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Ring around Vega? 14" ACF

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#1 MtnGoat

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Posted 31 August 2017 - 12:31 PM

Hi folks, 

I had an ACF conversion done on a 14" and just received it on Tuesday. I got it up on the mount but wildfire smoke was so intense Tues night I didn't get much of a test run, Vega barely showed, it focused fine, and I was very impressed with the amount of image shift during focusing..very little, even at 250x Vega was barely moving when I reversed direction. Boy will that make planetary imaging better!

 

Anyhow, yesterday we had a weather change and the smoke was gone last night, so I got to wring things out a bit. It does appear superior overall visually to the Schmidt configuration of my other 14" which I swapped off the mount...focus is easier, the image 'snaps' a little when it's close something I never had with the other one (that one was fine once you nailed it but finding the 'nailed' point was subtle). Stars are much better pinpoints and easier to get there. 

 

What I don't know about is this...bright stars like Vega show a very crisp, narrow ring around them. With my 14mm Meade series 5000 80 degree eyepiece (253x, 0.3 deg TFOV) , the ring is about 80% of the field wide. I think the ring expands and thins as I hit focus, but I'll have to recheck that. 

 

It was also detectable on alignment stars like Alpheratz. 

 

I don't recall ever seeing this on the other 14 with the schmidt style optics.

 

I looked at M13, M92, some other cluster type targets and the ease of focus and pinpoint stars are great...but the ring has me worried. 

 

Opinions? 


Edited by MtnGoat, 31 August 2017 - 01:06 PM.


#2 MtnGoat

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Posted 31 August 2017 - 01:52 PM

The thread here leads off with a pic of exactly what I'm seeing, in this example it's a 12" ACF. 

 

https://www.cloudyni...r-bright-stars/


Edited by MtnGoat, 31 August 2017 - 01:52 PM.


#3 nitegeezer

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Posted 31 August 2017 - 02:18 PM

WOW!  That ring would be ugly.  I am curious if you could mask off areas to see where the reflection is coming from.  Maybe make a cardboard mask that would stop it down to 10 inches, and maybe another that would make the central obstruction up to 6 or 8 inches.  If you can find out an area the causes the ring then maybe you could correct it.



#4 GlennLeDrew

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Posted 31 August 2017 - 03:17 PM

At first blush it looks to be a variation of a Schmidt ghost. But if this ring remains "centered" upon the light source anywhere the latter is placed in the FoV, it is not your typical Schmidt ghost. Such ghost reflections normally are centered on the opposite side of the field from the object producing them.

 

I don't think this is the result of any reflection/scatter from e.g. a baffle wall, because a centered object surely (?) has its light cone passing cleanly through the system without clipping any baffle aperture and such.

 

It would be interesting to see what this ring does when the object is panned to one side of the field. A distant streetlight might serve on a *ahem* cloudy night. ;)

 

If the ring remains fixed in angular size, thus changing in apparent size with magnification, this could suggest it results from an internal reflection within the Schmidt corrector. The aspheric curve could induce the bright rim aspect. Just a first guess...



#5 GlennLeDrew

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Posted 31 August 2017 - 03:19 PM

By the way, if Meade says this is normal, why don't they specify the cause?



#6 MtnGoat

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Posted 31 August 2017 - 03:36 PM

Thanks Glen and NG. 

 

Ok, ideas to  try...weather is looking excellent tonite so I should be able to git er done

 

mask down from full aperture, also mask upsize from central obstruction

move bright star around the field

 

observe effects, if any, upon the ring/halo


Edited by MtnGoat, 31 August 2017 - 03:36 PM.

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#7 carolinaskies

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Posted 31 August 2017 - 07:46 PM

You didn't mention if this occurs with other eyepieces.  Of course the basic approach is elimination, elimination, elimination.   I'd start with no diagonal just straight through the telescope.  Trying different eyepieces to see if it goes away or not.  If it goes away at all then you know it's not main optics.  Also, have you tried an ND filter to see if the ring disappears?  

One possible cause could be a secondary baffle reflection if the edge facing the main mirror somehow wasn't coated.  If you look through the back of the telescope at the secondary while looking toward a bright light source check to see if you're getting any reflection from that baffle.  

 



#8 MtnGoat

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Posted 31 August 2017 - 10:25 PM

It occurred with two eyepieces yes. I'll try more. I'm prepping the masks now and it's clear. 

 

Mainly I want to establish that is it not a glass or coatings flaw, since I have a limited time to return it. 



#9 MtnGoat

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Posted 31 August 2017 - 10:56 PM

Current scene at the observatory

IMG_5332sm sm.jpg



#10 MtnGoat

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Posted 31 August 2017 - 11:25 PM

ready

IMG_5334sm.jpg



#11 MtnGoat

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Posted 01 September 2017 - 01:13 AM

dew shield

dew shield+10' aperture mask

dew shield

no dew shield

no dew shield 10' aperture mask

no dew shield co mask on

no dew shield co mask off

no dew shield co mask on
no dew shield co mask off

 

ccw focus series

cw focus series

rotate diagonal 90

ccw focus series



#12 MtnGoat

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Posted 01 September 2017 - 03:43 AM

Vega. I was just snapping 'em off and it was windy, so some shots do not have perfect stars. Canon Rebel T3 at 6400 ISO, prime focus via a diagonal. Edit 9/1: Applied identical stretch to each image to bring out halo

 

Full aperture no dew shield (tried dew shield it made no difference). All images without dew shield. 

14 Meade ACF full aperture 8 sec.jpg

 

Stopping the aperture down to 10 inches did little.

14 Meade ACF 10in aperture mask  8 sec.jpg

 

 

A 6" secondary mask made a huge difference.

14 Meade ACF 6in secondary mask  8 sec.jpg

 

The halo follows the star off axis.

14 Meade ACF full aperture off axis 8 sec.jpg


Edited by MtnGoat, 01 September 2017 - 11:53 PM.


#13 MtnGoat

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Posted 01 September 2017 - 03:50 AM

All eyepieces show the ring. The ring's apparent size is dependent on the magnification. With two 30 mm eyepieces, of 80 and 60 degree AFOV, the halo stayed the same size (half field for the 80 degree unit). With a 14mm 80 degree eyepiece, the halo nearly fills the field. With a 5.5 mm 80 degree ocular, the halo appears gone..until you pan a bit and find the edge, it's still there all right. 

 

 

The ring tracks the and stays round off to the side in eyepieces as well. 


Edited by MtnGoat, 01 September 2017 - 03:51 AM.


#14 csauer52

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Posted 01 September 2017 - 05:29 AM

Looks to me like you're not at critical focus, you have collimation issues or both are at play.

 

I'm looking at the other stars in the images, none of which are round points. Even if tracking were bad I don't believe it would show up in such short exposures. The f8 ACFs are very sensitive to collimation and even the slightest misalignment causes funky image quality, including strange reflections. What's a slightly out of focus mag 3 or 4 star look like? Have you checked collimation?

 

The other stars in the image look like out of focus stars in my f8 14" when I'm running the auto focus routine in Maxim and it's approaching best focus (but still off).

 

I hope this helps.


Edited by csauer52, 01 September 2017 - 05:35 AM.


#15 MtnGoat

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Posted 01 September 2017 - 10:18 AM

Here's a pair from my focus runs, not very well matched...I'll have to look at my notes to figure out which is inside and which is outside, but it seems pretty closely collimated to me

 

collimation.jpg



#16 carolinaskies

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Posted 01 September 2017 - 11:03 AM

The 6" secondary mask essentially eliminates the issue appearing.  which is essentially like increasing the central obstruction.  This means 'near focus' the light cone from the primary is larger than the baffle and is reflecting off the corrector plate inducing the halo.  That's my assumption.  

*I'm assuming you're not using an external focuser but using the internal crayford.  

To test my assumption it is the light cone on this bright star, rack the primary either side of focus which varies the size of the light cone hitting the secondary. Then using your 30mm eyepiece, try to get it into focus on either side of the primary being racked in/out.  Do this by giving yourself about 1/4" of inward/outward movement in the eyepiece barrel.   If I'm correct, I believe on one side you should eliminate the reflection as the cone goes inside the baffle, and the halo should disappear.  

At least that's my supposition. 

 



#17 GlennLeDrew

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Posted 01 September 2017 - 11:13 AM

The optics are good enough. Here's what I'm thinking is going on, reinforced by the results with the aperture masks.

 

There is an internal reflection within the corrector. The strong blue tint would be due to the AR coating. The corrector over at least its central region acts as a weak convex lens, wherein the internal reflection is directed at a small inward angle, crossing the optical axis. The smoothly varying slope acts to concentrate some of this light into the prominent ring at the edge. The outer corrector, with its more rapidly varying slope making up the Schmidt curve, disperses most of its contributing light throughout the pattern.

 

And so the brighter ring results from the inner portion of the pupil's annulus, as shown by its marked reduction via the larger central obstructor.

 

To further ascertain that this is the case, a mask in the form of a broad occulting bar of width roughly 4" could be tried. Most telling would be a bar having one end given a concave shape of radius equal to that of the secondary obstruction. In use, you could have this curved edge just barely letting light through the narrow gap formed by the equally curved edges. Then increase this gap width in stages by pulling the bar outward.



#18 GlennLeDrew

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Posted 01 September 2017 - 11:20 AM

Note on which side of the image the occulting bar's impact occurs. If on the opposite side, this would accord with my theory of the internal reflection within what is effectively a weak positive lens.

 

It would be worthwhile to compare the form of the Meade vs Celestron Schmidt corrector... Perhaps a form starting from a weak positive lens is more prone to this reflection than is a corrector having a flatter underlying shape.

 

This illustrates the importance of ghosting analysis during optical design.


Edited by GlennLeDrew, 01 September 2017 - 11:22 AM.


#19 GlennLeDrew

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Posted 01 September 2017 - 11:35 AM

A better mask form. Start with a 90 degree wedge, like a fat slice of pie ;) , that obscures a 1/4 sector. Then cut out a central 'hole' that presents a narrow gap between its edge and the secondary. Then progressively cut a larger 'hole' so as to make for an increasing width in the arcuate gap. Of course, a set of such masks is best. The annular gaps could be made to widths of, say, 1/4", 1/2", 1", 2" and 4" (not forgetting no gap at all.) A string could be stretched across the front so as to support those masks whose inner edge does not rest against the secondary support, if not otherwise supporting the mask so that it does not touch the corrector.



#20 MtnGoat

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Posted 01 September 2017 - 12:33 PM

Hi guys, this is great stuff! thanks for your considerations, I don't want to have to send back a heavy OTA but I will if I have to, god knows I paid enough for it. I probly should have cranked the levels on the pics, on a different monitor this morning I can't even see the halo in them. anyway...

 

Ok, so if I understand the arguments, this is what's happening

SCT halo.jpg

 

The fatter part of the light cone is missing the secondary and baffle and striking the corrector instead, resulting in a halo which is the combination of the spherical primary curve and the corrector surface? Why isn't the AR coating knocking this down...or is it? 

 

 



#21 MtnGoat

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Posted 01 September 2017 - 12:37 PM

If I understand CS's suggestion, it involves racking the primary in and out and then making up the difference via manually shifting the eyepiece back and forth through the focal plane.

 

With the primary forward, the cone is fatter and misses the outer regions of the secondary and baffle, and shifting the eyepiece back to the focal plane physically will still result in the halo... due to the primary position.

 

With the primary racked backwards, the cone may shrink enough to fit within the secondary as intended with none of it striking the corrector, thus shifting the eyepiece physically to the focal plane will reveal the halo is gone

 

is that about right? 

 

My only question here is this...if the CO I put in is *external* to all the glass because it's taped to the secondary, how can it impact the reflections from the region of the corrector immediately surrounding the secondary? So light isn't entering the OTA from the blocked area around the secondary, isn't it still bouncing from everywhere else around it anyway? 

SCT halo secondary mask.jpg

 


Edited by MtnGoat, 01 September 2017 - 12:41 PM.


#22 MtnGoat

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Posted 01 September 2017 - 12:48 PM

The idea of an internal reflection from the rear of the corrector:

 

1) I do in fact have a micro focusser in place in the optical path, I just realized this..it adds at least two inches meaning the mirror is that much farther forward

2) I don't recall noticing this on the other non ACF OTA I just swapped out, I used the microfocuser there too

3) As mentioned, I don't get how a CO mask external to the system would change internal reflections from the inside near the CO from the inside

 

this all suggests I need to try the system with the microfocuser removed. Hitting focus in that condition will have the primary farther back and thus maybe the light cone down to it's narrower point. 



#23 MtnGoat

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Posted 01 September 2017 - 03:17 PM

Is this what you mean, Glenn? 

90 deg zone mask.png



#24 csauer52

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Posted 01 September 2017 - 04:02 PM

The idea of an internal reflection from the rear of the corrector:

 

1) I do in fact have a micro focusser in place in the optical path, I just realized this..it adds at least two inches meaning the mirror is that much farther forward

2) I don't recall noticing this on the other non ACF OTA I just swapped out, I used the microfocuser there too

3) As mentioned, I don't get how a CO mask external to the system would change internal reflections from the inside near the CO from the inside

 

this all suggests I need to try the system with the microfocuser removed. Hitting focus in that condition will have the primary farther back and thus maybe the light cone down to it's narrower point. 

By any chance is it the standard Meade microfocuser? I ask as I had really bad reflections on my CCD when using the Meade which disappeared entirely when I switched over to an Optec for the ability to run a TC model.

 

Also, collimation does appear to be slightly off in the image on the left (to my eyes). The 07:00 position appears ever so slightly larger than the 13:00 position. Of course it could also just be my eyes.



#25 MtnGoat

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Posted 01 September 2017 - 04:20 PM

Yup, it's the meade MF, sure enough. 

 

I can see that I'm going to be OTA swapping this weekend and examining the old one for this issue

 

Tonite I'm going to tear everything off the visual back and see if I can hit focus with cam or eyepiece directly at the back, shortest path length possible. Then I'll put the diagonal in without the microfocuser, etc and keep working the focal plane back as these devices require me to move the primary forwards


Edited by MtnGoat, 01 September 2017 - 04:56 PM.

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