Are you collimating your scope correctly?

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VinSnr
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Are you collimating your scope correctly?

Post by VinSnr »

Ok...this post applies to specifically to those with reflectors, Mak-Newts and S-Newts. People with refractors will find it useful too.

Very often, I have seen people using lasers to collimate the above mentioned scopes. Assuming you know that the laser must be well aligned first and you know how to collimate using a laser, is that really good enough?

What happen to the following tool? I have not seen a single person in singastro using this :

Image

The above is the Cheshire eyepiece with a cross hair.

You may think that having your laser hit the centre of the primary and adjusting the primary to hit the opening of the laser means your collimation is good enough. Is that really true?

The problem with laser is that you cannot determine if your secondary is correctly placed. How do you know if your secondary is squared with the focuser? And how do you know if your secondary is offseted correctly?

Scope performance is often robbed not because of collimation, but because of a poorly placed secondary. While your alignment of the primary may be dead on, a poorly placed secondary will waste those precious light, giving you sharp, but dimmer than what your scope should give.

The right way to collimate is to use the cheshire to check the secondary position first. Many people never do this step. They jumped straight to laser. Once the secondary position is corrected, you can then use the laser to check for the primary. Actually, the Cheshire hire eyepiece can do everything and is even more accurate than a laser, albeit less convenient especially when you are doing this at night.

The good news is that normally once you got the secondary in the right place, the secondary should hold that position quite well. But it always good to check that first before every observing session.

For those using refractors, the cheshire is a wonderful piece of instrument to check for collimation. You can't go wrong with it. With lasers, you have to make sure that the laser output is aligned and that you have to decide for yourself if that laser is coming out at the centre of the objective lens. It's quite a subjective thing. And how to do avoid parallax error when you can't look straight at the lens? (there is laser there...dun look straight at it).

The Cheshire can also check collimation in diagonals. It cost half the price of a laser, and is totally maintenance free.

It's time that you guys should take a good look at this under-rated and under utilise piece of instrument.
Last edited by VinSnr on Mon Jun 07, 2004 3:10 pm, edited 1 time in total.
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weixing
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Post by weixing »

Hi,
I also have one of this, but I have problem using it. For example, what position should I put my focuser draw tube?? I mean all the way in, middle, or all the way out. (I'm using a Newtonian).

Thanks and have a nice day.
Yang Weixing
:mrgreen: "The universe is composed mainly of hydrogen and ignorance." :mrgreen:
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VinSnr
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Post by VinSnr »

weixing wrote:Hi,
I also have one of this, but I have problem using it. For example, what position should I put my focuser draw tube?? I mean all the way in, middle, or all the way out. (I'm using a Newtonian).

Thanks and have a nice day.
Put to the extend that you can just see the outline of the secondary mirror.
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ariefm71
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Post by ariefm71 »

vince, what do you think of this sct collimation tool from digitec: http://www.digitecoptical.com/coll-sctlaser.htm ? normally i do collimation manually using 267x magnification and defocus a medium mag star.

btw, i think i will follow the instruction from http://www.rothritter.com/C11_Project/ to remove the corrector and take out the fillings.

cheers,
arief
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VinSnr
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Post by VinSnr »

ariefm71 wrote:vince, what do you think of this sct collimation tool from digitec: http://www.digitecoptical.com/coll-sctlaser.htm ? normally i do collimation manually using 267x magnification and defocus a medium mag star.

btw, i think i will follow the instruction from http://www.rothritter.com/C11_Project/ to remove the corrector and take out the fillings.

cheers,
arief
Don't waste your money on that collimator. If you have US$200++ to blow, treat some of us to a meal or something and you will still have some change. Collimating using a star is simple and fast..and it's free.)

But 267X is too little. At least 400X. Mr. Aw, a veteran around here uses 1000X. You ask Wira and you know. But for your alt-azi mount, 1000X will not allow you to track the star nicely. But you should go higher than 267X. (Harold Suiter in his book said 200X at least, and a mag 2 star. Mag 2 star doesn't work in Singapore, and 350X would be nice for your mount)

As for the corrector plate....yes you can follow the instructions, just make sure you keep the orientation of the corrector plate correct. Do it in an air-con room with plenty of light. Place the scope vertical and always mark the orientation. You loses the orientation, you loses the scope. As simple as that.

Next time, please don't drill your OTA. The only OTA that can be drilled are the reflectors because you can basically remove every single part and put it back together without any worry. If you want to mount a guidescope, get the proper mounting plate from losmandy. They have plates for Meade and Celestron SCTs..ranging from 8" to 16" scopes. The code for losmandy Celestron 8" SCT plate is DC-8 if i remember correctly. If you can even consider buying that collimator, then the price of that losmandy plate should be peanuts to you. Beats the hell out of me why you want to drill your SCT with mirror and everything inside.
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ariefm71
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Post by ariefm71 »

Beats the hell out of me why you want to drill your SCT with mirror and everything inside
the mirror was secured as i push them forward with the focuser first. i don't see any issue with drilling an sct, a lot of ppl do that (some even put a cooling fan inside), as long as you're be very careful and follow the instructions exactly. modifications is part of the fun :wink: . anyway, thanks for the suggestion.
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VinSnr
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Post by VinSnr »

ariefm71 wrote:
Beats the hell out of me why you want to drill your SCT with mirror and everything inside
the mirror was secured as i push them forward with the focuser first. as long you're very careful and follow the instructions exactly, i don't see any issue with drilling an sct. modifications is part of the fun :wink: . anyway, thanks for the suggestion.
No prob. Remember when you remove the screws around your corrector, feel and remember the tightness of each one of them. This to ensure you don't overtigthen them when you put them back or you risk pinching the corrector.

Once the screws are off, best take a photo of it so that you know the position of the shims.

then mark the corrector plate in relation to the tube. Make a few markings if you have to.

When you remove the filings, be very careful not to touch the primary.

I know modifications can be fun......but not with the primary mirror inside!!! It's too much of a risk!
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weixing
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Post by weixing »

Hi,
I was collimating my refractor using a cheshire eyepiece. The something strange... at least to me :P :P Below are two collimation result that I'm able to get, so which one is the correct one??

Thanks and have a nice day.
Attachments
Collimation Result
Collimation Result
refractor_collimation.jpg (26.3 KiB) Viewed 6682 times
Collimation Setup
Collimation Setup
collimation_setup.jpg (24.34 KiB) Viewed 6682 times
Yang Weixing
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Tachyon
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Post by Tachyon »

Is it the focuser?
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weixing
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Post by weixing »

Hi,
The focuser position is same for both... What I mean is that I can collimate to either 1) The smaller brightest white "donut" in the center, but the two bigger "donut" (one grey colour and one green colour) is slightly offset or 2) The two bigger "donut" merge to one, but the smaller brightest white "donut" is off center. All this done without touching the focuser, just adjusting the lens cell comillation screws.

Currently, I adjust the collimation to 2 which I think is correct, but just wonder why the brightest smaller "donut" is not in the center... is it because the hole on the reflective surface in cheshire eyepiece is not round or not in the center??? Or should I adjust to collimation 2??

Hope those refractor expert can give some advice.

Thanks and have a nice day.
Yang Weixing
:mrgreen: "The universe is composed mainly of hydrogen and ignorance." :mrgreen:
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