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I hit a brick wall collimating my Cat

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#1 Ballyhoo

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Posted 19 June 2019 - 12:22 PM

I think I understand the general concept, but no matter what I did last night, I could not get the secondary shadow to move from the left side of the de-focused star.  I put a pencil at the thin side, and I used the opposite two knobs to loosen, so I could tighten up the knob on the thin side. And I tried the reverse. But really, no matter what I did, I could not change where the secondary shadow was, except to make it worse.  What happens when we hit a brick wall collimating these?  I should have been able to solve this last night but I could not. 



#2 macdonjh

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Posted 19 June 2019 - 12:41 PM

Ballyhoo,

 

My guess is your scope's collimation is pretty far off, and the collimation screws are pretty tight.  With an 8" SCT you may be able to reach the collimation screws while you're looking through the eye piece.  I'd "start over" with collimation.  By that I mean loosening all three collimation screws quite a bit.  I would start by aiming your scope at your favorite star for collimation, then start loosening the collimation screws.  Loosen the screws a little at a time while you're looking through your eye piece and keeping your target star centered.  Don't loosen the screws so much your secondary mirror falls out.  When you think your collimation screws are loose enough, adjust your finder scope (so you can find your target star if you lose it in the main scope).  

 

Now you have lots of adjustment range in all three screws so you can begin collilmating again.  This is what I had to do with my C11: it's secondary mirror was "locked down" tight.  A friend helped me with it the first time.  It only took us about ten minutes to get my scope collimated as well as seeing that night would allow.  Over the next several months I tweaked collimation every night I thought seeing was better than the last time I collimated.  I only spent a couple of minutes tweaking (or not), then got on with observing.  Eventually I got collimation dialed in pretty well.  Once collimated, most SCTs hold collimation well so you shouldn't have to do it again.

 

This is a really tedious process if you can't reach the collimation screws while you're looking through the eye piece.  If that is the case, do you have a friend that can help you?  The process will go ten times faster with help than without.  


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#3 Ballyhoo

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Posted 19 June 2019 - 12:59 PM

Ballyhoo,

 

My guess is your scope's collimation is pretty far off, and the collimation screws are pretty tight.  With an 8" SCT you may be able to reach the collimation screws while you're looking through the eye piece.  I'd "start over" with collimation.  By that I mean loosening all three collimation screws quite a bit.  I would start by aiming your scope at your favorite star for collimation, then start loosening the collimation screws.  Loosen the screws a little at a time while you're looking through your eye piece and keeping your target star centered.  Don't loosen the screws so much your secondary mirror falls out.  When you think your collimation screws are loose enough, adjust your finder scope (so you can find your target star if you lose it in the main scope).  

 

Now you have lots of adjustment range in all three screws so you can begin collilmating again.  This is what I had to do with my C11: it's secondary mirror was "locked down" tight.  A friend helped me with it the first time.  It only took us about ten minutes to get my scope collimated as well as seeing that night would allow.  Over the next several months I tweaked collimation every night I thought seeing was better than the last time I collimated.  I only spent a couple of minutes tweaking (or not), then got on with observing.  Eventually I got collimation dialed in pretty well.  Once collimated, most SCTs hold collimation well so you shouldn't have to do it again.

 

This is a really tedious process if you can't reach the collimation screws while you're looking through the eye piece.  If that is the case, do you have a friend that can help you?  The process will go ten times faster with help than without.  

thanks I use an artifical star in daylight.   I will try it like this


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#4 TrustyChords

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Posted 19 June 2019 - 01:00 PM

A similar thing happened to me the first time I collimated my SCT. And then I overreacted and got it really screwed up. Not to worry, you can always get it back. Don't Panic. You don't want to do anything silly like drop a screw or screwdriver on the corrector plate.

 

Here's the most important part (the part I missed, despite everyone saying it): Always center the star after each adjustment. I somehow missed that at first, and it set me back. With that advice, collimation should be a breeze.

 

Also, I would hesitate doing anything other than very minor adjustments until you have tried the above. However if you do get it very very off centered, a  good tip I found here is: "Stand back about 8 feet back from the scope during the day with the business end level with your eyesight.  You should see concentric circles." This will ballpark you pretty close enough to resume fine adjustments through the eyepiece (or on screen if using a camera).

 

Here's a great link that bailed me out:

https://www.cloudyni...help/?p=7570881

 

Good luck!


Edited by TrustyChords, 19 June 2019 - 01:03 PM.

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#5 Migwan

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Posted 19 June 2019 - 01:05 PM

Agree with loosening and starting over.  Was going to suggest doing in daylight, then you posted again.

 

You can also stand with your eye squarely in front of the OTA and look into to check if everything is concentric.  

 

Last year I helped a friend with an old C8 and we had so much difficulty that I finally pulled the corrector off and just adjusted the secondary till it was parallel with the base plate.  Man was that sucker off.  Was an easy job collimating after that.

 

jd


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#6 Ballyhoo

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Posted 19 June 2019 - 01:45 PM

A similar thing happened to me the first time I collimated my SCT. And then I overreacted and got it really screwed up. Not to worry, you can always get it back. Don't Panic. You don't want to do anything silly like drop a screw or screwdriver on the corrector plate.

 

Here's the most important part (the part I missed, despite everyone saying it): Always center the star after each adjustment. I somehow missed that at first, and it set me back. With that advice, collimation should be a breeze.

 

Also, I would hesitate doing anything other than very minor adjustments until you have tried the above. However if you do get it very very off centered, a  good tip I found here is: "Stand back about 8 feet back from the scope during the day with the business end level with your eyesight.  You should see concentric circles." This will ballpark you pretty close enough to resume fine adjustments through the eyepiece (or on screen if using a camera).

 

Here's a great link that bailed me out:

https://www.cloudyni...help/?p=7570881

 

Good luck!

I think I got it!

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Edited by Ballyhoo, 19 June 2019 - 01:47 PM.

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#7 barrett_flansburg

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Posted 19 June 2019 - 01:55 PM

Did the star move in the field of view when you turned the collimation screws? It should - you want to drive the star in the direction of the fat side star. Recentering after each change, of course. 

 

Also,  I believe some SCTs have screws in addition to the collimation screws on the secondary holder. The others hold the secondary to the corrector plate. If so, make sure you are turning the right screws. 



#8 ToxMan

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Posted 19 June 2019 - 02:09 PM

I unscrewed the secondary locking ring and pulled out my secondary to see what was going on with mine during a similar experience. Everything was definitely too tight and there was insufficient travel left to accomplish anything. Backed out all the screws evenly and put secondary back into its position. Things went very easy after that little check.


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#9 carolinaskies

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Posted 20 June 2019 - 06:37 AM

If you do astrophotography with camera/laptop simply turn on Sharpcap and use it to constantly take images and you can stand at the front of the telescope and make adjustments with the laptop screen faced toward you while making adjustments.  If you have a DSLR with a flip screen you may be able to use it if the screen rotates 180* or hook it's output to a larger screen for adjustments.  


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#10 Migwan

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Posted 20 June 2019 - 08:06 AM

I think I got it!

Looks good.   How did you get 5 images in the pics?  Pretty cool!

 

To get the best possible performance wait for that great night of seeing, be sure your scope is acclimatized and use as high a power as you can.    Barely go out of focus, maybe even less than the bottom image of the second pic.  Basically, as close to focus as you can get a poisson point.

 

Then check at around 45 and 90*.   My 1100 has a touch of mirror flop that I can't really see till I approach 350x.  My final collimation is at 460x (though I check it even higher than that).   I am happy when there is virtually no difference in slightly out of focus tests between the two positions.  My 2080 has more flop and I just do final collimation at around 75* on it.   

 

Once you get there, leave it alone.  Check, but don't touch unless its really out of whack.  Better to wait for a great night at high power. 

 

jd


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#11 Bill Barlow

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Posted 20 June 2019 - 09:56 AM

It looks pretty good to me.  I would slowly defocus the stars airy disc to see how the first diffraction ring breaks out.  If it breaks out evenly, then you are done.

 

Bill



#12 WadeH237

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Posted 20 June 2019 - 12:54 PM

How did you get 5 images in the pics?  Pretty cool!

He's using an artificial star that presents 5 points of light at different brightness levels.  It's probably something like this.


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#13 Migwan

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Posted 20 June 2019 - 08:09 PM

Thanks Wade.  Never saw one like this before.  I have a single hole artificial star I got with my 2080.  It's pretty worthless.  This one makes sense.   jd 



#14 Ballyhoo

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Posted 20 June 2019 - 09:09 PM

He's using an artificial star that presents 5 points of light at different brightness levels.  It's probably something like this.

yeah I was lucky to have an artificial star and that really saved me here. It was the first time I ever used it. Had grabbed it out of my box of stuff I have. I was looking at it before, really wondering why it has five little lights. I set up my SCT in my little office downstairs, through two doors, out the laundry room, into my garage.  It worked great. I think two stars at the bottom were distorted from when I cropped the pic.  

 

 

edit

 

as luck would have it, last night I got everything set up --excited to try it out. And guess what, the clouds rolled in. June gloom is real. 

 

 

Edit, also, last time I used my scope, like four nights ago, it was way out of collimation. Of course DSO's were horrible.  But I was not too happy, or unhappy with my view of the moon. Does anyone know how collimation effects the moon?

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Edited by Ballyhoo, 20 June 2019 - 09:19 PM.


#15 jjack's

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Posted 21 June 2019 - 02:59 AM

And take care everybody about the diagonal.  Some are far off collimation.


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#16 jjack's

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Posted 21 June 2019 - 03:04 AM

My first 99% dielectric diagonal was 1) obviously not 99% reflectivity 2) collimation was good in my SCT without it and off with it.



#17 WadeH237

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Posted 21 June 2019 - 07:05 AM

Does anyone know how collimation effects the moon?

Miscollimation affects fine detail and harms contrast.  The greater the miscollimation, the greater the damage to the image.

 

With stars and planets, all of the details are small, so a small miscollimation is noticeable.  With planets, the contrast is subtle and even a very slight miscollimation can harm the views.  With diffuse DSOs, miscollimation harms the image, but since your eye generally doesn't see the fine details, it can be harder to notice the damage to the view with a slight miscollimation, but it's there.

 

The moon is unique.  It has very high brightness, very high contrast, and tons of detail at many image scales.  A miscollimation will reduce contrast and wipe out fine detail.  But since there is so much contrast and details at medium and large image scales, your eye will find something to see.  So even though the view might look decent, it will be missing the potential sharpness and fine detail that would be there if collimation was spot on.

 

Note that I am talking about fairly small levels of miscollimation here.  A large miscollimation will prevent the telescope from reaching anything that looks like a sharp focus.

 

For what it's worth, my observation over the years has been that many people with SCTs are using their instruments with slight collimation errors, not realizing that they are not reaching full potential that they can deliver.


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#18 macdonjh

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Posted 21 June 2019 - 07:17 AM

 

For what it's worth, my observation over the years has been that many people with SCTs are using their instruments with slight collimation errors, not realizing that they are not reaching full potential that they can deliver.

 

But, "be careful what you wish for", "ignorance is bliss" and all those cliches.  Once you view through an SCT that is well collimated, you won't tolerate poor collimation.  I guess it's a good thing SCTs are relatively easy to collimate.


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#19 Ballyhoo

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Posted 21 June 2019 - 11:46 AM

Miscollimation affects fine detail and harms contrast.  The greater the miscollimation, the greater the damage to the image.

 

With stars and planets, all of the details are small, so a small miscollimation is noticeable.  With planets, the contrast is subtle and even a very slight miscollimation can harm the views.  With diffuse DSOs, miscollimation harms the image, but since your eye generally doesn't see the fine details, it can be harder to notice the damage to the view with a slight miscollimation, but it's there.

 

The moon is unique.  It has very high brightness, very high contrast, and tons of detail at many image scales.  A miscollimation will reduce contrast and wipe out fine detail.  But since there is so much contrast and details at medium and large image scales, your eye will find something to see.  So even though the view might look decent, it will be missing the potential sharpness and fine detail that would be there if collimation was spot on.

 

Note that I am talking about fairly small levels of miscollimation here.  A large miscollimation will prevent the telescope from reaching anything that looks like a sharp focus.

 

For what it's worth, my observation over the years has been that many people with SCTs are using their instruments with slight collimation errors, not realizing that they are not reaching full potential that they can deliver.

my understanding is that to fine tune collimation, one would move from the medium-high powered EP, like 14mm, to a high power. So the pics I posted of my  artifical star, well, maybe there is a false sense of security since that was not an EP, but my camera.   I should probably now fine tune the collimation. But my wife and daughter are back from from the in-laws and it would be uncanny right now to set-up everything like I had it.  But later I Can do it.

 

edit,

 

incidentally, how does one compute the power/ magnification of a telescope -imaging camera? I did not see a way to do that in astronomytools


Edited by Ballyhoo, 21 June 2019 - 11:53 AM.


#20 Toddeo

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Posted 21 June 2019 - 12:12 PM

If you still want to use your camera to help with collimation, use a barlow with the camera.


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#21 Ballyhoo

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Posted 21 June 2019 - 12:14 PM

If you still want to use your camera to help with collimation, use a barlow with the camera.

oh yes, good point.   

 

Edit,

 

And I guess we really do not use "magnification"  as a measurement in imaging, but I am thinking in terms of FOV equivalent. 


Edited by Ballyhoo, 21 June 2019 - 12:19 PM.


#22 WadeH237

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Posted 21 June 2019 - 12:23 PM

Collimation is an iterative process.

 

If you are way, way out of collimation, it's best to start in the daytime by looking into the front of the scope for concentricity.  For a better collimation, it's fine to use an artificial star that is out of focus.  For fine collimation, it's best to use a real star that is at least 45 degrees high.

 

With the real star, you can start with it a bit out of focus.  It's best to defocus clockwise, so that you are turning clockwise to achieve focus.  This way, you will be using the focus rod to push the primary mirror against gravity.  Center the star in the eyepiece (very important!) and *slowly* move closer to focus.  As you do this, the star image will get smaller and non-concentricity will become easier to see.  At this point, make sure that the star is still centered (very important!) and adjust collimation to make the rings concentric.  After *each* adjustment, recenter the star in the eyepiece (very important!)  Keep doing this over and over, using higher eyepieces as necessary, until the star is in focus.  If seeing is good and you are at a high enough power, the diffraction rings may be visible (and they may be popping in and out and shifting around due to seeing), and you want them to be concentric.

 

Often times, what we think is a seeing problem, might actually be a non-acclimated OTA.  So it's best to ensure that the scope has had time to adequately reach temperature equilibrium.  And after each adjustment, make sure that the star is centered (very important!)

 

As for your question about imaging and "power", the term does not apply.  Magnification and power reference how the image from the scope is presented.  In the case of an image, this is obviously controlled by the size of the screen that you use to look at it, or the size of the image on a printed page.  In imaging, the figure to be concerned with is image scale.  This is a measure of the number of arc seconds of sky captured by each pixel.  It's been pretty thoroughly discussed in all of the imaging forums.

 

It's probably worth mentioning that the Airy disc and diffraction rings are very, very small.  Depending on the aperture of the scope, they could be less than 1 arc second in size.  So if you are using a wide field imaging setup, you'll never see it.  It's also why you need a very high power eyepiece to see it visually.

 

Oh, and did I mention that it's very important to center the star in the eyepiece each time that you make a collimation adjustment?  Not doing this can lead to high frustration and insanity...


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#23 bobharmony

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Posted 21 June 2019 - 12:29 PM

oh yes, good point.   

 

Edit,

 

And I guess we really do not use "magnification"  as a measurement in imaging, but I am thinking in terms of FOV equivalent. 

I use this site to calculate field of view for AP:  http://celestialwond...eScaleCalc.html  It gives you a quick answer once you know your equipment parameters.

 

Bob



#24 Ballyhoo

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Posted 22 June 2019 - 02:56 PM

 This is a measure of the number of arc seconds of sky captured by each pixel.  It's been pretty thoroughly discussed in 

This is actually quite an interesting point but I do not think we can dispose of the concept of magnification in imaging.  When I first bought a digital camera, I think it was in 1998, my camera had two types of magnification that were highlighted in the sale of the prodcut: 1) optical zoom, 2) software zoom.

Now obviously the latter is no zoom at all. But the optical zoom, is real zoom, I believe we mean, "magnification." that is, the glass in the lens is bending the photons in such a way as to increase resolution. And isn't increased resolution the same thing as magnification?  When you have more pixels, per arc second, isn't that increased resolution (increased detail) which is if not identical to magnification, very close it it? Less pixels per unit of space on the screen means less detail. 

 

When we were kids playing with magnifying glasses, the bigger magnifying glass was able to gather more light to bend the photons to a smaller area (increased resolution). 

The morale of the story is that "software zoom" is a fraud.  I am sure I do not know as much as you about this, I never took the time I should to really study the intricacies. But I still think that  image scale is not a completely different thing to magnification. As image scale increases, there s more fine detail and resolution per arc second of area. I think that is similar to magnification, which also shows more detail per arc second space in the sky. I hope I am not way off here.



#25 WadeH237

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Posted 23 June 2019 - 07:13 AM

This is actually quite an interesting point but I do not think we can dispose of the concept of magnification in imaging.

Sorry.  It's not my opinion.

 

It is well established.


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