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C8 SCT - Bob’s knobs V original collimation screws


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A few days back I thought I’d tweak my C8 SCT collimation to see if I could get it better than it was. To be honest the stars were nicely shaped in the SCT already, but I though what the hell and went ahead and did it anyway. Had a cheap artificial star that I’d used before (about a year ago). As the SCT came with Bob’s knobs when I bought the scope off Grumpy Martian I performed the collimation with them as per before. Thought I’d done a good job on the collimation after about 25 mins or so, so the next clear night I got the scope out to do some observing, and had a heart attack when I saw what the stars where actually like after doing this re-collimation. It was way out! So after doing a live collimation on Altair, and tweaking the Bob’s knobs collimation screws I eventually got it pretty much near back to how it was previously, but did notice that fine tuning  with the knobs did not allow what seemed like a very accurate setting, so checked an online video which suggested getting a much more accurate SCT collimation with the original screws in place as you can fine tune it better with the screws rather than with Bob’s knobs. 

So a little long winded but has anyone gone backwards and re-fitted the original collimation screws on their SCT’s to get a better collimation rather than using the famous Bob’s knobs at all?

 

Edited by Knighty2112
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No SCT here, but Ive often considered the upgrade myself, but can't fathom out how much better the mirror cell would sit  on my scope, under spring tension, against  the original screws and compressed washers!
It's still my assumption the cell would hold collimation better with the original setup.
If there was an issue in the first  place, surely the manufacture would  have made amends.

The other main reason for not upgrading is the reach! I cant adjust and look down the focuser at the same time?

Edited by Charic
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When I had a C8 SCT I fitted Bob’s Knobs which made collimating a lot easier than the standard screws. Not the slightest problem with getting the collimatiion spot on and no danger of scratching the corrector plate with a screwdriver. 

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17 hours ago, johninderby said:

When I had a C8 SCT I fitted Bob’s Knobs which made collimating a lot easier than the standard screws. Not the slightest problem with getting the collimatiion spot on and no danger of scratching the corrector plate with a screwdriver. 

That sums it up I think. Wouldn't be without mine.

Peter

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+1 for others.  Make the thumb screws yourself, they are socket head cap screws with plastic caps, and for just a few pennies each at hardware suppliers. Caps are simply squeezed on.  So why pay outrageous prices for what you now know ?

 

PICT0001 (Medium).JPG

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The secret with SCT collimation is that you must finish with all 3 tight. If they are not tight the collimation will not hold.

I had bobs knobs on both my SCTs and they were much easier to handle in the dark than the screwdriver/screws.

its very easy to think you are finished with collimation but the knobs are not tight. You have to loop around until they are tight. And I mean tight! I often put plasters on two fingers so I could get them properly tight :) 

I also found that collimating on a cold night helped everything stick in place as metal expands as it warms up.

Also remember that collimation should be done when the scope has cooled and everything is proper. Collimating early in your session may yield poor results once the scope has cooled.

Alan

Edited by alanjgreen
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3 minutes ago, alanjgreen said:

The secret with SCT collimation is that you must finish with all 3 tight. If they are not tight the collimation will not hold.

I had bobs knobs on both my SCTs and they were much easier to handle in the dark than the screwdriver/screws.

its very easy to think you are finished with collimation but the knobs are not tight. You have to loop around until they are tight. And I mean tight!

I also found that collimating on a cold night helped everything stick in place as metal expands as it warms up.

Also remember that collimation should be done when the scope has cooled and everything is proper. Collimating early in your session may yield poor results once the scope has cooled.

Alan

Excellent advise Alan. Thanks! :) 

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59 minutes ago, L8-Nite said:

+1 for others.  Make the thumb screws yourself, they are socket head cap screws with plastic caps, and for just a few pennies each at hardware suppliers. Caps are simply squeezed on.  So why pay outrageous prices for what you now know ?

 

PICT0001 (Medium).JPG

I can see Bob cacking his undies ?

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21 hours ago, Knighty2112 said:

Any body used a Duncan mask before to collimation an SCT? Just found this online. Looks kind of interesting, and seems relatively easy to perform.

http://alpha-lyrae.co.uk/2013/12/31/schmitt-cassegrain-collimation-made-easy-using-a-duncan-mask/

Thanks for that..never heard of that one before..might try it in the future...

For ease to find  the right collimation screw I use a 90 degree piece of card held in place with a clothes peg..

My scope holds its collimation very well but I'm very  careful to handling it

20180713_234631.jpg

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2 hours ago, knobby said:

I've gone kind of midway, changed mine to socket head screws so no chance of driver slipping and fine adjustment is easy.

Good idea. Have a small Bosh ratchet that I could then use to tighten or slacken them off with. :) 

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On 14/08/2018 at 17:30, L8-Nite said:

+1 for others.  Make the thumb screws yourself, they are socket head cap screws with plastic caps, and for just a few pennies each at hardware suppliers. Caps are simply squeezed on.  So why pay outrageous prices for what you now know ?

 

PICT0001 (Medium).JPG

Great idea. Have you got a link for those red caps? Can't find them anywhere.

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I bought boxes of the ones shown in my photo many years ago over seas,   For a source here in the UK, google " essentracomponents.com " and search for..... Thumb Screw Knobs - Knurled.  You can buy any quantity, depending on size, a box of fifty can be less than a couple of (£) Pounds.

Also, google " Images for plastic thumb screw caps " which will give you an idea of the types that are available for various purposes.  

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  • 2 months later...
On 13/08/2018 at 21:46, Knighty2112 said:

A few days back I thought I’d tweak my C8 SCT collimation to see if I could get it better than it was. To be honest the stars were nicely shaped in the SCT already, but I though what the hell and went ahead and did it anyway. Had a cheap artificial star that I’d used before (about a year ago). As the SCT came with Bob’s knobs when I bought the scope off Grumpy Martian I performed the collimation with them as per before. Thought I’d done a good job on the collimation after about 25 mins or so, so the next clear night I got the scope out to do some observing, and had a heart attack when I saw what the stars where actually like after doing this re-collimation. It was way out! So after doing a live collimation on Altair, and tweaking the Bob’s knobs collimation screws I eventually got it pretty much near back to how it was previously, but did notice that fine tuning  with the knobs did not allow what seemed like a very accurate setting, so checked an online video which suggested getting a much more accurate SCT collimation with the original screws in place as you can fine tune it better with the screws rather than with Bob’s knobs. 

So a little long winded but has anyone gone backwards and re-fitted the original collimation screws on their SCT’s to get a better collimation rather than using the famous Bob’s knobs at all?

 

Starizona suggest staying with the screws 

https://starizona.com/tutorial/collimating-a-schmidt-cassegrain/

but as others have said, a screw driver in the dark is nerve wracking?

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  • 1 month later...
On 02/12/2018 at 14:30, Beammeup said:

If u use a Duncan mask, it greatly reduces the risk of scratching the connector.

 

 

Diagram-for-use.jpg

Never heard of these,  might give it a go. Are they any good. Find my collimination a bit of hit and miss. 

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