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C11 Defocus Test - Opinions Please

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#1 MarMax

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Posted 05 August 2021 - 12:43 PM

Here is an image showing inside and outside defocus for the C11. Star is Polaris and I'm using a 1.9x barlow with the IMX464 sensor camera. Perhaps the magnification is too much since it's probably close to an equivalent 700x.

 

The defocus amount is one focuser turn (0.75mm) in each direction. Not the normal way you do this with an eyepiece but 0.75mm of primary mirror travel must still translate to an eyepiece equivalent.

 

gallery_332504_14303_88064.jpg

 

What's the diagnosis?

 

EDITED to say that I'd like comments on aspects of the optics besides collimation.


Edited by MarMax, 05 August 2021 - 01:53 PM.


#2 Tapio

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Posted 05 August 2021 - 01:03 PM

This looks good but it might need to be smaller - or what does the jury say ?

 

Are you just to confirm or do you have trouble with star shapes ?



#3 Robindonne

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Posted 05 August 2021 - 01:05 PM

Congrats. Its going to be a boy
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#4 WarmWeatherGuy

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Posted 05 August 2021 - 01:05 PM

When you turn the focus knob clockwise one turn it would be better to turn it clockwise two turns and then counter clockwise one turn. You always want the last turn of the focus knob to be ccw. That way the mirror tilt will be the same for both cases and gravity won't cause a change later on. The top photo seems to show more out of collimation than the bottom photo.


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#5 MarMax

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Posted 05 August 2021 - 01:09 PM

Thanks for the replies so far. I always finish any focus adjustment with a CCW movement. The above images reflect this.

 

No trouble with star shapes. I'd just like to know what those with more knowledge of defocus images have to say. Like the bean shaped area on top and bottom, what is this called?

 

To me it looks pretty good, collimation is good with both a Duncan and Tri-Bahtinov mask, just looking for some feedback from folks more into ATM and optics.


Edited by MarMax, 05 August 2021 - 01:12 PM.

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#6 Mark Lovik

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Posted 05 August 2021 - 01:20 PM

I collimate an 8" Meade SCT visually -- it was the first thing I needed to really learn.  I received a scope so uncollimated you could not even see a star in the sky.  The difference after collimation was astounding.  So I naturally obsessed on the collimation process ..  even after it became habit and routine.

 

I never have needed to defocus by a whole turn for collimation (this is millimeters of change in the focal plane).  I suggest to try this

 

1. Find you focus point using the sensor.  Get it close but do not obsess over it.

2. Then defocus counter clockwise the minimum amount where the secondary spot is first clearly seen.

3. Clockwise (as suggested earlier) - over defocus clockwise a turn, then gradually turn the focuser counter clockwise.  Do this until the image is about the same size as seen in step 2.

 

You may need to change the exposure (and magnify the image displayed on the sensor) for the changed image spot size.

 

For final collimation (when seeing is good) -- try for the diffraction pattern and collimate it.  This may be hard on your bigger scope.


Edited by Mark Lovik, 05 August 2021 - 01:41 PM.


#7 Bill Barlow

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Posted 05 August 2021 - 01:44 PM

Your collimation looks very good, maybe a slight amount off in the top image between 3 and 9 o'clock.  But if the focused stars show a nice sharp and round airy disc with a faint evenly illuminated diffraction ring, I wouldn't touch it.  The bean inside the diffraction rings might be an oddly shaped heat plume?  

 

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Edited by Bill Barlow, 05 August 2021 - 01:47 PM.


#8 MarMax

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Posted 05 August 2021 - 01:47 PM

I collimate an 8" Meade SCT visually -- it was the first thing I needed to really learn.  I received a scope so uncollimated you could not even see a star in the sky.  The difference after collimation was astounding.  So I naturally obsessed on the collimation process ..  even after it became habit and routine.

 

I never have needed to defocus by a whole turn for collimation (this is millimeters of change in the focal plane).  I suggest to try this

 

1. Find you focus point using the sensor.  Get it close but do not obsess over it.

2. Then defocus counter clockwise the minimum amount where the secondary spot is first clearly seen.

3. Clockwise (as suggested earlier) - over defocus clockwise a turn, then gradually turn the focuser counter clockwise.  Do this until the image is about the same size as seen in step 2.

 

You may need to change the exposure (and magnify the image displayed on the sensor) for the changed image spot size.

 

For final collimation (when seeing is good) -- try for the diffraction pattern and collimate it.  This may be hard on your bigger scope.

Thanks for the reply Mark, but my question is not about collimation. I'm looking for some more information about the defocused star images. The C11 is collimated to 95% of the best it can be. If I had some excellent seeing I could bump that up to 98%.

 

The defocus star test was to get a sense of the optics quality but I don't know how to interpret what I see.


Edited by MarMax, 05 August 2021 - 02:00 PM.


#9 Electronchuck

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Posted 05 August 2021 - 03:53 PM

It is an oddly shaped plume. The plume should go upwards towards the sky in one photo as you show assuming that is skyward. If you reorient the tube clockwise or counter-clockwise along the eyepiece axis using the mount it should again point skyward even with tube rotation.

#10 MarMax

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Posted 05 August 2021 - 05:29 PM

I'm thinking there is a low or high area on the primary mirror. Considering the way they grind and polish the mirror it may  be more of a coatings thing. I would imagine a half a thousandth might make a difference. Something is a tad thicker or thinner in that bean shaped area. It seems to have a high center and low edge or a high edge and low center.



#11 Mike Spooner

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Posted 05 August 2021 - 08:59 PM

If you can halve the defocus distance (or even a third) it would provide a better sensitivity for the star test. The blotch may be dust on camera or Barlow lens. Rotating them between a couple of shots may help isolate where it is. This far outside focus it actually looks decent but closer to focus will give a more reliable assessment.

 

Mike Spooner


Edited by Mike Spooner, 05 August 2021 - 09:00 PM.

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#12 Asbytec

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Posted 06 August 2021 - 06:05 AM

I'm thinking there is a low or high area on the primary mirror. Considering the way they grind and polish the mirror it may  be more of a coatings thing. I would imagine a half a thousandth might make a difference. Something is a tad thicker or thinner in that bean shaped area. It seems to have a high center and low edge or a high edge and low center.

Yes, that makes sense. It's bright (with a dark ring) on one side of focus and dark (with a bright ring) on the other side. This tells me the defect focuses separately from the mirror to the same side of focus as the upper image. The edge of the defect (bright ring) focuses a little longer on the opposite side of focus. Looks almost like a very small area of correction error. If the upper image is inside focus, then the defect looks to be under corrected, i.e., a small depression. But, it also covers only a small area, so it may not have much of an affect on the image as a whole. I've never seen anything so dramatic in a star test, so it would be interesting to note how it behaves as you scroll through focus. 


Edited by Asbytec, 06 August 2021 - 06:23 AM.

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#13 MarMax

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Posted 06 August 2021 - 10:52 PM

I'll try it again as Mike suggests with a half focuser turn. I'll also be using the scope at f/10 (no barlow). To cover all the bases I'll take AVIs of 1/4, 1/2 and 1 turn for comparison. I was going to do it tonight but I'm too tired and lazy, so hopefully tomorrow.


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#14 rmollise

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Posted 07 August 2021 - 02:31 PM

You are a little too far out for a star test. And you need to let the scope spend more time acclimating to ambient temperature.


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#15 MarMax

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Posted 10 August 2021 - 11:06 AM

Conditions were not good but I did run the defocus tests again, this time with 1/2 turn and 1/4 turn. Star was Altair. No barlow so the scope is at f/10 and equivalent magnification is 308x.

 

Top left is 1/2 turn CW, Top right is 1/2 turn CCW

Bottom left is 1/4 turn CW, Bottom right is 1/4 turn CCW

 

gallery_332504_14303_76608.jpg

 

Interesting how the 1/4 turn exaggerates the mis-collimation in the 11am/5pm region.

 

So other than a collimation adjustment, what does this say to you experts about the state of the optics. And let me know if I need to do this again with some better conditions and better collimation.



#16 Asbytec

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Posted 10 August 2021 - 04:09 PM

Ah, okay, that looks very much like a thermal plume. though I've never seen a blob like that. Someone mentioned above, your scope needs to cool down to ambient temperatures. Or use an insulating wrap like reflectix. If those images are in the center of the filed of view, your scope need a little collimation tweak, too. The 1/4 turn is closer to focus at about 10 waves defocus. That's where you want to me for a star test. Possibly some (under?) correction error, but not bad from those images. 


Edited by Asbytec, 10 August 2021 - 04:36 PM.

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#17 MarMax

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Posted 10 August 2021 - 04:19 PM

Ah, okay, that looks very much like a thermal plume. Someone mentioned above, your scope needs to cool down to ambient temperatures. Or use an insulating wrap like reflectix. If those images are in teh center of the filed of view, your scope need a little collimation tweak, too. 

So that oval area is a thermal plume? Since it's top and bottom it could be. I'm wrapped in Refectix. So I'll run the cat cooler for an hour first and then try it again. And I'll collimate before more AVIs. It's amazing that I can get pictures like my 3rd try post in the Planetary forum with thermals like that. That just makes figuring this out much more interesting.

 

It also seems like the 1/2 turn defocus is about the place to be to collimate and evaluate things.



#18 Asbytec

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Posted 10 August 2021 - 04:45 PM

So that oval area is a thermal plume? 

 

It also seems like the 1/2 turn defocus is about the place to be to collimate and evaluate things.

Yes, it does appear to be a thermal artifact. See how it seems to originate at the obstruction shadow and stretch toward the edge. That's a classic thermal plume.

 

You can start collimation at a 1/2 turn, but the closer to focus the more precise your collimation will be. Really, in focus is the best if seeing allows a good look at the focused image at higher magnification. But, slight defocus to see the Poisson spot in the center of one diffraction ring is more precise. And the scope should be thermally stable, too. The plume will confuse the results. Always remember, we collimate on diffraction, not the image of the shadow. Simply centering the shadow is not good enough. 


Edited by Asbytec, 10 August 2021 - 04:48 PM.

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#19 MarMax

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Posted 11 August 2021 - 02:01 PM

I had the rig out early last night, ran the cat cooler for a good 90 minutes and then the clouds rolled in. 

 

I had just taken a 30 minute drive to buy a old Meade 2080 8" f/6.3 Wide Field OTA so I should have known better. It's looking like clouds for a couple days.



#20 Migwan

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Posted 11 August 2021 - 09:51 PM

MarMax, my compliments on the pictures and to think they can only get better as you resolve those pesky collimation and thermal issues. 

 

Regarding the thermal issues, what's your temperature delta between storage and viewing?   Also, I'm thinking that you are probably not using a diagonal in the pics of the star tests.  Is that correct?

 

To my eyes, there's a big difference between the original star test with the Barlow and the later ones without the Barlow, that wouldn't be expected even with the different number of turns out of focus.  Something changed and suggests that the Barlow misaligned the collimation in a manner that at least in part, corrected it.  

 

I have read that one should collimate with the diagonal that will be used or without one, if one will not be used.  I have a cheap Barlow that does not alter collimation and a Denk power switch that also does not appear to alter collimation, but does shift the image's position significantly.  I can't say with any certainty what caused the difference between those shots, but think you might want to check that.  Maybe the Barlow was just loose.

 

One last bit of wind, I find it much easier to collimate and more often than not, to just check collimation, at high powers in focus on a mag 3 to 4 stars.  The closer you can get to focus the better.    



#21 MarMax

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Posted 11 August 2021 - 10:31 PM

Thanks for the feedback jd.

 

MarMax, my compliments on the pictures and to think they can only get better as you resolve those pesky collimation and thermal issues. 

 

Regarding the thermal issues, what's your temperature delta between storage and viewing?   Also, I'm thinking that you are probably not using a diagonal in the pics of the star tests.  Is that correct?

 

The C11 is in the garage which goes from cool to warm each day. I have a routine to open and ventilate the garage each day so the inside does not get much warmer than ambient. The scope has a Reflectix wrap and is covered in a sheet with another piece of Reflectix draped over it (to reduce the radiant heat that comes off the metal roof). Temps lately have been 85-90 during the day and 68-73 at night. My feeling is that the primary mirror is tracking somewhere in between the extremes.

 

To my eyes, there's a big difference between the original star test with the Barlow and the later ones without the Barlow, that wouldn't be expected even with the different number of turns out of focus.  Something changed and suggests that the Barlow misaligned the collimation in a manner that at least in part, corrected it.  

 

The second round at f/10 used quite a long bunch of M42 spacers to put the camera back where the primary was at the mid-point of travel for focus. It's possible that all this stuff is the cause of the obvious mis-collimation in the photos. Good catch!

 

I have read that one should collimate with the diagonal that will be used or without one, if one will not be used.  I have a cheap Barlow that does not alter collimation and a Denk power switch that also does not appear to alter collimation, but does shift the image's position significantly.  I can't say with any certainty what caused the difference between those shots, but think you might want to check that.  Maybe the Barlow was just loose.

 

I'd trust the 2" BARADV more than the f/10 setup and I'm fairly certain the collimation is not off like it shows in the second round of photos. Collimation that far off would have showed up as some softness in my last round of Jupiter and Saturn pics.

 

One last bit of wind, I find it much easier to collimate and more often than not, to just check collimation, at high powers in focus on a mag 3 to 4 stars.  The closer you can get to focus the better.    

 

I've not been able to do that yet so I must be missing something. I can barely distinguish the airy disc when in focus.

Thanks for pointing out the collimation difference between the barlow and no barlow sessions. I had not even processed that since I knew that nothing changed with the collimation. The collimation has held up for months now without a change. I was even surprised by the small amount in the barlow photo because it looks spot on with the Tri-B and Duncan masks.

 

 



#22 Migwan

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Posted 12 August 2021 - 06:01 AM

Thanks for the feedback jd.

 

Thanks for pointing out the collimation difference between the barlow and no barlow sessions. I had not even processed that since I knew that nothing changed with the collimation. The collimation has held up for months now without a change. I was even surprised by the small amount in the barlow photo because it looks spot on with the Tri-B and Duncan masks.

I did catch that you were insulated.  I stored in an unheated pole barn and found that if the delta was greater than around 6° or if the temperature drop significantly during viewing, the insulation ultimately fails.   That is, initial star tests were great, but later ones showed thermals.   I added fans and my views of the planets and airy disks became much more consistent, though still very dependent upon good seeing. 

 

Pickering scale.   You should be able to evaluate an airy disk, though it is unlikely to be as clean as with the 5" refractor shown in the animations, due to the larger aperture of the C11.  One difference is in the flaring.  I've had two nights where the refractor on top showed 10/10, but the C11's first ring had some slight flaring (8/10), though it was very well formed and very still.  Still is good!  Especially with the central disc.  Another difference is that when seeing is bad 1 to 3/10, your central disk will become quite large and broken/broiling within.  In good seeing and proper acclimation, it is smaller than what is shown in the animations, again, due to the aperture.

 

The point is not really about about the Pickering Scale, it's that thermals within an OTA are going to appear much the same as bad seeing.   It's hard to tell them apart unless you happen to have a refractor to compare with.  Not saying you should use one, rather be aware and look for consistency in an occasional in focus or out of focus star test.

 

Also, if you have a nice well formed first diffraction ring that is concentric to the central disk, you know your collimated.  Flaring to one side, not so much.  These in focus star test are much easier to see in mag 3-4 stars than mag 1 stars and at ridiculously high mags due to how small the central disc gets with more aperture.  I generally use 667x in my C11, but 500x might get it done.  


Edited by Migwan, 12 August 2021 - 06:03 AM.

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#23 Asbytec

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Posted 12 August 2021 - 06:31 AM

"To my eyes, there's a big difference between the original star test with the Barlow and the later ones without the Barlow, that wouldn't be expected even with the different number of turns out of focus. Something changed and suggests that the Barlow misaligned the collimation in a manner that at least in part, corrected it."

Yea, I agree. That's a pretty significant change in collimation. I'm not so sure, and could be wrong, but that much decenter is probably not anything in the focuser shifting. I believe the focuser is responsible for focal plane tilt, not coma. The latest images look like primary axial error from the secondary or the image was not near the center of the FOV.
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#24 MarMax

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Posted 12 August 2021 - 10:16 AM

Thanks for the continuing discussion, it's very helpful. It makes me think it's time to go back to the basics and the airy disc. I just grabbed this off the web and did a quick notation.

 

gallery_332504_14303_37486.jpg

 

Maybe I'm not even defining terms correctly. With good seeing in the C11 the two things that are clear (easy to see) are the airy disc and the first larger clear (empty) ring. I have not detected the first or other diffraction rings. Perhaps this is where 600x comes into play. And perhaps with the camera and an AVI it may come out. And in the C11 the airy disc is much smaller.

 

I'm always doing a star check for the airy disc and empty ring. It's my method of focus to shoot for the cleanest empty ring. But it's all so small that I don't think I could make my fine collimation adjustments using it. I prefer the Tri-B mask.


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#25 Migwan

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Posted 12 August 2021 - 12:14 PM

Just keep in mind that for the airy disc and first refraction ring to look that perfect, there has to be perfect seeing, acclimation within about 2° and near perfect collimation.  I've never seen that in the C11 (though I have in my little 80ED).  With my C11 I always have some slight flaring off the first diffraction ring.  As long as that flaring is equal on all sides, I know I'm collimated.  That and the ring is concentric to the airy disc.

 

I was never satisfied with the result from using a mask.  I could get the ring concentric, but the flaring was never quite so.   Maybe you'll have better luck with it than I did.

 

Norme's comment about having the image centered is also a consideration of note.


Edited by Migwan, 12 August 2021 - 12:15 PM.

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