Refractor collimation

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bwh
Posts: 189
Joined: 06 Feb 2007, 23:08
Location: Dalgety Bay

Refractor collimation

Postby bwh » 21 Oct 2008, 19:22

anyone got a chesire to collimate a refractor ?

could someone bring one to a meeting ?

regards
brain
bwh

Meade 8inch (228mm) LX5 Smidt-Cassegrain fl=2000
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bwh
Posts: 189
Joined: 06 Feb 2007, 23:08
Location: Dalgety Bay

Postby bwh » 23 Oct 2008, 17:18

paul lent me his laser collimator...... thanks

So refractor now collimated - nice and easy. Lesson dont clean it too often. But i found that my focuser needs to be locked to get best alignment - did not know that doh!

Now using laser on me SCT - which different kettle of fish, and i dont get single spot on the return - more a red hase..... does this say anything about my secondary mirror (is it dirty???) as placing laser in eyetube only points at secondary.

Help Moi!
bwh



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astrogeorge
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Postby astrogeorge » 23 Oct 2008, 21:09

From what i've read on collomating an sct, its not recomended to use  a laser collomater. This could be the reason for your problems.
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Cosmic
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Collimation

Postby Cosmic » 24 Oct 2008, 09:49

Yep george is right

Laser for SCT   = Bad
laser for newt  = Good.

and not forgetting Laser for eye = VERRRRRRY BAD


Cosmic
Meade 8inch  SCT. Observatory mounted on an HEQ5 Pro using EQMOD Canon 1000D modified DSLR,200mm Dobsonian, 200mm Celestron C8, 15x70 Astro Binoculars. 2x Mintron ccd cameras, lots of other stuff.

Cosmic
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Postby Cosmic » 24 Oct 2008, 09:56

Hi Brian found this for you hope it helps:

Before collimating, a brief explanation of SCT optics is required. Commercially made SCTs all have small errors in optical alignment, figure and mechanical alignment. Typically the optical error is in the centering of the secondary mirror. Consequently, when a laser is placed into the back of a Schmidt Cassegrain Telescope the beam will seldom return dead center, even though star testing would indicate perfect collimation. The secondary mirror of an SCT magnifies any error by about 5 times. Typically, a well collimated SCT will return a laser beam .125" to .250" off center.

Due to the slight alignment errors of SCT optics and mechanics, it is not possible to use a Laser collimator in the same manner as it would be used with a Newtonian telescope. Regardless, very accurate collimation of an SCT is easily accomplished using a laser if the following procedures are accurately followed. This is a one time process and after this the laser may be used exclusively to bring your SCT quickly and easily into accurate optical alignment.


Some Do's and Don'ts

Only try to collimate on solid ground, concrete or asphalt. NEVER try to collimate on wooden floors, carpeted floors or any other surface that will flex or vibrate.

NEVER make adjustments to your secondary mirror or to the laser placement on the paper target that are greater than 50% of the observable error. In fact, we recommend making adjustments that are within 25% to 40% (described in detail below).

Only use the provided SCT Adapter to hold the laser and eyepiece. NEVER remove the adapter during the collimation process or involve other accessories that are not described in the process.

When focusing the laser onto the paper target, turn your focuser SLOWLY. The beam will come into and go out of focus quickly and it is easy to miss if you are focusing quickly. The beam is not an intense red beam. It is fairly easy to see but do not expect to see the usual type of brightness associated with a laser.

If you cannot resolve the laser on the target you may either be focusing too quickly, too close to the target or it may be too bright outside. Either slow down your focusing, move your target farther away or wait until the evening to collimate when it is darker outside.

Collimating...
Tape the Paper Target that came with your collimator up on a wall at a distance that corresponds to slightly more than your telescope's close focus. "Close focus" is the point of closest focus you can use on your telescope. Check the measurements indicated on the piece of paper that the target is found on with your instructions to determine close focus for your telescope. In order to address the common problem of mirror shift in SCTs, it is always advisable to turn the focuser clockwise after any counter clockwise adjustments. This will set the mirror at a common point when taking the scope in and out of focus during the collimation process.

It is necessary to attach the SCT adapter first and with the use of a 2" to 1.25" adapter (included with most 2" focusers or available through Kendrick Astro Instruments) insert your eyepiece and then center the target. IMPORTANT!! You must connect the laser and the 2" to 1.25" adapter (with the eyepiece inserted into it) with the included 6" steel fishing leader. The top of the laser has a small hole in it through which the hook of the leader may be placed. The other end of the leader must be secured to the thumbscrew of the 2" to 1.25" adapter. Having these items connected in this manner will take up any slack in the declinaton clutch brought about by the weight of the laser and eyepiece. The weight must be constant. This is CRUCIAL for good collimation. Take a look at an SCT Connections Diagram for further clarification.

Focus on and center the paper target visually in the eyepiece. Start with a low power eyepiece so that the target may be located easily. Then go to a high power eyepiece or use a guiding eyepiece and center the target in this eyepiece. If you are using a guiding eyepiece then simply put the crosshairs of the paper target in the crosshairs of the eyepiece. If your eyepiece does not have cross hairs then use the circles on the target as a visual reference against the edge of the field of view within the eyepiece. Leaving the telescope directed at the target, and being careful not to bump the scope out of alignment from the target, insert your laser into the adapter.

Turn the laser on and using your binoculars, a small telescope or your friend, look at the target. You may notice that the target pattern now has a diffuse ball of light around it. If you do not, turn your focuser clockwise (usually about 4.5 turns) until you focus the laser down to a ball of light that is about 10mm in diameter. This ball of light is being projected onto the target by the primary mirror of the SCT and is following the precise optical path that your telescope is currently collimated on. This light is the extraneous scattered light that surrounds all laser beams. It is being bounced off the secondary mirror onto the primary mirror and out through the front of the telescope while the main beam of the laser is being directed back to the face of the collimator. The ball of light on the paper target will be quite obvious. If it is not obvious and you have tried to focus then you are still too close to the target. You will need to move your telescope farther back from the target and start again.

Ideally, the ball of light should be exactly centered on the paper target. If it is not, adjust the secondary mirror to compensate for NO MORE than 50% of the error. In other words, if you can see that the ball of light is .5" off center, then make an adjustment that is no more than 50% of .5" towards center, or .250". This previous point is extremely important and is the primary reason people have difficulty collimating using a laser. If you keep your adjustments to a maximum of 50%, and preferably 25% to 40% you will do fine. Use your friend or your binoculars to check your adjustments. This procedure is very accurate, and very tight collimation can be achieved if you are careful. Click here to see a visual description of this procedure.

Having made your first adjustment to the secondary mirror, remove the laser and insert the eyepiece again (of course, as you remember, the eyepiece is in its 2" to 1.25" adapter and is secured to the laser by a length of string or thin wire). Look through the eyepiece at focus on the paper target. You will now observe that the target is no longer in the center of the eyepiece. Using your fine adjustment knobs for Declination and Right Ascension, make adjustments that will compensate for NO MORE than 50% of the difference of error you observe through the eyepiece. In other words, if the target appears to be .250" off center when looking through the eyepiece, then make adjustments that will compensate for no more than .125". DO NOT PLACE THE TARGET DEAD CENTER. As I have described in the previous paragraph, adjustments that are greater than 50% of the observable error will decollimate your telescope.

Remove the eyepiece and put the laser back into the SCT adapter. Focus the laser light on to the paper target. Again, adjust for NO MORE than 50% of the observable error from dead center. Remove the laser and repeat the process with the eyepiece. Keep doing this until no more adjustments can be made. You will know when you have reached this point as you will begin to notice that adjustments now seem to be larger instead of smaller.

It is likely that you will never get the laser to be exactly dead center on the target. This is due to minor optical and mechanical axis errors inherent within the scope.

Once you are satisfied that you have centered the ball of light on the target accurately, check the face of the laser collimator located in the adaptor on the back of your telescope. Look for the return beam on the face of the collimator. It will be a blob of light about 5mm in diameter. Stick the small paper label included with your SCT collimator onto the face of the laser where this beam returns to. This will resolve the beam to a small red dot or black circle surrounded by a couple of diffraction rings. Mark the central spot within the diffraction rings. This spot is where your will collimate to in all further collimation procedures. The laser may now be used exclusively to collimate your SCT without having to go back to any of the above procedures.

The advantages of this process is that you are collimating at at least 5 times the focal length normally used when collimating using the single star method. Consequently, collimation is at least 4 to 5 times more accurate. The farther you put the target away, the more accurate it will be as you are increasing the F ratio of the collimation beam.



Seems a lot of messing around needed

Regards
Cosmic
Meade 8inch  SCT. Observatory mounted on an HEQ5 Pro using EQMOD Canon 1000D modified DSLR,200mm Dobsonian, 200mm Celestron C8, 15x70 Astro Binoculars. 2x Mintron ccd cameras, lots of other stuff.

bwh
Posts: 189
Joined: 06 Feb 2007, 23:08
Location: Dalgety Bay

Postby bwh » 24 Oct 2008, 17:12

cosmic - thanks i had found that article too - and it seems to suggest that you can use the laser to collimat SCT 5 times (5!!!) better than star test.

Have tried it - pinning target 25 feet away from static scope and the laser (WARNING - not for kids or looking at as very dangerous) - and could more easily collimat the SCT (3 screws on secondary) as stationary target and no like a star which moves.....

Will await next clear night - to confirm if SCT now collimated.

I think laser can more easily collimate Newtonian as beam hits secondary then to primary, back to secondary and back to target. So you collimating 2 elements precisely.

In SCT laser hits secondary and comes back to target - so in effect main use is to align secondary to a fixed reference point (spot on target where secondary in collimation with primary).

took a while on internet as far to many articles are sales pitches for laser collimators -rather than how to use it.

b
bwh



Meade 8inch (228mm) LX5 Smidt-Cassegrain fl=2000

Skywatcher 5inch (120mm) Refractor fl=1000m

Meade DSI Pro

Canon 300D

Toucam Pro II 840k

Cosmic
Posts: 364
Joined: 12 Jun 2008, 11:39
Location: Dunfermline
Contact:

Postby Cosmic » 24 Oct 2008, 20:04

Hi Brian

Glad it was of use to you. I have ordered a laser collimator and will be interested if
the hype about my scope was true that the collimation is not affected by extending
and retracting tube. I find this very hard to believe.

perhaps we could do a short demo on collimation for those that are interested at
one of meets. I would be interested in a live demo at the star party. Should be easy on the Fly with a laser collimator.

cheers
Cosmic
Meade 8inch  SCT. Observatory mounted on an HEQ5 Pro using EQMOD Canon 1000D modified DSLR,200mm Dobsonian, 200mm Celestron C8, 15x70 Astro Binoculars. 2x Mintron ccd cameras, lots of other stuff.

Cosmic
Posts: 364
Joined: 12 Jun 2008, 11:39
Location: Dunfermline
Contact:

Collimation

Postby Cosmic » 05 Nov 2008, 10:13

Collimated my 12" Dob last night. Used Laser Collimator then found
that when I went to take the collimator out it twisted round a qurter of a turn and was 2" off the sweet spot.
Checked with my Cheshire collimator and it was way off.
It's getting a bit much when it seems I will have to collimate the Laser collimator. :shock:

Oh well thank goodness for good old cheshire.

Cosmic
Meade 8inch  SCT. Observatory mounted on an HEQ5 Pro using EQMOD Canon 1000D modified DSLR,200mm Dobsonian, 200mm Celestron C8, 15x70 Astro Binoculars. 2x Mintron ccd cameras, lots of other stuff.


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