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edge hd collimating dillema?

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#1 ray cohen

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Posted 16 February 2011 - 12:06 AM

in a previous post i stated that i attempted to install bobs knobs and was sold the wrong ones which were a standard thread.i now have the right ones which are a metric thread.i made the mistake of completely taking out the screws.so my question is,before attempting to collimate,how should the screws be put back,how tight should they be,and should the screws be screwed in evenly into the back of the secondary before collimating? :bawling:

#2 matty

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Posted 16 February 2011 - 08:07 AM

Did your secondary mirror fall out? I'm sure someone will chime in that has some experience with readjusting the secondary. For now, at least keep the scope pointed down somewhat to keep the secondary from hitting the primary.

#3 David Pavlich

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Posted 16 February 2011 - 08:42 AM

When I found collimation way out, I would pick a bright star, put in my 27 Panoptic and take it out of focus enough to see the secondary shadow. When it's that far out of collimation, the secondary shadow is off center. Center it and that will give you a rough collimation. From there, you can put in a higher power eyepiece and do a standard star collimation.

David

#4 Ed Whitney

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Posted 16 February 2011 - 09:26 AM

Ray, I hear you!

Thou I'm not at all sure if the secondary mirror will "fall" into the primary if all 3 screws are removed. I do know the sec mirror does sag or hang on the other two screws when removing one of them making it extremely hard to put another screw back in the hole. I had to hold the entire correcter plate assembly upside down to be able to thread in the replacement screw!

I changed out the 3 Phillip's screws and replaced them with 3, #3 x 1/2inch metric SS socket-head cap screws. Best thing I could have done. BUT, to do this job, removed the corrector plate and worked on it on my desk. I was afraid the secondary might come loose and fall out as well.

In the process, did clean both mirrors, but won't go into this now.

Here is how I re-collimated my 9.25 EdgeHD.

File: TIB-038 - SCT Collimation Basics.txt

Title: SCT Collimation Basics for the C-9.25 EdgeHD SCT
-------------------------------------------------------

Note: This procedure applies to the C-9.25 EdgeHD but can be used with any similar SCT.

This will help you determine which of the three collimation screws to turn to
achieve perfect collimation in the least amount of time.


1. Set up scope aimed at Polaris and cool down for 2 hours before starting.
(or use another star with your mounts tracking on)

2. With a TV 15mm Plossl, (156x) turn focus knob CW to make a defocused star image
OUTSIDE of focus. Then turn focus knob 1 turn CCW to "set" the mirror position as to
prevent any image shift.

The CCW turning PUSHES the mirror UP the tube into focus and holds it there.
A CW turn PULLS the mirror DOWN into focus, but due to the slop in the system,
will not stay at your desired focus. Failure to turn CCW into focus for collimation
will result in your forever "chasing" after the correct collimation!

The mirror is now in the correct viewing or defocused star collimation position.

3. Now, observe the defocused image. The dark center spot is the shadow of the
secondary mirror and should be centered in a perfectly collimated SCT using a
high power eyepiece.

4. Now, while observing a defocused star image that's centered in the eyepiece,
take one hand and place it on the top edge of the scope so your fingers show up in
the eyepiece image. Do not touch the corrector! Now slide your hand around the edge
of the tube to where the shadow of the secondary is closest to the edge of the
defocused star at the point of the thinnest part or skewed area of the defocused star.

Locate the collimation screw closest to your hand. This is the screw to adjust first.

5. Use the hand control buttons to move the de-focused star image to the edge of the
field of view, in the same direction that the central obstruction of the star image
is skewed or the thinnest part. In other words, put the edge of the part of the
skewed image at the edge of the FOV.

6. While looking through the eyepiece, use an Allen wrench to turn the collimation
screw in the direction that moves the defocused star image TOWARDS the center of
the FOV.

If the star image moves out of the field of view in the direction that the central
shadow is skewed, than you are turning the collimation screw the wrong way.

7. Continue this procedure until perfect collimation is achieved.

NOTE: These instructions work like a charm and are taken directly from Celestron procedure, etc.

PS - This is my first SCT and first collimation job ever done on an SCT. I found it much easier to work with an Allen wrench than a Phillip's screw driver. Also did notice the "tightness" of the 3 Phillip's screws BEFORE removing them so as to determine how tight the cap screws should be. I also do feel that when you DO achieve good collimation, that it will hold it very well. The 3 screws on my C-9.25 are now quite tight. You must judge for yourself how tight they must be.

GOOD LUCK! :)

#5 ray cohen

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Posted 16 February 2011 - 09:36 AM

the secondary did come completely loose,but did not fall or hit anything,i lined up the secondary mirror with the back of the secondary plate,and then put back the secondary lined up the set screw with the notch on the corrector and tightened the locking ring.thanks for your concern.

#6 ray cohen

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Posted 16 February 2011 - 09:50 AM

Ed,thank you for that post,but what do you mean by CW,and CCW. :foreheadslap:

#7 Eddgie

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Posted 16 February 2011 - 10:18 AM

Clockwise and Counter-Clockwise.

#8 ray cohen

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Posted 16 February 2011 - 10:31 AM

"DUH".didnt get much sleep lastnight!

#9 Ed Whitney

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Posted 16 February 2011 - 10:34 AM

Oh, Sorry!

"CW" is "clockwise" rotation and "CCW" is "counter-clockwise" when turning the screws as when you are behind the bolt/screw and turning it.

Your final TURN into focus or defocus when collimating, MUST BE "CCW". There should even be an "arrow" on your focus knob to remind you to do this. It's a funny little "arc" with two tiny circles at the end.

It's not enough to just defocus the star image. You would defocus it turning "CW" with say, 1 whole turn of the focus knob, and then go back "CCW" 1/4 turn actual rotation. This "sets" a positive upwards pressure against the mirror so it can't slip backwards.

It is vitally important to remember to turn the screws in the right direction when collimating.

#10 ray cohen

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Posted 16 February 2011 - 11:24 AM

Ed,the 11 inch OTA has mirror locks,do they come into play when collimating?

#11 Ed Whitney

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Posted 16 February 2011 - 11:40 AM

No, leave the two mirror locks loose a turn or two.

Essentially, the telescope is always pointing at an upwards angle so the mirror is resting on the focusing rod only. Using the locks would only add another frustration.

You know, some may argue this point, but no where in the collimating instructions do they say to lock the mirror.

They're only to lock the mirror in position for AP.

#12 ray cohen

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Posted 16 February 2011 - 02:34 PM

Ed,so are you saying ,for general observing without doing any AP the mirror locks should not be used,and left one turn loose?

#13 mike174

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Posted 16 February 2011 - 04:01 PM

Ray,

Focus in SCT's is achieved by moving the primary mirror up and down the baffle tube so the mirror must be unlocked while focusing. Once you reach critical focus I guess you could then lock the primary but you would have to remember to unlock it when you change eyepieces and have to refocus.

Mike

#14 Ed Whitney

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Posted 16 February 2011 - 04:58 PM

Ray,

Sorry for the delay, but had to get a tooth filled.

Yes, leave the locks loose. But, your last turn to bring a star into focus, should be CCW. For visual use it really doesn't matter at all which way you achieve focus. At higher powers, you will notice the image shift, quite a bit I might add. This is normal and demonstrates the fact that the mirror is "tilting" on you. That's why when collimating, it's critical to defocus the star pattern and make it bigger than you really want, turning CW. Then turn a small amount CCW to "fix" the mirror in the "correctly tilted" position for good collimation.

As the mirror rides up and down the baffle tube, it "slops" or tilts from side to side, but only a tiny unseen amount. This is why you must be consistent with which final direction the mirror is going, before it comes to a stop. This is only necessary for collimating purposes.

When using the scope, if you achieve sharp focus with a CW turn, the collimation is actually off a slight amount!

#15 gnowellsct

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Posted 18 February 2011 - 09:21 PM

http://www.astromart...?article_id=718

This article tells you how to bring a grossly misaligned SCT back into alignment. It's pretty easy.

Greg N

#16 Cliff Halliwell

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Posted 20 February 2011 - 12:05 AM

One can also get close initially just doing a check on the how concentric the various circles are in daylight. See the notes from Robin Cassady.
http://www.robincasa...ges/collim.html

#17 rmollise

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Posted 20 February 2011 - 10:15 AM

One can also get close initially just doing a check on the how concentric the various circles are in daylight. See the notes from Robin Cassady.
http://www.robincasa...ges/collim.html


This is in fact the easiest and most effective way to get a grossly miscollimated scope back in the ballpark so you can do a normal star-collimation. I've done it more than once for anguished newbies on a star party field. ;)

Just stand a couple of meters away and look down the tube, straight down the tube. Does everything look concentric? If not, adjust the screws till everything does, and then fine-tune on Polaris after nightfall.

#18 dobsoscope

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Posted 03 December 2012 - 09:38 AM

thanks very useful






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