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I've been given a lovely Dobsonian, the balance isn't quite right, I need slightly more weight at the back, how do I alter the weight? Also, I need advice in collimating it, without a collimator, is there any other way to get it all sorted? 

Thanks in advance. 

 

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You should be able to collimate it by looking at a star using a high power eyepiece. The out of focus diffraction rings should be concentric. You may need the assistance of friend for turning the collimating screws/knobs while you keep the scope on target, until the correct screws have been slightly adjusted to create a concentric diffraction pattern.

There are adjustable counterbalance kits available that you could attach to the scope to adjust its balance as needed.

Edited by mikeDnight

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It's £99 from FLO but an Astrozap artificial star is very useful for testing both collimation and optics.

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For collimation, depending on f ratio, I'd start with a collimation cap. Easy to make, or you can buy one for less than a tenner. Think FLO sells the Rigel aline collimation cap for about £6, and it does the job for basic collimation. If you need more than a combined Cheshire and sight tube would be my choice.

As Mike noted, you can also collimate on a star, but this can be tricky with Dobs, because:

* The star needs to be reasonably bright.

* The magnification needs to be high. Over x200 for sure, about x500 seems to be regarded as good over on CN.

* Seeing needs to be very good. At x250 the image should be very stable (not really any appreciable scintillation). If it's not you'll struggle (seeing is very rarely this good in the UK)

* The star image needs to be assessed at the centre of the eyepiece. Tracking a star with a manual dob and keeping it centred at x250, while tweaking collimation knobs (which will move the star around a huge amount relativeto the FOV) is going to be very fiddly.

You can avoid some of these issues by using Polaris, as it stays relatively still, but I think it'll be much easier to get the alignment roughly right with some basic tools.

Billy.

Edited by billyharris72

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Personally, I would buy a suitable collimation device. I've found it pretty hard to do using a star unless the scope is small enough that you can look through the eyepiece and tweak the primary mirror at the same time. Also, IIRC, you can only do the more critical primary alignment with a star test. With a collimation tool (or tools) you can sort out the alignment during the day and leisure. 

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