at the correct distance (the so-called eye-relief) from the glass, you will see a clipped image of the exit pupil which might behave in the ways you describe. Note that the full extent of the glass surface is required to provide the extended apparent field of view, which is defined as an angle (in this graphic it is the angle "ε" http://www.telescope-optics.net/eyepiece1.htm). So you will never "see the image filling the lens."

My suspicion is either the corrector lens is in the wrong location (or possibly inverted, as suggested by RonVen), or there's a focusing issue, or the secondary is mis-placed, or there is nothing wrong at all.

[Oz Ramos]

(Oops I accidentally turned off notifications...)

Sorry, the illustration is of the moon. I ignored your first rule and just started messing around...serves me right! You're all right, it looks like there's a barlow lense at the bottom of the focuser.

I feel silly saying this, but I think the root of the problem was my hastiness to get it collimated, so it's probably me. After Astro Baby asked me to list the steps I took, I started to and noticed I made more assumptions on some of the steps than I should have.

I'll report back in a few days when I have a chance to try again. Thanks everyone!

[haitch]

How di

(Oops I accidentally turned off notifications...)

Sorry, the illustration is of the moon. I ignored your first rule and just started messing around...serves me right! You're all right, it looks like there's a barlow lense at the bottom of the focuser.

I feel silly saying this, but I think the root of the problem was my hastiness to get it collimated, so it's probably me. After Astro Baby asked me to list the steps I took, I started to and noticed I made more assumptions on some of the steps than I should have.

I'll report back in a few days when I have a chance to try again. Thanks everyone!

How did you get on? Is it in the trash yet?