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Does this look collimated?

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#1 rkelley8493

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Posted 22 July 2019 - 06:59 PM

I had very good seeing conditions last night, but I didn't get a very clean view of Saturn. In my 5" Apo, I can easily see the Cassini Division as a thin line encircling the ring system, and the northern equatorial band is a darker tawny colored band. In my SCT, the division seemed kind of stuttering its way around the rings, and the NEB was mottled. It wasn't near as clean or crisp using the same magnification as the refractor. So.. does this look like it's out of collimation? Or is this just a factor of the SCT design vs Apo refractor?

 

collimate.png

 

Edit: I'll also add that when viewing the Wild Duck Cluster [M11], the stars were pinpoint and very detailed. It didn't look at all like a fuzzy ball but looked like fireworks expanding outwards. I could have probably counted the individual stars it was so detailed. But I know stars behave differently compared to the planets.


Edited by rkelley8493, 22 July 2019 - 07:09 PM.

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#2 theApex

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Posted 22 July 2019 - 07:33 PM

From what I see here in my smallish phone screen, isn't the direction towards the 1 o'clock mark more compressed than the 7 o'clock one, so to speak?


Sent from my Mi A1 using Tapatalk

Edited by theApex, 22 July 2019 - 07:34 PM.

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#3 fullotto

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Posted 22 July 2019 - 07:36 PM

i agree with the above poster. but, i really have never been too picky when i got that close.

seems when i try to tweek a tad better, i only make it a little worse.

 

last time i collimated it wasnt much worse than that pic and i was completely satisfied


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#4 S.Boerner

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Posted 22 July 2019 - 07:38 PM

I always had trouble deciding so long ago I found this page:

http://sweiller.free...ollimation.html

 

At the very bottom is a piece of software called Mere de Collimation that draws an overlaying bulls eyes that makes judging easier.

 

It shows what the two prior posts suggest.


Edited by S.Boerner, 22 July 2019 - 07:45 PM.

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#5 J A VOLK

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Posted 22 July 2019 - 07:54 PM

Collimation is slightly off, as 'theApex' stated. Closed tube SCT/ACF scopes are hard to cool and suffer from internal heat currents, causing what you describe. Planetary viewing is the most discriminating type, and highlights issues that are less apparent than on other objects.
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#6 Chris MN

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Posted 22 July 2019 - 08:30 PM

Try this.  Start with the star out of focus like your pic.  Then bring the star into focus and watch the dark center spot in relation to the outer rings.  In perfect collimation, the image above should "collapse upon itself" with the dark center spot remaining centered until the star is focused.  Think of it kinda like a funnel.  If the star does not collapse upon itself like a funnel and the dark spot is not centered, adjust collimation until it does.  It doesn't take much tweaking.  But the results are worth it.  My SCT is a carbon fiber C11 about 2006 vintage.

 

My 2 cents.

 

Chris N

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#7 whizbang

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Posted 22 July 2019 - 08:53 PM

I think it's off at 2 o'clock and 8 o'clock.

 

If it was mine, I would give it a slight tweak using higher magnification closer to focus.


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#8 Astrojedi

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Posted 22 July 2019 - 09:17 PM

I had very good seeing conditions last night, but I didn't get a very clean view of Saturn. In my 5" Apo, I can easily see the Cassini Division as a thin line encircling the ring system, and the northern equatorial band is a darker tawny colored band. In my SCT, the division seemed kind of stuttering its way around the rings, and the NEB was mottled. It wasn't near as clean or crisp using the same magnification as the refractor. So.. does this look like it's out of collimation? Or is this just a factor of the SCT design vs Apo refractor?

 

attachicon.gif collimate.png

 

Edit: I'll also add that when viewing the Wild Duck Cluster [M11], the stars were pinpoint and very detailed. It didn't look at all like a fuzzy ball but looked like fireworks expanding outwards. I could have probably counted the individual stars it was so detailed. But I know stars behave differently compared to the planets.

 

Your collimation seems off. There also seem to be thermal issues. Cassini division should be very easy in the 10” SCT.  

 

Collimate precisely and make sure the OTA is in thermal eqilibrium. 


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#9 rkelley8493

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Posted 22 July 2019 - 11:09 PM

Thanks for the replies everyone! Great feedback waytogo.gif  



#10 watchplanets

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Posted 23 July 2019 - 02:38 PM

Agreed bit off but not horrible try again to get bit closer
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#11 Nippon

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Posted 23 July 2019 - 02:54 PM

I have found that if the seeing is steady so I can really get the magnification up like 400 in my Edge 8 and I get a complete evenly illuminated first diffraction ring I'm good. You can play chase me Charley all night centering the donut. As soon as I get the donut close I go to a smaller donut get that looking centered then high power in focus and check the diffraction rings.


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#12 fullotto

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Posted 23 July 2019 - 04:11 PM

I have found that if the seeing is steady so I can really get the magnification up like 400 in my Edge 8 and I get a complete evenly illuminated first diffraction ring I'm good. You can play chase me Charley all night centering the donut. As soon as I get the donut close I go to a smaller donut get that looking centered then high power in focus and check the diffraction rings.

ditto !  exactly what i do.


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#13 starman876

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Posted 24 July 2019 - 04:54 PM

In an SCT all you have to be is just a little off and you no longer will see the detail you would if it was spot on.  The scope goes from 1/4 wave to 1/2 wave in a heartbeat if collimation is not spot on.  


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#14 rkelley8493

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Posted 28 July 2019 - 10:55 PM

Try this.  Start with the star out of focus like your pic.  Then bring the star into focus and watch the dark center spot in relation to the outer rings.  In perfect collimation, the image above should "collapse upon itself" with the dark center spot remaining centered until the star is focused.  Think of it kinda like a funnel.  If the star does not collapse upon itself like a funnel and the dark spot is not centered, adjust collimation until it does.  It doesn't take much tweaking.  But the results are worth it.  My SCT is a carbon fiber C11 about 2006 vintage.

 

My 2 cents.

 

Chris N

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Okay.. so an update on the collimation "issue". I use quotations because I wasn't sure if that was the problem or not. Turns out, that's exactly what it was.

When I had the star [Altair] severely out of focus, it still looked like a perfect donut to me. However, when I would bring it to focus, it did not "collapse upon itself" perfectly. The bottom part of the concentric rings was the last to pull into focus [this would explain the "stuttering" Cassini Division]. A couple of minute adjustments took care of the issue.

I was able to use up to 357x magnification on Saturn tonight without blurring or softening the image! Saw excellent detail, similar to that of my Apo, but at much higher magnification. I think the Apo may have had better contrast & color, but the differences were very subtle. Needless to say, the SCT really performed to its potential tonight waytogo.gif


Edited by rkelley8493, 29 July 2019 - 01:28 PM.

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#15 Nippon

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Posted 04 August 2019 - 12:01 PM

My Edge 8 is well collimated and on typical nights either of my 4" refractors seem to provide as much detail as the Edge 8 although dimmer in the refractors. But if it's a really steady night it's all over the Edge leaves the refractors behind.


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#16 rkelley8493

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Posted 04 August 2019 - 12:28 PM

My Edge 8 is well collimated and on typical nights either of my 4" refractors seem to provide as much detail as the Edge 8 although dimmer in the refractors. But if it's a really steady night it's all over the Edge leaves the refractors behind.

I've had a similar experience. Last night was very clear & dark. It seems like the skies are darker after a storm passes thru during the afternoon or early evening.

Anyways, I decided to use my SCT last night since I had used the refractor the last 2 or 3 nights out, and I was only planning on viewing Saturn in between the rolling clouds. But the clouds parted and it turned into a beautiful night. My first thought was, "I should've tried collimating this months ago." I was getting just as sharp of a view as I do in my Apo. The illusive moons Mimas & Enceladus were showing very clearly as two white points of light. The northern polar region was showing very distinct cloud banding, and I could make out the darker colored polar vortex.

Other objects that showed very good detail [UHC filter was used]:

Eagle Nebula - the "eagle's head" was very distinct, and I could make out the pillars in the center with another darker region off to the right [mirror image]

Swan Nebula - the clouds were very dynamic with structural differences

Crescent Nebula -  I had never seen it before. First sight was last night.



#17 Astrojedi

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Posted 04 August 2019 - 08:21 PM

In an SCT all you have to be is just a little off and you no longer will see the detail you would if it was spot on.  The scope goes from 1/4 wave to 1/2 wave in a heartbeat if collimation is not spot on.  

Correct... also note that this is true for all optical designs not just SCTs. Sometime we forget we are using very high precision scientific instruments.


Edited by Astrojedi, 04 August 2019 - 08:23 PM.

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#18 jhayes_tucson

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Posted 04 August 2019 - 09:49 PM

It's difficult to say for sure if that scope is aligned for a couple of reasons:

 

1)  In order to evaluate the defocused image, it must be precisely in the center of the field.  How close to center of the field was that image taken?

 

2)  Proper alignment is not determined by the defocused shadow position.  The defocused shadow position only shows roughly how well the system is aligned--and it starts with the assumption that the secondary is well centered mechanically within the entrance pupil, which may or may not be true.  In order to accurately evaluate system alignment, you have to bring the image into focus to evaluate the shape of the Airy disk.  On-axis coma will be minimized when the system is precisely aligned so you are looking for any small asymmetry in the Airy disk as you bring the image into focus.   This process is difficult on its own but it's especially difficult if the seeing is poor--so try to align the scope when it is thermally stable and the seeing is good.  As others have pointed out, the wavefront quality is very sensitive to small misalignments of the secondary so it's important to be very picky to get it just right.

 

 

John


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#19 starman876

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Posted 05 August 2019 - 07:20 AM

Correct... also note that this is true for all optical designs not just SCTs. Sometime we forget we are using very high precision scientific instruments.

Very good point.  I wonder how many tales of how bad my scope is was just the scope not being aligned properly.   


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#20 rkelley8493

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Posted 05 August 2019 - 09:11 AM

It's difficult to say for sure if that scope is aligned for a couple of reasons:

 

1)  In order to evaluate the defocused image, it must be precisely in the center of the field.  How close to center of the field was that image taken?

 

2)  Proper alignment is not determined by the defocused shadow position.  The defocused shadow position only shows roughly how well the system is aligned--and it starts with the assumption that the secondary is well centered mechanically within the entrance pupil, which may or may not be true.  In order to accurately evaluate system alignment, you have to bring the image into focus to evaluate the shape of the Airy disk.  On-axis coma will be minimized when the system is precisely aligned so you are looking for any small asymmetry in the Airy disk as you bring the image into focus.   This process is difficult on its own but it's especially difficult if the seeing is poor--so try to align the scope when it is thermally stable and the seeing is good.  As others have pointed out, the wavefront quality is very sensitive to small misalignments of the secondary so it's important to be very picky to get it just right.

 

 

John

Chris MN was very helpful with the "collapse upon itself" analogy. Although it looked collimated at first glance, the bottom part of the Airy disc was the last to pull into focus which made the sharpness off just enough to not give a crisp image. 



#21 Astrojedi

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Posted 05 August 2019 - 09:43 AM

As John mentioned, the airy disk is the best way to get precise collimation. But you must ensure it is centered in the fov to ensure optimal alignment of the secondary. Without collimating precisely and getting the scope to thermal equilibrium you cannot evaluate performance.

 

If the features seem like they are “jumping” around as you mentioned a lack of thermal equilibrium or poor seeing could also be factors here.

 

Irrespective of optical design larger apertures require better sky conditions to perform to their full potential. As the conditions get worse it becomes harder and harder to see the difference and people wrongly assume that ‘a small scope can match a larger scope’ or ‘X vs Y diff is aperture could not be seen at the EP. In fact in poor seeing a smaller aperture performs better. In truly steady skies the planetary images produced by my EdgeHD 8 and 10” & 14” dobs are truly stunning. If you experience poor seeing at your location a small refractor may be a better scope for you.


Edited by Astrojedi, 05 August 2019 - 09:46 AM.

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#22 Slashzero

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Posted 11 August 2019 - 05:15 PM

Glad I found this post. I’ve been comparing my 12 inch dob and my 8 inch SCT. I had Saturn razor sharp on the dob, but I could not get Saturn focused on the SCT. Note: this was just after I received the SCT, it was shipped. I did a star test, and the SCT needed collimation. I did a rough collimation, and focus was much better, but still not as sharp as the dob. I had thought perhaps that was just because I was comparing a 12 inch dob to an 8 inch SCT. 

 

Thanks to this post, I realize my collimation of the SCT must still be slightly off. I’m going to do a star test tonight, and use the suggested “funnel” / collapse into focus to see just how off I am. 

 

One question: when I did my collimation, one of the adjustment screws on the secondary felt like it had bottomed out. Would this warrant backing off all three adjustment screws to reset them and re-collimate from scratch?


Edited by Slashzero, 11 August 2019 - 05:18 PM.

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#23 rkelley8493

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Posted 11 August 2019 - 05:51 PM

Glad I found this post. I’ve been comparing my 12 inch dob and my 8 inch SCT. I had Saturn razor sharp on the dob, but I could not get Saturn focused on the SCT. Note: this was just after I received the SCT, it was shipped. I did a star test, and the SCT needed collimation. I did a rough collimation, and focus was much better, but still not as sharp as the dob. I had thought perhaps that was just because I was comparing a 12 inch dob to an 8 inch SCT. 

 

Thanks to this post, I realize my collimation of the SCT must still be slightly off. I’m going to do a star test tonight, and use the suggested “funnel” / collapse into focus to see just how off I am. 

 

One question: when I did my collimation, one of the adjustment screws on the secondary felt like it had bottomed out. Would this warrant backing off all three adjustment screws to reset them and re-collimate from scratch?

When I was collimating, I adjusted each of the screws slightly doing no more than a quarter turn on any adjustments. I wouldn't recommend backing off all of them at once, but just do minor adjustments until the Airy disc looks even all the way around. I also found that it was easier to make adjustments when the star was a little out of focus. When it was severely out of focus, it was more difficult to see the changes that were made.

Glad this thread helped you some, and good luck to you waytogo.gif

 

RK


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#24 Slashzero

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Posted 11 August 2019 - 05:56 PM

When I was collimating, I adjusted each of the screws slightly doing no more than a quarter turn on any adjustments. I wouldn't recommend backing off all of them at once, but just do minor adjustments until the Airy disc looks even all the way around. I also found that it was easier to make adjustments when the star was a little out of focus. When it was severely out of focus, it was more difficult to see the changes that were made.
Glad this thread helped you some, and good luck to you waytogo.gif

RK

The first time I tried to collimate my SCT, I wasn’t sure how out of focus it needed to be. I went too far, and got a perfectly round and even donut. Then I realized that couldn’t be right and brought it closer to focus. Then I could see it was way off, towards 11 o’clock. I got it pretty centered, just with minute, less than quarter turns. One of the screws felt like it was not going to move any further, however, so I left it alone at that point as “close enough”. After reading your post, I realized I should have kept going (I don’t mean going as in trying to force that one adjustment screw).

The sky is looking pretty cloudy right now, so I am not sure I will get to fine tune the collimation tonight. I want to get razor sharp focus, darn it! I suppose I could set it up anyway in the hopes that by the time it cools down the sky clears up?

Alternatively, is there a way to collimate indoors using some sort of artificial light source?

Edited by Slashzero, 11 August 2019 - 06:49 PM.

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#25 rkelley8493

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Posted 11 August 2019 - 07:10 PM

Alternatively, is there a way to collimate indoors using some sort of artificial light source?

I tried this with a flashlight at the end of the living room, but it was too close. I think you probably could if you were about 30-40 yards away, but not 30-40 feet away. I think street lights would work too, but I haven't tried with them before.

I usually set my SCT out an hour before darkness falls to try and get it thermally acclimated. I also set a fan on my back porch and have it oscillating, but that's more just to cool off the area where I'll be sitting  than trying to cool the scope.


Edited by rkelley8493, 11 August 2019 - 07:15 PM.



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