Author Topic: Collimation on a small scale - expert needed.  (Read 2629 times)

Longjohn54

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Collimation on a small scale - expert needed.
« on: February 15, 2014, 10:30:23 PM »
I bought a mini-dob for my nephew for Xmas, this one in fact - http://www.firstlightoptics.com/heritage/skywatcher-heritage-76-mini-dobsonian.html

Had a go with it this evening and something's not right.  Nothing focuses properly and everything is just mush.   Back in the house I looked down the eyepiece tube and it seems like the secondary mirror is a bit squint.  I couldn't see all of the main mirror at all.   There's no centre dot on the mirror either so I cannot tell how far out it is.

So, is there a collimation expert with nimble fingers who wouldn't mind tackling this for me please?

John
John

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stev74

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Re: Collimation on a small scale - expert needed.
« Reply #1 on: February 16, 2014, 10:37:34 AM »
I got one of the Heritage 76 scopes as the main prize for the astro raffle that I organized for the stargazing night I did at Kirkliston primary and I'll be collimating that one anyway when me and James go to give the family that won a hour or two's tuition on how to use it. So if you like i can have a look at your nephews as scope too, when would suit you?  :)


One thing to check first is make sure the secondary spider vane support isn't bent as I believe that it can happen in transit sometimes with these scopes as there is only the one holding the mirror?
Steve
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Longjohn54

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Re: Collimation on a small scale - expert needed.
« Reply #2 on: February 16, 2014, 12:58:24 PM »
I don't think the spider vane is bent but the secondary mirror is not pointing to the centre of the main mirror. any time that suits you will be fine. I can bring it over to you.
Thanks Steve.

Sent via phone - excuse spelling!
John
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G1ZmO

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Re: Collimation on a small scale - expert needed.
« Reply #3 on: February 16, 2014, 08:12:05 PM »
The one I got for Daniel last year was the same. There's no adjustment on either primary or secondary which is pretty hopeless
Paul

Longjohn54

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Re: Collimation on a small scale - expert needed.
« Reply #4 on: February 17, 2014, 12:30:08 AM »
Primary is fixed and looks to be aligned in the tube.

Secondary is attached by a threaded rod into a captive nut which is held in place with a locknut.   The locknut is a standard 7mm nut however the captive part is round with two flats on it.   The flats are 90degrees from the tube entrance so will need a spanner with a right angled offset to hold it.   See photo.

The secondary is pointing at about 30mm to the left of the main mirror's unspotted centreline.   Once straightened it should be reasonably easy to collimate with the normal three screws (somewhat fiddly though).

John

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« Last Edit: February 17, 2014, 12:33:21 AM by Longjohn54 »
John

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