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Collimation of an SCT from scratch, a photo guide

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#1 Tom Glenn

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Posted 30 June 2018 - 01:25 AM

Collimation is critical for any attempt at high resolution imaging.  On this occasion of my replacing the collimation screws on my C9.25 Edge, I thought I would document the process as an example of collimating an SCT from scratch.  This will be far more out of alignment than a typical scope would be, but collimation can be daunting for the beginner, so perhaps some may find this helpful, as questions about collimation have come up recently on this board.  

 

I had a collimation screw that has finally stripped, which gave me the opportunity for a long overdue project of replacing the factory collimation screws on my C9.25 Edge.  I don't know why Celestron uses Phillips head screws, as these are the last choice you would want for any fastener.  The Phillips head guarantees that your tool will cam out under even moderate tightening, leading to stripping.  It doesn't help that the Celestron screws are very soft.  Using socket head screws (also called Allen key or Hex key) is a much better choice, and I have been postponing this project, but it was finally becoming a huge PITA to collimate because of stripping.  I avoided using Bob's knobs, as I feel they attempt to solve a problem that doesn't exist, and I like using a tool to achieve proper tightening.

 

My first comment is that if you are going to replace the screws, this is easiest done during daylight, with the scope on the mount and pointed horizontally.  You don't want a disaster happening with anything falling into the OTA onto the primary!  Replace the screws one at a time.  DO NOT remove all screws at once, to avoid disaster.  Once all the new screws are in and tightened up to a reasonable degree, you need to make a rough alignment.  The easiest way to do this is simply to look into the OTA and see how the mirrors reflect in one another.  You want everything to form a nice straight line of reflections.  Note the starting position here, in which the reflections are NOT in alignment.  

 

Before_collimation1.jpg

 

As a complete aside, this photo is much cooler than I originally anticipated!  Lots of weird optical effects going on with strange colors from the interaction of the reflection off the corrector plate with the mirrors and my iPhone lens.  But as it pertains to collimation, notice how the reflection of the mirrors do not make a straight line.  After a few minutes playing around with slight turns of the collimation screws (complete trial and error), we arrive at this position.

 

Rough_alignment.jpg

 

Note that the pattern of reflected mirrors make a rough line.  It's not perfect but this is good enough to get a star into the field of view.  If you don't do this rough alignment by eye, you will have a heck of a time trying to find a star to complete collimation.

 

I used Vega for this collimation, because I needed to start before it was completely dark, and Vega was the only star visible at the time.  It helps to have a camera with a large sensor (I have an ASI183mm), but if you do not have one, then the initial steps can be done with an eyepiece, since you are likely to be WAY off when starting.  But it is far easier and more accurate to finish collimation with a camera.  The star will move when you turn the collimation screws, which is a PITA, but you will get used to it.  This is where it also helps to have a larger sensor camera.  After each adjustment, recenter the star in the center of the field of view.  As to which screw to turn, there are many other collimation guides that address this, but honestly, there are only three screws, so you can use trial and error and see if the pattern gets better or worse.  In general, you want each screw to be tight, so start by only tightening, but you don't want to over torque anything, so if one screw gets too tight to turn, loosen the others slightly (fraction of a turn) before tightening again.  Here is what Vega looked like, greatly defocused, after this rough alignment shown above. 

 

Collimation_1.jpg

 

The reason this image is so washed out is because this is a hugely defocused image, requiring a long exposure (300ms) and high gain, and there is still sunlight in the sky at this time.  You might look at this image and think that collimation is close to aligned.  You would be wrong.  Look at what happens when we focus tighter on Vega to bring the diffraction rings closer.  

 

Collimation_2.jpg

 

Note that in all cases, when you focus tighter, you will want to reduce the ROI around the star and "zoom in" in the live view to make things easier to see.  Each time you tighten focus you will also need to reduce exposure as more energy is deposited into a smaller diffraction pattern.  At this point I made a small adjustment, changed the exposure, and arrived at this.

 

Collimation_3.jpg

 

Still out of alignment.  Another adjustment yielded this.

 

Collimation_4.jpg

 

This is starting to look better, but you cannot stop here.  You need to tighten the focus even more, zoom in again, and adjust exposure.  

 

Collimation_5.jpg

 

Here you can see that we are still out of alignment.  This is why you cannot use strongly defocused stars to collimate an SCT.  You need to get much closer to focus.  Another small adjustment gives this.

 

Collimation_6.jpg

 

And we keep going.

 

Collimation_7.jpg

 

Still not quite right.  Another adjustment and tightening of focus and we arrive here.

 

Collimation_8.jpg

 

Now this is looking pretty good (these still images don't quite convey how it looks in live view, some asymmetry in the still shot is not representative of the average view, which moves around depending on the seeing).  Any closer in focus and we arrive at the Airy pattern, which can be used on some occasions, but on this occasion, as Vega was still pretty low in the sky at the time, it was not very helpful.  It's worth noting that collimation shifts somewhat as you slew about the sky, so you really want to collimate using a star closest to the object to be imaged.  For Jupiter, you could use Spica, although even better would be Alpha Librae, which is just a few degrees away from Jupiter right now.

 

Please note:  All of the star images are screen grabs and were not captured with the camera itself, so the pixel dimensions are not representative of the actual relative sizes, but are greatly zoomed in to depict more clearly what I am trying to show. 


Edited by Tom Glenn, 30 June 2018 - 01:28 AM.

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#2 RedLionNJ

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Posted 30 June 2018 - 07:29 AM

Nice set of images to illustrate, especially the "looks pretty good, but really isn't" phase.

 

I was reading, looking, scrolling down, waiting for the proof of perfect collimation with either a cleanly-separated close double star or a textbook set of diffraction rings around a tiny Airy disk... 

 

Doesn't look like seeing was up to it, however, last night?

 

 

Grant



#3 astrolexi

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Posted 30 June 2018 - 07:38 AM

Thank you Tom. This is an excellent guide for SCT collimation.

It reminds me of collimating my old C8 (34 years old!) again as soon as possible.

With Bob's Knobs btw...wink.gif



#4 kevinbreen

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Posted 30 June 2018 - 09:23 AM

This is great Tom. Thanks for posting. Collimating has always been a pain in the asteroid for me. I do it in my house where I have one 50+ foot space with a crappy imitation “star” gizmo. In a year of AS I’ve done it twice😬
Where I live it’s not always easy, Wexford Town is 3km away and there’s some light pollution, plus I’m always eager to start capturing and couldn’t be asteroided even finding a decent star to do a collimation. And, it’s bad enough in the house in daylight, never mind in the dark with my dog dropping a ball at my feet every 30 seconds and me trying to fumble around with a screwdriver (and sometimes a can of beer👎). I’m also tree-confined - I’d cut them down right now (they’re not indigenous, they’re Scandinavian,) but they’re my neighbour’s.
Just a thought, given that the star moves around the FOV with tweaking of the screws(akin to the planet’s movement upon tweaking the ADC levers), a focal reducer might help? A back-of-an-envelope calculation tells me a f/6.3 focal reducer will increase the FOV by close to 60%. If the aperture is big enough it shouldn’t make a difference. But as I type I’m remembering all I’ve read here about the importance of collimating with the complete optical train in tow. Never mind. It’s time I collimating again. Thanks for the reminder.
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#5 GeorgeInDallas

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Posted 30 June 2018 - 10:45 AM

Tom,

 

Great description!

 

Here is a technique that has greatly helped me with collimating my C14.

 

When I have all of my components (focuser, filter wheel, telescope, multiple cameras) wired up to my laptop it is not very moveable. Cables are short and tend to come loose when moved. When my C14 is on it's tripod and wheeleybars I must climb a ladder to reach the collimation knobs. Prior to implementing this solution I would look at the out of focus star image, decide on a collimation adjustment, climb the ladder and attempt the adjustment, then climb down the ladder and check the results on the laptop screen. As you can imagine, this was very painstaking and did not work very well. 

 

The solution that has helped is to get  a separate laptop and run TeamViewer on both laptops. I can take the second laptop to the top of the ladder and monitor the screen of the laptop that connected to all of the equipment and cameras. With sufficient wifi bandwidth, the screen updates at display refresh speeds and gives immediate feedback on collimation adjustments. This has worked extremely well for me. I happened to have an older laptop and 100 megabit router available for this purpose so I was not out any cost. The TeamViewer that I downloaded with basic features for home use was free. 

 

TeamViewer is an application that is designed for, among other things, remote screen sharing.

https://www.teamviewer.us

 

Below is a photo that might help explain my problem.

 

 

 

Hope this is useful,

George

Attached Thumbnails

  • C14Config.jpg

Edited by GeorgeInDallas, 30 June 2018 - 10:50 AM.

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#6 RedLionNJ

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Posted 30 June 2018 - 10:51 AM

Funny Bob's Knobs get mentioned again here. I'm one of those who replaced my original hex screws with Bob's Knobs "for ease of collimation" and while it's definitely true they're easier to handle in the dark, even with gloves on if necessary, and don't require tools, they do have one major disadvantage - there's no concept of extremely fine tweaking. I always found it nearly impossible to adjust BK finely enough as I became accustomed to the difference between very good collimation and "maybe good enough" collimation.

 

That's where the hex head with the Allen wrench comes in - and the longer the Allen wrench, the better. With patience and a steady (or streadied) hand, I can make a rotational adjustment somewhere in the vicinity of a single degree (might even be half a degree - hard to measure, precisely). That would just be impossible with Bob's Knobs. And it can be enough to make a tangible difference in collimation, even at f/15.


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#7 Tom Glenn

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Posted 30 June 2018 - 11:05 AM

Nice set of images to illustrate, especially the "looks pretty good, but really isn't" phase.

 

I was reading, looking, scrolling down, waiting for the proof of perfect collimation with either a cleanly-separated close double star or a textbook set of diffraction rings around a tiny Airy disk... 

 

Doesn't look like seeing was up to it, however, last night?

 

 

Grant

 

I was hoping to having something to better conclude the post, but the problem last night was clouds.  It was actually somewhat overcast during this test, and you can see the beginnings of this in the reflection of the sky in the corrector plate in the first photos.  Vega was bright enough to shine through the clouds to allow collimation, but sadly, after getting this rough collimation done I was unable to do any imaging.  And the Airy pattern around Vega was not pretty enough to post, as it was still fairly low in the sky during the test.  Seeing was probably pretty good near zenith, so I could have done a double star test on an appropriate target if not for the clouds.

 

One question I have for others who image with SCTs, is how much collimation shift is expected when you point in a different direction or do a meridian flip?  I ask because I always recheck the collimation in these cases, and am usually disappointed that I do have to make some adjustments.  Especially doing a meridian flip it is usually significantly off, at least by my standards.  From reading some other forums on CN, most notably the Cats and Casses forum, I seem to get the impression that this is atypical, and that collimation should hold much better than that.  However, from reading this forum, I get the impression that what I see is typical.  So what gives?  Do we just have higher standards for collimation than the folks posting in the other forums?  I see others saying that if collimation doesn't hold reasonably well when slewing about, and the screws are tight, then the baffle holding the primary could be loose allowing it to shift (different from the typical "flop").  I've just taken for granted that I have to constantly adjust collimation, but now I'm wondering what others experiences are.  


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#8 kbev

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Posted 01 July 2018 - 06:34 AM

Tom, I have noticed that when I do a meridian flip my collimation is off enough to warrant adjusting it.  However when I stay on one side of the meridian I don't notice as much shift going from say Jupiter to Saturn or Mars, the angle changes a bit but my collimation holds well enough that I seldom make an adjustment - I'll check it to be sure it's OK and once I see that it is I'll get on with my night.

 

And I understand what you say about Bob's Knobs, the scope I currently use came with them installed but I keep thinking it might be better to try some Allen head cap screws.  If I don't like how they work or feel they're too cumbersome I can always switch back.


Edited by kbev, 01 July 2018 - 06:34 AM.


#9 Andrewf

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Posted 01 July 2018 - 07:45 AM

I was hoping to having something to better conclude the post, but the problem last night was clouds.  It was actually somewhat overcast during this test, and you can see the beginnings of this in the reflection of the sky in the corrector plate in the first photos.  Vega was bright enough to shine through the clouds to allow collimation, but sadly, after getting this rough collimation done I was unable to do any imaging.  And the Airy pattern around Vega was not pretty enough to post, as it was still fairly low in the sky during the test.  Seeing was probably pretty good near zenith, so I could have done a double star test on an appropriate target if not for the clouds.

 

One question I have for others who image with SCTs, is how much collimation shift is expected when you point in a different direction or do a meridian flip?  I ask because I always recheck the collimation in these cases, and am usually disappointed that I do have to make some adjustments.  Especially doing a meridian flip it is usually significantly off, at least by my standards.  From reading some other forums on CN, most notably the Cats and Casses forum, I seem to get the impression that this is atypical, and that collimation should hold much better than that.  However, from reading this forum, I get the impression that what I see is typical.  So what gives?  Do we just have higher standards for collimation than the folks posting in the other forums?  I see others saying that if collimation doesn't hold reasonably well when slewing about, and the screws are tight, then the baffle holding the primary could be loose allowing it to shift (different from the typical "flop").  I've just taken for granted that I have to constantly adjust collimation, but now I'm wondering what others experiences are.  

Tom

 I have the same issue with my C11, losing collimation after the meridian flip.  My C11 is about 1-1/2 years old.  A few months ago I called Celestron and asked questions regarding this problem.  They didn't hesitate at asking me to send the ota back for service.  I shipped the ota to Celestron and they turned it around very fast.  It is much better but not perfect.  All they told me was they cleaned and regreased.  I think Celestron knows they have a problem here.  The reason I say this is they emailed me, without my asking, a UPS prepaid shipping label for the trip out and they shipped it back to me on their dime.  No shipping cost to me both ways.  Again, it is way better now requiring just a tad of adjustment after a flip and I will add that they cleaned the optics.  Wow, did they ever.  Not a hint of dust or anything on the primary or the inside of the corrector.

Question for you, where did you get your hex screws?  I want to make this mod on my C11.  I agree with you, I don't care for my free Bobs Knobs either and those phillips screws are not the trick.

 

Andy



#10 GeorgeInDallas

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Posted 01 July 2018 - 10:19 AM

I am amazed to hear the complaints about Bob's Knobs. For me, they beat trying to use an allen wrench in the dark on top of a ladder by a mile. I am not aware of any difficulty making fine adjustments. Maybe I just have not reached that level of criticality in my collimation adjustments?

 

Now, if someone would just invent a battery powered, wireless, motorized, digital collimation device I would be very happy. 

 

George


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#11 Tom Glenn

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Posted 01 July 2018 - 07:14 PM

Tom

 I have the same issue with my C11, losing collimation after the meridian flip.  My C11 is about 1-1/2 years old.  A few months ago I called Celestron and asked questions regarding this problem.  They didn't hesitate at asking me to send the ota back for service.  I shipped the ota to Celestron and they turned it around very fast.  It is much better but not perfect.  All they told me was they cleaned and regreased.  I think Celestron knows they have a problem here.  The reason I say this is they emailed me, without my asking, a UPS prepaid shipping label for the trip out and they shipped it back to me on their dime.  No shipping cost to me both ways.  Again, it is way better now requiring just a tad of adjustment after a flip and I will add that they cleaned the optics.  Wow, did they ever.  Not a hint of dust or anything on the primary or the inside of the corrector.

Question for you, where did you get your hex screws?  I want to make this mod on my C11.  I agree with you, I don't care for my free Bobs Knobs either and those phillips screws are not the trick.

 

Andy

Thanks Andy.  I'm sure Celestron could take a look at it, although I've had my scope for over two years so it's not under warranty anymore, and also I couldn't imagine being without the scope, unless perhaps in the winter, but even then, the performance of the scope is great, it just needs constant attention to collimation, which is something that I would be checking anyway, so it hasn't risen to the level of me doing anything about it yet.  It might still be within normal limits.  

 

As for the screws, they are M3 machine screws, which is a fairly small size and not every hardware store will have them.  The first Home Depot I went to did not, but an Ace Hardware did. The length of the screws is different for the various OTAs.  For the C9.25, they were 10mm long.  For your C11, I think they are 12mm.  So you will want M3x12mm socket head screws.  I would opt for stainless. 


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#12 Tom Glenn

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Posted 01 July 2018 - 07:16 PM

I am amazed to hear the complaints about Bob's Knobs. For me, they beat trying to use an allen wrench in the dark on top of a ladder by a mile. I am not aware of any difficulty making fine adjustments. Maybe I just have not reached that level of criticality in my collimation adjustments?

 

I'm sure they are fine, and probably if I had them I would like them!  I'm just a bit particular when it comes to threaded fasteners, because you can never achieve the same level of tightness or fine control with your fingers then with a tool.  Although this is probably more of a theoretical argument, because if your star patterns look good, then there's nothing wrong with your method! 


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#13 kbev

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Posted 01 July 2018 - 08:17 PM



I am amazed to hear the complaints about Bob's Knobs. For me, they beat trying to use an allen wrench in the dark on top of a ladder by a mile. I am not aware of any difficulty making fine adjustments. Maybe I just have not reached that level of criticality in my collimation adjustments?

George

My Bob's Knobs work ok, it just gets a bit frustrating sometimes making very small adjustments. But once adjusted the scope hold collimation quite well, it probably helps that I leave it set up under a cover and don't have to adjust it everyone I want to use the scope.



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#14 RedLionNJ

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Posted 01 July 2018 - 08:42 PM

My Bob's Knobs work ok, it just gets a bit frustrating sometimes making very small adjustments. But once adjusted the scope hold collimation quite well, it probably helps that I leave it set up under a cover and don't have to adjust it everyone I want to use the scope.



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As stated way above somewhere, that's exactly the problem with Bob's Knobs. They're fine for non-critical (gross) collimation, but there's simply no way to apply a fine adjustment without modifying them in some way - or replacing them with hex heads where you can use a long (at least two inches, preferably longer) Allen wrench to fine-tweak the screws.


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#15 Guest_djhanson_*

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Posted 01 July 2018 - 09:06 PM

 

One question I have for others who image with SCTs, is how much collimation shift is expected when you point in a different direction or do a meridian flip? 

Hi Tom, I image with a 14" classical cass.  Like my previous C14HD it has a bit of collimation dependency on imaging angle. (i.e. primary collimation)

 

In my case the primary tweak is about a delta of 0.05mm on one of the 3 primary collimation knobs located on the rear backing plate.  (they have graduated markings so it's easy to see where they're set)  In this case I am presently collimating for about a 35 degree planet at low (25 deg), medium (30 deg) and high (35 deg) angles as the planet rises from east to meridan.  In each case I'm able to characterize the collimation needed on the single primary knob.  So from 25 to 35 degrees it's about 0.05mm delta if this makes sense, where 35 degrees is the planet at highest as it crosses the meridian.  cheers, DJ



#16 Tom Glenn

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Posted 02 July 2018 - 02:56 AM

Thanks for the data points, DJ!



#17 Tom Glenn

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Posted 02 July 2018 - 02:59 AM

Tonight, before Jupiter was available, I checked collimation on Spica, and took a few second long recording and then stacked 250 frames.  This is what the pattern looks like in focus.

 

Collimation_infocus.jpg

 

You could quibble, and say that the central point of the Airy pattern needs to move a hair up, but in live view you can't appreciate this, and honestly chasing this refinement of collimation will take a lot of time for diminishing returns.  I called this good enough.  And again, I didn't know what this would look like stacked until hours later.  In live view it was dancing and strobing a bit.  


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#18 astrolexi

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Posted 02 July 2018 - 01:24 PM

I am amazed to hear the complaints about Bob's Knobs. For me, they beat trying to use an allen wrench in the dark on top of a ladder by a mile. I am not aware of any difficulty making fine adjustments. Maybe I just have not reached that level of criticality in my collimation adjustments?

 

Now, if someone would just invent a battery powered, wireless, motorized, digital collimation device I would be very happy. 

 

George

 

George,

There is a chance that your (and my) dream of such a motorized collimator comes true soon.

See Bud Martin Budzynski's article "Motor-Driven Collimation" in the april 2018 issue of Astronomy Now...



#19 roelb

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Posted 06 July 2018 - 07:13 PM

I did a collimation adjustment on my Evolution 8".

Is their improvement necessary?

Here are some frames:

 

2018-07-06T00_58_35_Capture_00001 00_58_35.jpg

2018-07-06T00_58_35_Capture_00002 00_58_43.jpg

2018-07-06T00_58_35_Capture_00003 00_58_55.jpg



#20 Guest_djhanson_*

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Posted 06 July 2018 - 07:32 PM

Tonight, before Jupiter was available, I checked collimation on Spica, and took a few second long recording and then stacked 250 frames.  This is what the pattern looks like in focus.

 

attachicon.gif Collimation_infocus.jpg

 

You could quibble, and say that the central point of the Airy pattern needs to move a hair up, but in live view you can't appreciate this, and honestly chasing this refinement of collimation will take a lot of time for diminishing returns.  I called this good enough.  And again, I didn't know what this would look like stacked until hours later.  In live view it was dancing and strobing a bit.  

That looks pretty good.  I definitely agree with you about the point of diminishing returns...or one keeps spending another hour dinking around and low and behold you might be missing out on some cyclical moments of very good seeing.

 

Darryl showed a pic of his collimation once a few years back, next to his Saturn image.  As I recall it was just a skosh off, but yet his image looked amazing.  So I think once you get it about this close, then you can do quite good.  Your recent images definitely show this! :)



#21 Tom Glenn

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Posted 07 July 2018 - 01:45 AM

I did a collimation adjustment on my Evolution 8".

Is their improvement necessary?

Here are some frames:

 

Roel,

 

Your scope is at the very least, in pretty good shape, however, your images are still quite a bit out of focus.  They correspond to images that were towards the middle of my series above.  This will get you close enough to take some decent images.  I understand that you are working at very low elevations for the planets, so probably collimation will not be the limiting factor for you based on your image above.  Also, in order to improve upon your collimation, you will need good seeing conditions.  The central bright spot inside the diffraction rings (called the Poisson spot) becomes difficult to see when seeing conditions are not good, and this makes it difficult to proceed past the steps in your images you posted above.  I myself had some poor conditions tonight, and was able to appreciate the struggles that many of you have when collimating because you cannot see the Poisson spot, while close to focus.  You just have to do the best you can under the conditions.  Remember that when you get closer to focus, you need to reduce the exposure so that the diffraction rings are not so overexposed as to wash out the central Poisson spot.  When you are in complete focus, what was the Poisson spot becomes the Airy disk, although this is very difficult to see under most conditions.  To achieve near perfect collimation when you cannot see an Airy disk, you need only center the Poisson spot inside the rings, with only a slight defocus.


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#22 Tom Glenn

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Posted 07 July 2018 - 01:56 AM

That looks pretty good.  I definitely agree with you about the point of diminishing returns...or one keeps spending another hour dinking around and low and behold you might be missing out on some cyclical moments of very good seeing.

 

Darryl showed a pic of his collimation once a few years back, next to his Saturn image.  As I recall it was just a skosh off, but yet his image looked amazing.  So I think once you get it about this close, then you can do quite good.  Your recent images definitely show this! smile.gif

Thanks DJ.  You will find all kinds of mathematical models that are used to spit out numbers that tell us how much performance of the scope is reduced at various levels of miscollimation.  A very slight miscollimation will have a certain amount of theoretical reduction in performance, but this pales in comparison to the effects we get from variations in the seeing conditions.  On Thierry Legault's webpage, he gives a value that says a scope with a 20% central obstruction that has a level of miscollimation that corresponded to slightly more than what mine shows above performs as if it had a 27% obstruction.  This is negligible for resolution, and almost negligible for contrast (which can be adjusted in post anyway).  It would also introduce a slight spherical aberration, but again, this is the stuff that would show up in a laboratory test but not in real world conditions.  On a good night, my scope can resolve 0.5 arcseconds.  On what I consider a bad night, this number grows to 1 arcsecond and above.  This corresponds to a 100%+ reduction in performance!  I think where some people get confused is what constitutes a minor level of miscollimation.  If collimation is severely off, the performance of the scope is completely crippled.  


Edited by Tom Glenn, 07 July 2018 - 01:57 AM.

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#23 Kokatha man

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Posted 07 July 2018 - 02:51 AM

Darryl showed a pic of his collimation once a few years back, next to his Saturn image.  As I recall it was just a skosh off, but yet his image looked amazing.  So I think once you get it about this close, then you can do quite good.  Your recent images definitely show this! smile.gif

No it wasn't DJ - I suspect you are just being diplomatic..! smile.gif

 

Unless I'm mistaken I believe you are referring to this thread/image: https://www.cloudyni...s/#entry5199185

 

The collimation was considerably out as the diffraction rings indicated...but with the clouds racing across the sky & about to blot out Saturn we couldn't spend a second longer on collimation - as it was we only managed one & a half r-g-b sequences...but as the records show it was enough for Pat & myself to become the first amateurs to image Saturn's North Polar Hexagon, something not done since the Pic du Midi's blurry images decades earlier when this aspect of the N. Hemisphere was last favourably revealed..!  

 

Nonetheless quite a lot of detail was revealed...but almost 2 months to the day later we obtained a much clearer image by being able to collimate at our leisure with plenty of cloud/storm detail present despite my much less developed sense of processing back in those days! lol.gif https://www.cloudyni...t/#entry5306210

 

My major criticism of Legault's et al collimation images is that none of them are real - they are all computer-generated simulations - as is the effects of miscollimation upon planetary detail/resolution: they give completely false notions of what actual collimating stages etc look like & can confuse a lot of folks - which is why I presented these images on our website together with the explanatory notes. http://momilika.net/...3Processing.htm

 

On the subject of "Bob's Knobs" et al I believe (like George) that larger knurled heads are a real benefit...so much so that I made my own with brass sheet so that each is 15-20mm in diameter allowing me to tweak each one miniscule amounts for fine adjustment by virtue of the greater mechanical advantage which dictates a turn of (say) 1/10 π on the larger head relates to 1/2.5 π on the standard (roughly) factory 5mm secondary-adjustment screw head. (I think I detail the maths correctly, but anyone should immediately grasp the basis... lol.gif )   


Edited by Kokatha man, 07 July 2018 - 02:52 AM.

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#24 Tom Glenn

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Posted 07 July 2018 - 03:21 AM

Nice post Darryl, and I like the reference to your old posts.  Those are some nice Saturn images!  Your collimation that is "considerably out" as you put it, is still much closer to correct collimation than what a lot of folks keep their scopes at because they never get past the stage of strongly defocused.  To what degree do you think collimation reflects the differences in your two Saturns linked, versus the usual variations in seeing, which can have a quite profound effect, enough to change a great image into a superb image?  I hope to one day get some good images of Saturn....I think Saturn will finally rise above 50 degrees for me in 6 years, and will reach 65 degrees in about a decade, and then we get a turn in the N. hemisphere for a little while at least.  

 

I agree with your criticism of the simulated collimation guides.  I don't even think the diffraction patterns provided look anything like the real ones.  Looking at diagrams like those were indeed confusing when I was first starting, even if the material provided is interesting from a scientific standpoint.  I guess nobody who wrote a book wanted to take the time to start miscollimating their scope on purpose, and then taking a few images to show what the effect was at various levels?  Not that I can blame them though......as informative as that might be, I sure don't want to be wasting valuable seeing conditions on that test! 


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#25 Kokatha man

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Posted 07 July 2018 - 04:36 AM

Nice post Darryl, and I like the reference to your old posts.  Those are some nice Saturn images!  Your collimation that is "considerably out" as you put it, is still much closer to correct collimation than what a lot of folks keep their scopes at because they never get past the stage of strongly defocused.  To what degree do you think collimation reflects the differences in your two Saturns linked, versus the usual variations in seeing, which can have a quite profound effect, enough to change a great image into a superb image?  I hope to one day get some good images of Saturn....I think Saturn will finally rise above 50 degrees for me in 6 years, and will reach 65 degrees in about a decade, and then we get a turn in the N. hemisphere for a little while at least.  

 

I agree with your criticism of the simulated collimation guides.  I don't even think the diffraction patterns provided look anything like the real ones.  Looking at diagrams like those were indeed confusing when I was first starting, even if the material provided is interesting from a scientific standpoint.  I guess nobody who wrote a book wanted to take the time to start miscollimating their scope on purpose, and then taking a few images to show what the effect was at various levels?  Not that I can blame them though......as informative as that might be, I sure don't want to be wasting valuable seeing conditions on that test! 

That would be well nigh impossible to say Tom..!

 

But looking at it again (& these 2 CN links are only accessible to me 'cos I stuck them in my "Favourites" list at the time I started them! lol.gif ) I think I was also being "diplomatic" with my assessment...although actually the inset with the Diffraction Rings (repro'd here with a boost in levels to brighten their appearance) did state "considerably out."

 

OldCollimationImage.png

 

I'd definitely concur with that in hindsight - in fact I can confidently state that although way out in collimation the appearance of this image indicated seeing must have been pretty good & proper collimation would have really benefited the image outcomes - we just didn't have the time & were glad we started imaging almost immediately to grab that image with the "hex" - the rest of the night was completely clouded out btw...

 

The other thread & image was an excellent night - we still remember it & I'm sure we collimated commensurate with the seeing...so possibly even though we grabbed the hex on Jan. 26th 2013 it would've been even better than on March 25th had we been able to finish collimating & the clouds had stayed away also - but it is all just conjecture now! wink.gif

 

George (in Dallas) made some comments later in that thread about the frustrations of the Legault examples vis what he was actually seeing during collimating, reinforcing my other comments above imo... wink.gif


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