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Aperture fever


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Hi all,

I have been very interested in purchasing a dobsonian scope but the only real issue that is stopping me is the whole collimation process. I have been looking at the Skywatcher dobs both the 8 and ten inch.

Is there anyone that can give me a good appraisal of these scopes and some advice regarding collimation. Also advice on which kind of collimating tool I need would be great.

Many Thanks in advance

Deacon

:)

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I wouldn't worry too much about learning how to collimate. When I got my 16" LB it was well out of whack. Had to adjust the secondary and primary. Once you get the hang of it, it's not that difficult. I use a Cheshire collimating eyepiece to help me. Basically though, as long as you get the secondary centred, the image of the primary centred and the reflection of your secondary vanes looking roughly equal when you look down the draw tube, you're 95% there. The Cheshire just helps me get that last 5%

Once I had set it up how I wanted - took me about 3 gos to get it right after star testing - it only requires very minor adjustments from time to time, unless the scope take an appreciable whack or jolt.

I wouldn't say my collimation is 100% perfect, but it works for me and in steady seeing its all sharp as a pin :)

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The 1st time it's a bit tricky and scary but collimation becomes something easy. I wouldn't be put off by it. A laser collimator does make the task easier and faster. I got the hotech and it works very well but before that I could achieve good collimation with just a cap, it just toke a bit more time and patience.

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I agree. I wouldn't let collimation put you off a Dob. Once you do it the first time it will be easier after that. I used just a lens cap with a hole in it to get mine sorted. You just want everything nice and centred.The laser, as Pvaz says, will get you 100% of the way there.

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Cheers guys for the advice. The Hotech is a bit over my budget for a collimation tool so I may just go for the cheshire!!

I live in an area of moderate light pollution so is the bigger the better or would a 8" dob suffice.

Many Thanks

Deacon

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According to the book "The Dobsonian Telescope" with a 8" you can aspire to see up to 1000 galaxies on a really dark sky.

For any beginner I think that suffices, at least I know I'm very pleased with mine, although I think in the future I may want a bigger one but there is still a lot for me to see before I outgrow my telescope. The 8" dob is considered to be a good balance between aperture, portability and ease of use.

That said, if you can afford/handle a bigger 10" it will make a difference and will still be fairly portable.

Edited by pvaz
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Yup - I agree with Luke - bigger aperture the better - but they do get heavier and bulkier.

If you're in reasonable shape the 8" will be a doddle, 10" easy enough and worth the little extra effort - and the 12" just managable without getting the missus involved lol

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Collimation is not something to worry about. With the Skywatcher reflectors I have looked at many and most are close to being aligned only needing the smallest adjustment of the primary mirror. Even if you make a complete mess of collimating initilaly it will not take long to get it right. You will not be taking the scope apart after all.

I use an Orion USA laser collimator (secondhand about £25) which although basic is accurate enough before doing the final star test.

Skywatcher have just announced a new range of Explorer relectors to replace the current range of 8" to 12". The dobsonians are bound to be change soon too.

Mike

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One point in favour of the 8" Skywatcher dobsonian is that they operate at F/6 wheras the 10" is F/4.7. I found the 8" dobs (I've owned a couple) kept their collimation better and were more "forgiving" of both small collimation errors and lower cost eyepieces than their larger aperture, faster, cousins.

That said, once you have got the hang of collimation it's easy to repeat the process and apply some final "tweaks" each time you observe.

The problem I found with an 8", great scope though it is, is that it's not quite enough "medicine" to cure the fever, it just arrests it for a bit .... :)

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