skywatcher collimator

jps · #1 24-11-2005, 01:58 PM

The only collimator in stock at my local telescope shop is a skywatcher collimation tool. I asked him if this was a sight tube/cheshire combination tool and he said ???????.

He described it as including a mirror and cross hairs. I had a look online and from the outside it does look the same as Orions sight tube/Cheshire combination tool.

Is it? Any reason not to buy this for my 8" F/3.75 newtonian?

slice of heaven · #2 24-11-2005, 06:24 PM

Quote:

Originally Posted by jps

Any reason not to buy this for my 8" F/3.75 newtonian?

At F3.75 collimation is pretty fickle, you would be better off with a cheshire that suits that f ratio.

Starkler (Geoff) · #3 24-11-2005, 06:48 PM

Ths skywatcher one if the same as the synta one is a loose fit to the focuser and wobbles. The Orion one is a better fit but still way too long for a f3.75 scope.

I would buy the short GS cheshire from Andrews and get/make a sight tube to fit your f ratio.

jps · #4 24-11-2005, 08:07 PM

Quote:

I would buy the short GS cheshire from Andrews and get/make a sight tube to fit your f ratio.

Appreciate the replies,

The buying of the short GS cheshire I'm sure I could handle.

As for the get/make a sight tube to fit my f ratio, I wouldn't have a clue.

janoskiss (Steve H) · #5 24-11-2005, 08:34 PM

The cheapo short-tube is the most inaccurate. The cross hairs and the outside diameter of the opening you are looking through may not not be central. But you can work around it by rotating in focuser and rechecking/readjusting whilst allowing for anything in the tool being off-centre. That's a very fast scope you have there. Maybe you should also get the sort of auto-collimator that Bird uses.

jps · #6 24-11-2005, 09:47 PM

auto-collimator?

janoskiss (Steve H) · #7 24-11-2005, 09:53 PM

Sorry, actually it's one word: autocollimator. Try google (or Bird).

jps · #8 27-11-2005, 09:11 PM

Contacted Jim Fly. I'm considering buying the catseye Tripplepack Kit which includes the blackcat cheshire, the teletube sight tube, and the infinity autocollimator. $196 US, so I'm thinking maybe $300 including postage.

The only reason I would do this is if I believe this will give me a noticable improvement in image quality compared to using a more everyday collimating set for my newtonian(turns out its an F/4).

Could a cheaper option give me the same results? Would I really notice the difference? It's a lot of money, but maybe its worth it.

Starkler (Geoff) · #9 27-11-2005, 09:32 PM

Your right it IS a lot of money.

Have you ever heard of the "barlowed laser" collimation technique?
http://www.globaldialog.com/~obsessi...#anchor9898989

slice of heaven · #10 30-11-2005, 10:12 AM

Quote:

Originally Posted by jps

Contacted Jim Fly. I'm considering buying the catseye Tripplepack Kit which includes the blackcat cheshire, the teletube sight tube, and the infinity autocollimator. $196 US, so I'm thinking maybe $300 including postage.

The only reason I would do this is if I believe this will give me a noticable improvement in image quality compared to using a more everyday collimating set for my newtonian(turns out its an F/4).

Could a cheaper option give me the same results? Would I really notice the difference? It's a lot of money, but maybe its worth it.

You get what you pay for jps. That is a top of the range, precision made set of tools. Adjustable to your F ratio, easy to use and you can check and adjust your collimation quickly anytime through the night.
With experience, I wouldn't doubt some people 'may' acheive the same standard of collimation with the cheapest of tools, but not as easily.

jps · #11 30-11-2005, 02:44 PM

Can anyone tell me what size sight tube I need for my F/4?

Thiink · #12 30-11-2005, 04:50 PM

Quote:

Originally Posted by ausastronomer

The sight tube really needs to be matched reasonably closely to the F-Ratio of the scope. It doesn't have to be exact ie An F5 sight tube on an F6 scope will be fine, an F7 sight tube on an F4.5 scope won't help much. My combination cheshire/sight tube is optimised for an F5 scope and the sight tube is about 5 inches long. The F-Ratio that Andrews device is optimised for is anyone's guess with the thing only being 3" long.

CS-John B

NB: I edited some unnecessary parts out.

This came from this thread. Hence you would need a Cheshire around 5" long, or there abouts.

Starkler (Geoff) · #13 30-11-2005, 07:10 PM

To match a sight tube up to the focal ratio:
length = (tube inner diameter) x f ratio.

Im getting a sight tube made for me optimised for f5.

hogly52 (Graeme) · #14 30-11-2005, 07:37 PM

Starkler,

With your formula, I'm assuming that the tube inner diameter you refer to is the focussing sight tube diameter, ie. it's either 1.25ins or 2ins? Also, once using the correct length sight tube, what is the correct position for the focusser, in or out?

Cheers,

Graeme

Starkler (Geoff) · #15 30-11-2005, 07:46 PM

Quote:

Originally Posted by hogly52

Starkler,

With your formula, I'm assuming that the tube inner diameter you refer to is the focussing sight tube diameter, ie. it's either 1.25ins or 2ins? Also, once using the correct length sight tube, what is the correct position for the focusser, in or out?

.

Graeme, for a sight tube to fit a 1.25 inch focuser, the length is F x the inside diameter of the sight tube. ie 1.25 inch minus 2x the wall thickness of the sight tube.

A correctly sized sight tube will alow you to visually align the secondary mirror to the edge of the sight tube, while simultaneously being able to see all of your primary mirror reflected in the secondary. Adjust the focuser so that this happens. This allows you to align your secondary easily and accurately.

jps · #16 30-11-2005, 07:56 PM

Could I buy a sight tube of the shelf and cut it to length?

Starkler (Geoff) · #17 30-11-2005, 08:11 PM

You could if you can make an accurate square cut and dont mind cutting the cross hairs off the end.

slice of heaven · #18 30-11-2005, 09:19 PM

http://groups.yahoo.com/group/bigdob/message/9677

Not the thread I was after but the link above touches on the subject of adjusting a sighttube to alter the F ratio.
Reducing the internal diameter can acheive the same result, depends which you think is easier/better. Sliding the sighttube in/out may give the final adjustment.

hogly52 (Graeme) · #19 30-11-2005, 09:31 PM

Starkler,

Thanks for that info.

Graeme

jps · #20 01-12-2005, 02:27 PM

Can anyone tell me how the barlowed laser technique compares to using an autocollimator for the primary mirror?