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Dobsonian and Collimation

Deacon

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Deacon

Deacon

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Hi

After enjoying the views from my SkyWatcher 130p I have taken the plunge and ordered from FLO a SW 8inch Dob.

Im really looking forward to using it but my main concern (surprisingly) is collimation. Not so much the process but rather the equipment. I have a Baader Laser Colli (the first one) which I used now and again with the 130p with good results. I have recenty read however that sometimes the knobs that fasten the eyepiece or laser in place can shift the laser.

I know the new HOTECH laser has a better way of fastning the collimator in place but its very expensive.

I guess my question is it worth investing in a centering adaptor or will the end result just using the Baader with the locking screws still give me decent views??

I hope that all makes sense.

Many thanks for your help.

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kniclander

kniclander

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IMHO it's very easy to get a bit over-fussy - on almost any mass produced scope there will be varianc i n the position of the eyepeice plus some focuser slop so however expensive your collimation tools are, you will still have to contend with that. In an F6 8" dob the "sweets pot" is quite large and uber-precise collimation is not something you need worry about. I would recommed a cheshire over a laser though. :D

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acey

acey

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I used an 8" f6 dob for about 10 years, collimated it maybe once or twice a year, and the only collimation tool I used was a film cannister with a hole in it.

I've now moved to a 12" and find a cheshire indispensible. I also have a laser which I've found to be pretty much dispensible - particularly because of the focuser issue just mentioned. If you've got to buy one tool then get a cheshire, but don't worry too much about collimation. It's basically a way for people to keep themselves busy on cloudy nights.:D

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umadog

umadog

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Centering the laser in the focuser isn't hugely important. The laser being miss-collimated or at and angle in the focuser a bigger problem. You don't need a Hotech, although it is nice. Your best (cheapest) bet is not to tighten the locking screw, as this can shift the laser. Buy a Cheshire/sight-tube combo tool to sanity-check what the laser is telling you. Will need this tool anyway to centre the secondary under the focuser. Check the alignment of the laser by rotating it in the focuser. Use the barlowed-laser technique (if you don't already) for adjusting the primary. This approach is insensitive to the collimation of the laser and so is much more accurate than using the laser's return beam.

As stated above, if you're at f/6 then the tolerances aren't too stringent. Furthermore, the tolerances you see quoted on the web are for high-power viewing. They are more lenient for low-power viewing. If the star test looks good then I wouldn't worry about it: if it ain't broke don't fix it!

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acey

acey

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You look through the small hole in the Cheshire and see a spot (the reflection of the cheshire itself) plus the ring/spot marked on the primary. You adjust the collimation bolts until these two things coincide. That's all there is to it: the cheshire has an angled face that catches light in order to illuminate the view: when using it at night I shine my red light on it.

You need to move away from the eyepeice in order to adjust the bolts. But by slightly twisting one bolt and re-checking you can easily see which way the ring and spot are moving in relation to each other.

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acey

acey

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The snag with the laser is that its accuracy depends on your telescope focuser, as has been mentioned. I find that when my cheshire tells me I'm collimated, my laser doesn't, and vice versa. And I know which is wrong, it's the laser, because my focuser is not perfectly square to the tube and doesn't hold the laser in exactly the same position every time. The "barlowed laser" technique, mentioned by Umadog, gets round this issue, but I prefer to use the cheshire, which I really find simpler (even if my explanation doesn't make it sound simpler!). If you've already got a laser, and a barlow, then use those.

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csarjeant

csarjeant

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I am new to all this. Just ordered a skywatcher 200P Dobsonian and need a collimator for it. Seen a site selling 2 types of cheshire collimator, a short tube one, and a long tube one. Does anyone know if a short tube or long tube cheshire collimator is better to collimate a skywatcher skyliner 200P Dobsonian, and whats the difference?

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Bodkin

Bodkin

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I use both a laser and cheshire, but would probably be happy with just the cheshire really. My problem with both lasers I have owned is that I had to collimate the laser before they were useful. In both cases the laser was displaced slightly. It caused all sorts of concern at first, with the focuser the main suspect. Probably just bad luck. The laser makes the collimation of the secondary easier, but I then revert to the Cheshire for the primary collimation.

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acey

acey

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I am new to all this. Just ordered a skywatcher 200P Dobsonian and need a collimator for it. Seen a site selling 2 types of cheshire collimator, a short tube one, and a long tube one. Does anyone know if a short tube or long tube cheshire collimator is better to collimate a skywatcher skyliner 200P Dobsonian, and whats the difference?

Ideally you want the "focal ratio" of the collimator (length divided by diameter) to match the focal ratio of the scope. Then when you look through the cheshire you see the edge of the primary mirror coinciding with the edge of the cheshire tube. If the cheshire is too long then you won't be able to see the edge of the mirror (or mirror clips), if too short then you'll see a gap around the edge of the primary and you just estimate when things are centred.

The short tube is really designed for short focal length scopes and your scope is f6. I think the long tube would do, though if it were a choice between too long or too short you're probably better off with too short (or else you use a hacksaw to cut down one that's too long).

Mine is a long one (I think), and just marginally too long for my f4.9, though perfectly useable, and I've never felt the need to resort to a hacksaw.

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