Simple collimation question (yes or no answer)

[Graffy]

I know collimation has been done to death, and I've spent hours reading probably hundreds of threads, but I haven't managed to find a simple answer. Every discussion seems to end up with people going into infinite detail on the intricacies of individual pieces of equipment and explaining why every other bit of equipment is useless. I know there's always going to be a better solution but I just want something that's going to give reliable, repeatable, good collimation.

Anyway, my question is: if I buy a simple chesire collimator along with a (separate) laser collimator (I'm thinking a Hotech), will that cover all my collimation needs? I have a 12" Skywatcher collapsible dob. I looked at barlowed laser collimators but as far as I can tell, as long as I use the chesire to ensure the secondary mirror is set up correctly, the self centering mechanism in the Hotech negates the need for a barlow...

Happy with a yes or no answer. If you want to go into more detail, feel free. Thanks everyone

[Shark Bait (Stu)]

In my opinion, for visual use....Yes!

[Steffen]

Yes.

I'd go as far as saying Cheshire for positioning the secondary, then star test for titling the primary But a well-collimated laser is very convenient.

The Barlowed lasers don't just remove errors due to off-centre holding of the laser but also due to crooked focusers, drooping draw tubes etc. But if you're sure your focuser is square any accurate laser will do.

Cheers
Steffen.

[Robh (Rob)]

Hi Graffy

It is impossible to achieve perfect collimation. The secondary mirror is approximately centred on the spider vanes and then as seen in the focuser draw tube. The secondary mirror minor axis is then roughly aligned at right angles to the focuser draw tube axis, which itself is roughly at right angles to the scope axis. Added to this, it is assumed the primary mirror is exactly centred to the scope tube.

Use the laser collimator as a rough alignment then the cheshire collimator to finish off. Often lasers are themselves only approximately collimated. Or because of some slack when in the focuser, will indicate a false off-centre alignment. Rotate it when in the focuser to see if the laser beam stays to a fixed point or inscribes a circle.

A cheshire collimator will, as your eye can see, return focus to centre of the eyepiece and essentially create an alignment which adjusts for all the other approximations. It is very forgiving and give you about as good as you're going to get.

Regards, Rob

[Graffy]

Thanks guys for the simple replies, that's exactly what I was after! Steffen, you've got me wondering whether maybe I should go the barlowed collimator, probably doesn't hurt to have the extra peace of mind.

At least none of you have told me I need to get a purpose-made, custom-built apparatus designed by NASA which can only be operated by an astrophysicist... that's the impression a lot of other forums give!

[asimov (John)]

Visual, yes. Imaging, nope.

[2stroke (Jay)]

I would say yes, as for imaging i would say yes as well, lol unless your going for image of the year which you won't be doing with a 12" dob. As an beginner amateur i would say its a fine setup for F5+ ratios for imaging, If you had a F4.5 or lower well i would go with better means. Have you looked at a Autocollimator http://www.catseyecollimation.com/index.html ?

[Steffen]

Originally Posted by Graffy

you've got me wondering whether maybe I should go the barlowed collimator

You could get the TuBlug, which is a short tube with Barlow lens and 45º screen. Bintel have them. Simply insert your existing laser collimator.

Cheers
Steffen.

[Jason D (Jason)]

Originally Posted by Graffy

Anyway, my question is: if I buy a simple chesire collimator along with a (separate) laser collimator (I'm thinking a Hotech), will that cover all my collimation needs?

You do not need the laser. A cheshire/sight-tube will suffice.

Originally Posted by Steffen

The Barlowed lasers don't just remove errors due to off-centre holding of the laser but also due to crooked focusers, drooping draw tubes etc.

Bear in mind that barlowed laser can only be used to align the primary mirror.

Originally Posted by Robh

It is impossible to achieve perfect collimation. The secondary mirror is approximately centred on the spider vanes and then as seen in the focuser draw tube. The secondary mirror minor axis is then roughly aligned at right angles to the focuser draw tube axis, which itself is roughly at right angles to the scope axis. Added to this, it is assumed the primary mirror is exactly centred to the scope tub

It is possible to achieve perfect collimation. Collimation is about aligning optics. Whether the secondary mirror or the primary mirror are well-centered in the OTA or not is irrelavent.

[Steffen]

Originally Posted by Jason D

Bear in mind that barlowed laser can only be used to align the primary mirror.

Indeed.

It is possible to achieve perfect collimation. Collimation is about aligning optics. Whether the secondary mirror or the primary mirror are well-centered in the OTA or not is irrelavent.

I tend to agree. You can always achieve perfect collimation by looking at star images in and out of focus and adjusting the mirrors to get as close to "textbook" as possible (leaving only mirror defects). The only problem is that this is very inconvenient if you can't be at the eyepiece and the collimation screws at the same time. Hence the multitude of collimation aids.

I'm often fantasising about servo motors at the primary collimation screws and a little hand-held pad for adjusting them while looking through the eyepiece. I'm afraid I'm not handy enough to make that myself – why is there no commercial solution for this?

Cheers
Steffen.

[PS19.1 (Greg)]

There's a really good article in the Feb/Mar edition of Aust Sky & Telescope on easy collimation and was a reminder to me to keep it simple.Easy to suffer from information overload on this subject.

[Jason D (Jason)]

Originally Posted by Steffen

You can always achieve perfect collimation by looking at star images in and out of focus and adjusting the mirrors to get as close to "textbook" as possible

Collimation consists of three different alignments:
1- Centering/rounding the secondary mirror under the focuser for optimal illumination
2- Eliminating any tilt between the primary/EP focal planes
3- Eliminating any shift between the primary/EP focal planes
The first two alignments are carried out by the secondary mirror and the last by the primary. Star collimation only takes care of the 3rd alignment. Granted is it the most critical alignment but you can’t achieve perfect collimation without perfecting all 3 alignments. Besides, star collimation is not as perfect as collimation achieved by quality collimation tools though it does bring the scope “within” collimation tolerance.

I'm often fantasising about servo motors at the primary collimation screws and a little hand-held pad for adjusting them while looking through the eyepiece. I'm afraid I'm not handy enough to make that myself – why is there no commercial solution for this?

Apparently someone holds a patent for it
http://www.google.com/patents/US20110116181
But I do not think it is a practical solution.

Jason

[Jason D (Jason)]

Originally Posted by PS19.1

There's a really good article in the Feb/Mar edition of Aust Sky & Telescope on easy collimation and was a reminder to me to keep it simple.Easy to suffer from information overload on this subject.

You can use simple steps and low cost collimation tools to bring collimation within acceptable tolerance and have great views at the eyepiece but good collimation knowledge is required.
On the other hand, with more expensive collimation tools and great collimation knowledge you can perfect your collimation.
Perfecting collimation allows your scope mechanical imperfections to use whatever collimation tolerance your scope has. That is, starting with perfect collimation the scope will remain within tolerance as temperature change and is you move the OTA around.
Everyone should make their own decision on what collimation tools to buy and which collimation method to follow. After all, this is only a hobby.
Jason

[Graffy]

Some interesting (and different) points of view! Good food for thought...

Another question to throw into the mix: my scope has adapters for 1.25" and 2" EPs. When using a laser collimator, should I collimate based on the size of the eyepiece I'm planning on using, or can I just use one size for everything? (ie. use a 2" laser collimator and then switch between 1.25" and 2" EPs during the evening.)

Thanks guys...

[Jason D (Jason)]

I do not believe it is worth it to have both a 2" and a 1.25" laser collimators. Just select one of the two depending of your EP usage. If you use 1.25" EPs more then get a 1.25" laser collimator and vice versa. If you are still undecided, just flip a coin. You can't go wrong with either but do ensure that you get a quality laser collimator. I have a Glatter which is the best commercially available laser collimator.
Jason