Short Tube Reflector Collimation

[Draconis]

Hi Guys,

Im looking for some advice re the collimation of a Celestron Nexstar 114 SLT.


Its a short tube style telescope with the built in barlow. I have a laser collimator which I have tried to use with the 114 but the inbuilt barlow spreads the beam into a big blob, should I be worrying about collimation on a small scope like this? The view through the eyepiece looks 'ok'
but it would be a shame if i'm missing out on an improved view because of dodgy collimation.

I also have a 10'' Dob which is fantastic and I am able to collimate this with the laser no problem.

The Nexstar was an expensive mistake, I got sucked into buying it for the go-to feature and didnt realise it really isnt worth the extra money for the fancy gadgets. However, i still bring it out from time to time as a quick grab and go instead of hauling the dob out for those nights when I just want a quick look at something and for that its not too bad.

Great site btw, been browsing here for a while now and always seem to find answers to anything I need to know here. I've done a search of previous threads for a similar problem but couldnt see anything, apologies if its already been covered and I'm just to blind to see it.

Cheers
Mick :D

[mental4astro]

Hi Mick,

Welcome to IIS, mate!

The short tube scope is certainly worth collimating if you use it. There is a gizzmo called a "cheshire eyepiece" you can use. They are inexpensive, and the added bonus is they are also used to collimate the secondary mirror, which laser collimators don't.

So, this way, you'll be getting the best performance of both your scopes!

Mental.

[Draconis]

Awesome, thanks Mental. Hopefully one day i'll get to actually use the scopes again when the clouds go away instead of them being a pair of dust collectors :P

[mental4astro]

I forgot to mention one thing and clarify another:

The cheshire eyepiece alone can be used to collimate the optics.

The laser can be used to align the secondary, but its failing is in not being able to opitmize the position of the secondary mirror. That is, the secondary's vertical and horizontal positions. Its best position for general viewing is having it centred.

If you want to get really, really freaky about its position, you can "off-set" the mirror to take into account the geometry of the path of light. We are talking off-sets in the order of 1 to 2 mm in each the vertical and horizontal, with 2mm for the largest of scopes.

Me, I'd stick to centring the mirror, as the % of light loss from not off-setting isn't noticeable to our human eyes. It is something you'd consider if you where doing serious imaging or building your own scope and want to be pedantic on details.

[barx1963]

I could be wrong but if there is a built in barlow in the focuser drawtube, wouldn't that get in the way of the cheshire? At least it would with my cheshire!

[Brian W]

Hi, I own an Orion 4.5' short tube and while it is not the same as your scope the views definitely improve with collimation.

I would suggest just worrying about the primary for the simple reason the secondary is very small and in a tight spot so adjusting it is a problem.

If you can find one, a collimation cap works quite nicely.
Brian.

[mental4astro]

Even if the built-in barlow gets in the way, a cheshire eyepiece would work brilliantly in Mick's larger scope.

There is another way, :lol: , always is, isn't there. It involves getting your hands on one of the old Kodak film canisters with its cap in place. If you make two holes, about 3 - 4mm in diameter, smack bang in the middle of the cap and the base of the canister, slip it into the focuser (it happens to be exactly 1.25"!), you've got yourself a very rudimentary collimator. It will work brilliantly for the positioning of the secondary mirror (it's what I use with my 17.5" dob), but its primary mirror collimating capability isn't as good as it doesn't have the crosshairs, but it will still do a significant improvement on what you've got. What a laser will improve upon from a cheshire or the "canister method", is splitting hairs. Very good though if you've got one, which I use too.

There are several sites that give very good instructions on collimating newtonians with cheshire's. Do a google search on "collimating a newtonian", :google: .

[mswhin63]

With my old SEBEN I used to remove the Barlow then collimate with a laser then re-insert the Barlow. But not sure if that is a good thing.