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"Offset" when collimating a fast reflector

wulfrunian

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wulfrunian

wulfrunian

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I've read the guides, so now I'm going to try some collimation! Just one thing is still confusing me though (easily done!) - the fact that fast scopes have an offset, and so you're not looking for a perfectly centred setup. How do I tell how far offset it should look? Or do I just get it as close as I can - will I just not be able to completely centre everything?

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Eyeball

Eyeball

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Have you had a look at my guide here Astro Babys Guide to Collimation

The final few pics show how 'offset' appears visually. Have a look at those. You will see that in a fast scope the two pale coloured circles are aligned as is the inner dark circle - only the outer of the two dark circles is offset.

I think my Heritage 130P is classified a fast scope f5 is this correct?

length 650mm, appeture 130mm

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wulfrunian

wulfrunian

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I'd read your guide, but I don't think I'd understood that bit, couldn't have read it properly :) think I've got it now. So the only stage of collimation that's affected by the offset it stage 1, the spider vanes and the secondary mirror holder. Thanks :hello2:

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Doc

Doc

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If you use a sight tube to centre the secondary, you have automatically offset the secondary toward the primary mirror, thus ensuring that the fully illuminated field is centered. When using the sight tube, you make the near and far edges of the secondary appear to have the same angular size. This means that the distance from the far edge of the secondary to the optical axis is greater than from the near edge to the axis, this constitutes an offset.

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Astro_Baby

Astro_Baby

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Eyeball your scope would be F5 which is 'fast'

Doc has it right - if the secondary is centred under the EP holder to create a perfect circle thats also centred to the EP holder then the offset is done.

The secondary offset is created by simply centring it under the EP holder. One the primary is also adjusted to be centre then all is well.

The offset is kind of built in as it were.

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wulfrunian

wulfrunian

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Thanks for all the help, it does all make sense :hello2: I've just been trying mine. The secondary seemed to me to be well collimated already, would this be normal from the factory? Primary was out, so I've adjusted that and it now all looks like the picture in the guide! Hopefully skies will be clear enough tonight for a final star test!

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  • 6 months later...
xmal

xmal

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For anyone who is struggling, Andy's Shot Glass has got to be the clearest guide on the interweb to collimating!

Wow, that was a really good tutorial. I bought a fast scope too and I know it will need adjusting one day. This guide answers a load of my questions, but I am not looking forward to the day I have to do it. The guide raises another question- is manual or laser adjustment the most accurate?

xmal

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pvaz

pvaz

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Laser is faster and can be accurate but to be honest I have the hotec (said to be the best on the market) and I still resort to a manual check after I use the hotec.

I think a cheshire is more then enough and it's much cheaper then a laser, although the process takes a bit longer.

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toml42

toml42

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Wow, that was a really good tutorial. I bought a fast scope too and I know it will need adjusting one day. This guide answers a load of my questions, but I am not looking forward to the day I have to do it. The guide raises another question- is manual or laser adjustment the most accurate?

xmal

The problem with laser collimation is that the laser itself is an optical instrument which should be well collimated!

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Astro_Baby

Astro_Baby

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Also a laser can't collimate the secondary mirror - for that you need a sight tube or a cheshire.

By the way the 'shotg'***' tutorial is useful but he doesnt cover mirror offset (or didndt last time I looked) as a result his collimation pattern is ok if you have a slow scope but a fast scope will show a real difference in the collimation pattern an this can lead to a lot of frustration (and usually lots of questions on here too ;) )

In part it was because of my own frustration trying to get an answer to the question as to why my own scope showed an offset pattern that I wrote my gude and systemised the knowledge. Jason took pity on me when I was stuck - fast scopes just werent around when I started in this lark.

Dont be afriad of collimation - the first time it will make you swear a lot but take your time, chill out and see it as a learning cycle. After the firts time its a lot easier.

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Jason D

Jason D

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In part it was because of my own frustration trying to get an answer to the question as to why my own scope showed an offset pattern that I wrote my gude and systemised the knowledge. Jason took pity on me when I was stuck - fast scopes just werent around when I started in this lark.

;)

To the OP, you might be interested in these two posts

http://stargazerslounge.com/beginners-help-advice/99007-collimation-grateful-some-advice.html#21

http://stargazerslounge.com/beginners-help-advice/99007-collimation-grateful-some-advice-2.html#47

Jason

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xmal

xmal

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I am going to start going to a local Astronomy club soon and hopefully someone there can show me how to do it first hand. Maybe at that venue I will be able to see the different types of collimating tools there too. It's a steep learning curve this astronomy hobby :)

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