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Collimation - Airy Disc intrafocal or extrafocal for star testing?

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#1 rkelley8493

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Posted 02 November 2019 - 01:51 PM

When collimating my SCT, do I need to judge the Airy Disc at its intrafocal or extrafocal position, or both? It seems like I can get it collimated at one end but not the other.. I got really frustrated the last time I took the SCT out bangbang.gif  Just couldn't get the collimation right fingertap.gif 

What are some best practices for collimating SCT's? Do Bob's Knobs work well, or do they make it where collimating is required more frequently?



#2 scopewizard

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Posted 02 November 2019 - 02:50 PM

First I would recommend is do not use a diagonal. Find a star that is not very bright, center it in an eyepiece with 200x to 300x. Defocus the star until you see 3 to 4 rings with the black circle showing. You can defocus in or out which ever is best. Now make a very small adjustment, 1/8 of a turn or less, once screw only. Recenter the star and see if you went in the right direction. Keep using this method until the circle is centered in the rings with the star centered in the eyepiece. Always recenter before making an other adjustment. Bob's know do help as you don't have to constantly fiddle with the allen key to adjust.


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#3 dweller25

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Posted 03 November 2019 - 02:50 AM

As above to get as close as possible with the airy disks when slightly defocussed, then go to best focus and final collimate.

Edited by dweller25, 03 November 2019 - 02:51 AM.

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#4 Ken82

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Posted 03 November 2019 - 06:25 AM

I’m assuming it’s the same principle for collimating with a camera attached ? In terms of star brightness what are we taking in terms of magnitude ? Thanks

#5 decep

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Posted 03 November 2019 - 09:08 AM

It makes little difference if you collimate using intra or extrafocal.  It will be impossible to have perfect collimation in both positions due to mirror flop.

 

You should probably collimate in the same direction you perform fine focus.  This means if you over shoot your focus, you have to over-defocus and then refocus in the original direction so the mirror is always shifted back to the same angle.

 

You could always try to collimate at the half-way point in your mirror flop for the best of both worlds.  :-)


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#6 Eddgie

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Posted 03 November 2019 - 09:15 AM

It makes little difference if you collimate using intra or extrafocal.  It will be impossible to have perfect collimation in both positions due to mirror flop.

 

 

I think that your advice is good, but not specific enough.

 

The "Same direction" should be modified to say counter-clockwise.

 

Using counter-clockwise turns of the focuser will lift the focuser side of the primary mirror against the viscosity of the grease film, and this will result in a more consistent mirror angle.

 

If counter-clockwise is used, when pressure is removed from the focuser knob, since there is a tiny amount of play between the threads on the mirror carrier and the threaded focuser rod, gravity can continue to pull the mirror down against the resistance of the grease film, and this can change collimation. 

 

So, for checking collimation, and for final focus, I agree with your advice to always approach from the same direction, but that direction should specifically be counter-clockwise.  If one overshoots, one should back up past the desired focal point using clockwise, then re-approach using counterclockwise.

 

And using this method, it is indeed possible to get collimation perfect. 


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#7 Magnetic Field

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Posted 03 November 2019 - 09:20 AM

When collimating my SCT, do I need to judge the Airy Disc at its intrafocal or extrafocal position, or both? It seems like I can get it collimated at one end but not the other.. I got really frustrated the last time I took the SCT out bangbang.gif  Just couldn't get the collimation right fingertap.gif

What are some best practices for collimating SCT's? Do Bob's Knobs work well, or do they make it where collimating is required more frequently?

Forget the intra- an extrafocal star test (and also forget the Suiter book for testing optical quality as it does not work).

 

You need to collimate (or double check) the star while it  is in focus. You keep collimating until you see a good first airy ring.

 

The extra- and intra focal pattern is only a very rough guide.


Edited by Magnetic Field, 03 November 2019 - 09:20 AM.

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#8 Eddgie

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Posted 03 November 2019 - 09:38 AM

To the OP, I would say that one thing that can cause this condition is having a secondary mirror that is not centered or if there is tilt in the system.

 

To check this condition, use a star at high power and defocus using counterclockwise.  You want to defocus only about 3 or 4 waves and note the position of the tiny point of light at the center of the secondary shadow.  

 

If the scope looks to be in collimation and this spot is not exactly centered inside the secondary shadow when it is defocused, this usually means a bit of decentering.

 

centering.jpg

 

Now this important message.  Generally, this does not matter if the scope is not used with a lot of different accessories with different amounts of flange to focal plane distance.  If you are always using the same diagonal, then a tiny amount of decentring is not going to have any effect as long as the collimation is good when the scope is at best focusSee, in normal use, the scope is not used out of focus, and if the scope is in perfect collimation when it is in perfect focus, then just don't worry about it.

 

Defocused testing is only useful for rough collmation.  It is not really precise to try to collimate using lots of defocus.  Once the rough collimation is complete, the fine collmation is done on an in-focus star.   As long as you show good collimation when in focus with the eyepeices you use, then you can ignore a slightly decentered secondary.

 

If on the other hand, you have configurations that change the back spacing (going from binoviewers to mono-viewing), if any change in collimation on an in focus star is noted, then this condition probably should be addressed. 

 

Mostly though, it can be ignored as long as when in focus, you see a Spurious disk with an even first diffraction ring.  If you can see that, ignore the fact that there is a slight asymmetry to the pattern inside and outside of focus (not centered Poission Point) and fine collimation should always be done with a perfectly focused star. 


Edited by Eddgie, 03 November 2019 - 09:49 AM.

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#9 Axunator

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Posted 03 November 2019 - 12:41 PM

...and also forget the Suiter book for testing optical quality as it does not work.

ohmy.gif Yeah right, laws of physics are useless nonsense. And forget mathematics for keeping book of your financial affairs, and medicine for treating curable but potentially fatal diseases, as they do not work bangbang.gif  

 

Apologies for the off-topic rant... 


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#10 Magnetic Field

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Posted 03 November 2019 - 02:04 PM

ohmy.gif Yeah right, laws of physics are useless nonsense. And forget mathematics for keeping book of your financial affairs, and medicine for treating curable but potentially fatal diseases, as they do not work bangbang.gif  

 

Apologies for the off-topic rant... 

I think you should refrain from ranting.

 

If you think you can test your telescope with the Suiter book more power to you.

 

And don't tell me about physics (okay good I forgot most what I have learned). I have got a Phd in physics and even work in the field.


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#11 rkelley8493

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Posted 03 November 2019 - 02:49 PM

Haha nice comebacks lol.gif  And good advice waytogo.gif I was using a diagonal when trying to collimate, so there's one thing I was doing wrong. 

It's supposed to be clear tonight.. I may give the SCT another shot laugh.gif



#12 decep

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Posted 03 November 2019 - 04:03 PM

The "Same direction" should be modified to say counter-clockwise.

 

Using counter-clockwise turns of the focuser will lift the focuser side of the primary mirror against the viscosity of the grease film, and this will result in a more consistent mirror angle.

 

If counter-clockwise is used, when pressure is removed from the focuser knob, since there is a tiny amount of play between the threads on the mirror carrier and the threaded focuser rod, gravity can continue to pull the mirror down against the resistance of the grease film, and this can change collimation.

I had to think about the direction I normally focus.  I *think* it is counter-clockwise, but I do not consciously think about it while focusing.  I will try to do so in the future.

 

At least with an alt/az mount, gravity is always the same direction.  In an equatorial mount, gravity can shift a full 180 degrees on the OTA.  GEMs are a worst case.  I am not show how that plays into this issue.


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#13 Eddgie

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Posted 03 November 2019 - 04:05 PM

Many dismiss Suiter's book, but I have identified a bunch of optical errors using the star test.  People that say it does not work have (in my experience) usually not read the book and do no know how to do it properly, and how to do the test and assess all of the optical errors covered in the text. That is after all what the book is for. 

 

I can spot a lemon  in 10 seconds (though I need to add an obstruction if it is a refractor). 

 

And I can tell the difference between a perfect scope and a not perfect scope in about as long. 

 

Now, telling the difference between a very good scope and a near excellent scope can be difficult but true excellence pretty much screams out.   

 

Now, if someone does not care about determining if a scope has excessive spherical aberration, zonal errors, a turned edge, astigmatism, or other common errors, then yes, reading the book is a waste of time.   


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#14 Eddgie

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Posted 03 November 2019 - 04:20 PM

I had to think about the direction I normally focus.  I *think* it is counter-clockwise, but I do not consciously think about it while focusing.  I will try to do so in the future.

 

At least with an alt/az mount, gravity is always the same direction.  In an equatorial mount, gravity can shift a full 180 degrees on the OTA.  GEMs are a worst case.  I am not show how that plays into this issue.

In my experience, it really does not matter all that much if the mount is GEM or Alt-Az.  The angle that the mirror comes to rest it is primarily a function of the support provided by the threaded focuser shaft. Gravity will almost always pull the side opposite the threaded rod in a downward direction with the angle of tilt between the mirror and baffle tube almost always being about the same.  Now there might be some tiny amount of sideways tilt in some orientations, but most of the error is the result of the latest possible angle that can occur, and this angle is the one between the mirror carrier and the baffle in a plane that passes through the inside of the mirror carrier hole opposite of the focuser rod, and the focsuer rod. No other tilt angle can be this great and mostly they are far smaller when the rod is fully supporting the carrier. 

 

Only when the scope is pointed at a low angle to the sky will this change in any meaningful way, because when aiming down low, gravity will tend to want to settle the mirror parallel to the baffle. (And this is why I don't recommend using an artificial star. If the scope has any meaningful play in the mirror carrier/baffle, it will be parallel when aiming low, but then when the scope is pointed into the sky the collimation sometimes changes because as the mirror settles into the grease film the focuser rod holds up one side and the other side settles through the grease film at winds up being at a slight angle. 

 

I had a C14 and if I collimated the way I suggest, for most of the sky, the scope would be in collimation.  

 

And once collimated my C14 would go for years without needing collimation again. 

 

Once set properly and the collimation screws are properly torqued, unless the scope is handled with less than reasonable care, the collimation should rarely need to be adjusted.  When I say rarely, I mean time frame on the order of years. 


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#15 luxo II

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Posted 04 November 2019 - 05:03 AM

... but then when the scope is pointed into the sky the collimation sometimes changes because as the mirror settles into the grease film the focuser rod holds up one side and the other side settles through the grease film at winds up being at a slight angle.

Excellent point Eddgie, I suspect this could be part of the reason why many SCT owners have issues with collimation - many (self included) would collimate it pointing down low to access the secondary alignment screws. But as the scope rotates over the sky the slight shifts in mirror position affect it unpredictably.


Edited by luxo II, 04 November 2019 - 05:04 AM.


#16 rkelley8493

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Posted 04 November 2019 - 08:34 PM

So.. I totally *fuzzed something up [pun intended].

This cursed thing is all out of some wack. The Airy Disc looks like a diamond ring now.. One side is bright and shiny while the other is dim and slanted. I must have the 2ndary mirror off centered confused1.gif

I'm bout ready to give up on this thing  smashpc.gif


Edited by rkelley8493, 05 November 2019 - 02:37 PM.


#17 Eddgie

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Posted 05 November 2019 - 11:22 AM

Sometimes if you get really far out of collimation, it can be difficult to get back in because you can't be sure which way the mirror needs to be tilted.

 

Try defocusing a lot.  I mean like 30 waves or so (25 rings visible but at this much defocus, they tend to run together).  At some point, you should get a big enough pattern to easily see where the secondary is displaced (assuming it is that far out of collimation).

 

Also, you can sometimes get a line on what is happening by pointing the scope towards you and walking back away from it while looking at the various refectsions of the edges of the primary, secondary, and baffles.  At every distance, everything should look concentric.  The high negative power of the secondary will reveal any serious tilt and give you a clue as to where you need to make your adjustments.

 

Best though is simply to defocus enough so that you can see a lot of rings and the secondary shadow..  Too much and you loose sensitivity but 20 to 30 rings will probably be sensitive enough to show you which way the mirror is out if it is really far off. 

 

I have seen a lot of people get their collimation whacked out, but I can't ever remember anyone not getting it back in shape. Just be patient.   Take you time.   Small turns of the collimation screws.  Re-center after every adjustment. 


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#18 KerryR

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Posted 05 November 2019 - 01:04 PM

I'll offer up 2 tips that I like:
 

1) For figuring out which screw(s) to turn, place your hand in front of the scope, near the edge while you're looking at the blur-circle. You'll see the shadow of your hand in the blur-circle. You can move your hand around the circumference of the aperture 'til you see your hand's shadow aligned with the point around the blur circle that the secondary shadow is closest (or furthest) from. You can leave you hand in this position and move your view to the front of the scope. Note the screw that's closest to or opposite of your hand, and that's the screw to turn. Note that you may have to loosen or tighten the 2 opposite screws to allow adjustment to the selected screw. Bob's Knobs make this easier because you can leave you eye over the eyepiece and just reach around and feel for the relevant knobs to turn, but they're not necessary. Some folks have made objects that clip on the front of the ota to replace the use of the hand for casting the shadow. 

2) As Ed mentions, it can be very helpful to defocus quite far if things are way out, perhaps using a longer focal length eyepiece. Then, while looking through the scope at the blur-circle (defocused star), move the ota, which will move the blur-circle in the field of view. Somewhere in the field, the secondary shadow will be more centered in the blur-circle than elsewhere (or perfect if things were already somewhat close). Avoid placing the blur-circle at the very edge of the field, where eyepiece astigmatism may be pronounced. Once you've moved the blur-circle to this position, lock the scopes 2 movement axes if possible; the scope has to stay put while you turn the collimation screws. You then use the collimation screws to move the improved blur-circle to the center of the field of view, and repeat the process again. Once you have a nicely centered secondary shadow with the blur circle centered in the field of view using a large defocus, you can repeat the process ever closer to focus at increasing magnification, 'til you finish the process in focus if seeing and tube currents allow. If seeing and thermal currents won't support this, I've found the next best thing is centering Poison's spot as close to focus as conditions allow.

 

Regarding the condition of a centered shadow/Poison's spot on one side of focus and not the other: I ran into a severe case of this issue after having my 8" SCT completely apart for work (adding active venting). The fix ended up being very careful centering of the secondary housing relative to the corrector cell. I used Vernier calipers for this, and, luckily, the secondary mirror must've been well centered within it's housing, or this probably wouldn't have worked. (There were no shims around the plate).


 


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#19 AtmosFearIC

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Posted 07 November 2019 - 11:23 PM

I replaced the screws on a second hand Mewlon 180 a couple of years ago and I used a Glatter with Circular Holographic for getting collimation close. This allowed it to be done inside at night with the lights off when it's cloudy. It is not sensitive enough for proper collimation but I found that when the collimation is WAY off and it is difficult to see what direction you should be moving, it takes 5 minutes inside to get reasonable close and you can see what you're doing in real time.



#20 jhayes_tucson

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Posted 09 November 2019 - 04:45 AM

I don’t want to steal his thunder but you guys should pay attention to the talk that Gaston (IFI) is presenting at AIC.  He has come up with something potentially quite significant with regard to telescope alignment.

 

John


Edited by jhayes_tucson, 09 November 2019 - 04:46 AM.


#21 WadeH237

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Posted 09 November 2019 - 08:44 AM

I don’t want to steal his thunder but you guys should pay attention to the talk that Gaston (IFI) is presenting at AIC.  He has come up with something potentially quite significant with regard to telescope alignment.

 

John

Thanks for the heads up.  I will be sure that catch that one.



#22 rkelley8493

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Posted 10 November 2019 - 03:36 PM

Okay, so here's an update.. I ordered a set of Bob's Knobs for the SCT and installed them the other day. The instructions had some pictures of how the mirrors should look when properly aligned. It kind of reminded me of the "Hall of Mirrors" Effect where you can line up the mirrors to make them reflect to infinity. But in this case, it was more of lining them up so the reflection of the secondary mirror would be roughly parallel to the primary. See link for images.

http://www.bobsknobs...ons_English.pdf

I took out the SCT for a minute last night to see how well I did with the rough collimation. I'm a better judge at eyeballing other things lol.gif  but at least it got me in the ball park. Anyways, the knobs made it much easier to view through the eyepiece while adjusting the collimation. I think I was making it worse by blindly adjusting the collimation before. It was much easier to get the adjustment right while viewing the Airy disc pattern while adjusting the knobs vs going back and forth from the eyepiece to the screw driver.

I think I finally got the hang of it smile.gif, now I can use this big A* scope to its potential waytogo.gif  [A* = aperture grin.gif]


Edited by rkelley8493, 10 November 2019 - 09:40 PM.

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#23 petersellis

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Posted Today, 02:37 AM

I don’t want to steal his thunder but you guys should pay attention to the talk that Gaston (IFI) is presenting at AIC.  He has come up with something potentially quite significant with regard to telescope alignment.

 

John

Hi John,

Sounds very interesting if it is a new collimation technique. Could you please elaborate, what is AIC,  and what is Gaston's initials, and when is this conference taking place?

 

                 Cheers

                 Peter


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#24 WadeH237

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Posted Today, 07:16 AM

Hi John,

Sounds very interesting if it is a new collimation technique. Could you please elaborate, what is AIC,  and what is Gaston's initials, and when is this conference taking place?

 

                 Cheers

                 Peter

Who: https://www.innovationsforesight.com/

 

What/Where: https://www.advanced...onference.com/?


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