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Best Collimation Tool for a CAT

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#1 LivingNDixie

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Posted 28 November 2005 - 10:25 AM

I have been trying all my tricks, and it just seems the scope isn't quite there. Its odd too, usually I'm happy with the images but it seems after a bump or something I can't get the scope perfectly collimated. So any ideas on a tool I should get?

#2 garyc11

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Posted 28 November 2005 - 12:58 PM

i have allways used a night of good seeing and polaris. no matter what collimation tools you buy they are not going to replace a star at over 300x.

#3 Psa19one

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Posted 28 November 2005 - 02:09 PM

In my (limited) experience and (not so limited) research, star collimation is the best tool for SCTs.

#4 LivingNDixie

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Posted 28 November 2005 - 02:11 PM

I'm begining to think its just been bad seeing that has been the culprit, I usually do the star thing too. But I was wondering if anyone used anything else.

#5 JerryWise

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Posted 28 November 2005 - 03:06 PM

Preston, I have used the look through the front method but that seems to only work well when you have had the corrector plate off for cleaning and you want to get things close. Also made a LED star. It works well but the best thing I have found is a star at zenith and a Nagler 3-6 zoom. Get things right at 6 then move to 3 and check. The only thing that really works for me is to slowly bring the star to focus and make sure all "spikes" fold into the star at the same time. Especially at the 3mm setting. If the spikes are dancing off to one side even slightly collimation is still off. If it still doesn't look right then I change diagonals and start playing with other EPs.

Just like Gary, I like Polaris too. Its low enough you can remove the diagonal and check collimation without needing a back brace later. Where I am it does dance more from seeing problems though.

#6 Derwin Skotch

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Posted 28 November 2005 - 07:43 PM

Make sure the scope is adequately cooled before checking collimation or making adjustments.

#7 snorkler

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Posted 28 November 2005 - 09:16 PM

I'm in the star test camp, too, with my SCT.

#8 garyc11

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Posted 28 November 2005 - 09:43 PM

do you have bob's knobs on your scope?

#9 Guest_**DONOTDELETE**_*

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Posted 29 November 2005 - 12:58 AM

Hi Preston,

Cooling can be an issue which can throw off the image. You already have an aluminum scope so that is great for cooling. A little more time having your scope sit out may clear things up.

It is best not to use a widefield like a Nagler or a Panoptic to collimate. You can do it but if you are just a bit off center it can be difficult.

I have used the Kendrick Laser collimator to a good degree of sucess. The manual has you go through a fairly tedious 1 time proceedure. Instead, I marked mine when my scope was new and it works great.

It's expensive for certain.

However, I find it easier to collimate outdoors with a star test and Bob's knobs.

The Kendrick's Laser collimator only gets used when I feel like collimating indoors during daytime or cloudynight time.

#10 Matt Looby

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Posted 29 November 2005 - 07:07 AM

Hi Preston,

I have had good luck with this adapter for a mag light
flashlight. Great for daylight collimation and the
price is right $9.


http://www.astromart...ified_id=387479


Thanks,

Matt

#11 LivingNDixie

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Posted 29 November 2005 - 12:25 PM

Piano,
tube currents aren't too bad usually, though worst in the winter since the temp keeps droping and the scope just can't keep up.

Jerry,
I have tweaked it the using the indoor method as described in the Meade manual before (even did it last night), I think I'm going to have to just try again tonight or tommorow night. I too have a obstructed view of polaris.

Thanks guys for the responses I appreciate it.

#12 Starman1

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Posted 29 November 2005 - 04:53 PM

Preston,
1. Use the daylight collimation method first: here.
2. Use an articial star, or a Christmas tree bulb on a tree 200-300" away--the sun's reflection looks like a star, and you're only looking through 200-300' of air, so seeing isn't an issue.
3. Buy/use and artificial star (available from Digitec and others).
4. Use Polaris (doesn't require the scope to be aligned and driving). I have over 50 websites bookmarked for SCT collimation. Type "Schmidt Cassegrain telescope collimation" into Google and read all the reports.
5. It's not necessary to use extremely high magnifications. The optimum is about 1X per mm of aperture. The only reason you use a power that high is to see the Airy disc when it's focused. With care, and good seeing, you can collimate the Airy disc into a round shape. Normally, seeing the Fresnel rings and Poisson's point all concentric is close enough.
Best to you,
Don

#13 NeoDinian

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Posted 29 November 2005 - 05:14 PM

If possible, you should Collimate WITHOUT a diagonal in place. You may need to add an extension tube to reach focus though.

#14 Rusty

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Posted 29 November 2005 - 10:24 PM

I'm of the opinion that Bob's Knobs are the most useful collimation tool - since they make collimation easy, regardless of the light source.

#15 Nocturnal

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Posted 30 November 2005 - 01:23 PM

Well I had an odd experience last night. I have a 30 year old Super C8+ that appears to be in great shape looks new except for some minor spots on the primary) but has an unknown maintenance history. So reading all this stuff about people collimating 5 times a day I figured my collimation must be terrible. Yet every time I check the collimation with a star the focus circles are nice and concentric.

So when I saw that link to a daylight collimation technique I figured I'd give that a try. It was overcast anyway. And whadayanno, the scope was quite a bit out of collimation according to this technique. Fixed that up pretty quickly and took the scope outside. There were some breaks in the clouds and Polaris was easily found. Well, found after I put a wide angle eyepiece on because the finder scope was no longer aligned with the scope. Forgot about that little detail, doh!

So this is where it gets interesting. The star image was waaay out of collimation. I mean terrible. True text book example of rings overlapping each other and moving to the side when near focus and the out of focus ring was way distorted as well. So I recollimated it using Polaris.

Any ideas what could have happened?

#16 jrcrilly

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Posted 30 November 2005 - 01:27 PM

Any ideas what could have happened?


That points out one disadvantage of daytime colimation. The mirror location on the baffle tube is different for the focus point for a near object compared to that when focused for infinity. Any shift in the mirror tilt between the two positions will result in collimation shift.

#17 Guest_**DONOTDELETE**_*

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Posted 30 November 2005 - 03:11 PM

I also think real star collimation is the best thing to do.
But for lazy people (like me) an artificial star is also a good alternative to do your collimation during daytime. For this daylight collimation procedure A proper build artificial star is a must!
Bonzo

#18 Nocturnal

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Posted 30 November 2005 - 05:10 PM

It sounds like an artificial star daylight collimation would be preferable over the daylight procedure that doesn't use one. The result I got was so startingly bad that I'm wondering if anyone else ever really used it with success. It may work for some scopes but there must be something different mechanically about C8s that prevents it from working right.

#19 Nocturnal

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Posted 30 November 2005 - 08:13 PM

Hey Bonzo,

how would you define a 'properly built artifical star'? Is a shiny ball in the sun enough?

I checked out your site, very nice. Especially the little jabs at the Leiters from Berlin. Haha. I've had similar experiences. Much more of a class hierarchy than we're used to in Holland or the US.

#20 Starman1

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Posted 30 November 2005 - 11:19 PM

It sounds like an artificial star daylight collimation would be preferable over the daylight procedure that doesn't use one. The result I got was so startingly bad that I'm wondering if anyone else ever really used it with success. It may work for some scopes but there must be something different mechanically about C8s that prevents it from working right.

When I used the daylight technique by using a file card (it had a 1/8" hole punched in it) taped to the top of a chair so I could hold my head steady enough, I got great results. Under the sky, the total extra collimation necessary was 1/32 turn of one screw to make collimation perfect.
What is the case on some SCTs, though, is that the secondary mirror is not perfectly centered in the secondary housing. By using the daylight collimation technique, you make the secondary reflection of all internal circular images concentric, but this does not result in the image being centered in the baffle tube or final focal plane. Because of the 5X magnification of the secondary, a very slight offset from center will result in a huge difference at the focal plane.
The daylight collimation technique (looking in the front of the scope) is merely a starting point when the scope is reassembled or after the installation of Bob's Knobs.
It's better to use an ornament in a tree several hundred feet away.

#21 LivingNDixie

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Posted 01 December 2005 - 06:42 PM

How far does the chair have to be?

#22 Starman1

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Posted 01 December 2005 - 11:26 PM

I put the chair at the correct distance to make the outline of the secondary only slightly smaller than the reflected image of the secondary. On my 8" SCT, that was, oh, 6-8 feet.

#23 Guest_**DONOTDELETE**_*

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Posted 02 December 2005 - 04:22 PM

Hey Bonzo,

how would you define a 'properly built artificial star'? Is a shiny ball in the sun enough?



Oeps, that wasn’t the intention ....... :foreheadslap: :lol:

See the next link for some pricy one
Artificial star

But there are cheaper way's to lay your hand on one.
A friend of mine made one.
Simply said he punched a hole in some aluminium foil and build a high tension LED behind it. The complete battery powered box can be mounted on a small tripod.
But don’t make mistakes! This is more difficult than it looks.
The pinhole really needs to be a pinhole! You better stack more layers of aluminium tape and punch them carefully with a very fine needle! Only use the bottom layer hole! This will be the smallest

To build one yourself just hit the next link
Build one yourself

I'll hope this will do

Bonzo

#24 LivingNDixie

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Posted 03 December 2005 - 01:00 PM

OK I got the collimation alot better (pratically perfect), I think my problem was the seeing the nights before. Thanks for all the imput in the thread!

#25 BWDenver

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Posted 04 December 2005 - 06:10 PM

Preston,

I found during my shooting sessions of Mars the star collimation was close - but not on the money.

Prior to each session I hook up my WEB cam and a 5X Powermate. Then I focus on the planet and look for a telltale ghost under one portion of the target. Then I very carefully adjust the collimation screws to get rid of the ghost. Adjustments have to be very small as the target will move out of the field of view. It’s best to set up the computer so you can see the effect of each adjustment as you make it. Then if you need to bring the scope back on target, you know where it went.

I’ve also done several sessions where I throw the scope out of collimation then see what it takes to get it back in. that way you get a better idea just how much and in what direction you need to moves the screws.

Regards,

Bryan
Denver


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