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SkyWatcher’s 20" goto truss dob!

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#201 TonyStar

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Posted 12 April 2018 - 10:40 AM

 you can see the actual mirror structure in the image! 

Thanks for sharing this info, you could run a Roddier test on your optics to get quantitative information on the defects as print-through etc. As I mentioned earlier in this thread, (see here) Ciel & Espace (French magazine) did an interferometric test of the stargate in 2016 and found the print-through was indeed an issue, although the bigger problem was astigmatism. Chances are the secondary was the culprit in the model under test. If you have seen no astigmatism at the eyepiece, your secondary is probably not as bad. Still, it would be cool to get some numbers with Roddier....


Edited by TonyStar, 12 April 2018 - 10:42 AM.


#202 PETER DREW

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Posted 12 April 2018 - 11:07 AM

I guess my question is why they can't just use the 3/4" disc without the ribs, and provide a quality mirror cell to support it.

 

Maybe 3/4" thickness over 20" is not feasible even with a 27+ point mirror cell though. 

I think mounting a 3/4" thick 20" mirror would be a solvable engineering problem and once achieved should be repeatable. However, there's nothing much repeatable in the production of a good quality mirror of those dimensions let alone quantity production. Just ask the current thin mirror wizards.



#203 vhinze

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Posted 12 April 2018 - 11:29 AM

I see that the notion of "print-through" has thoroughly grabbed some respondents imaginations and they are unreceptive to other proposed explanations.  It also seems safe to conclude that few participants in the conversation own or have access to Star Testing Astronomical Telescopes.

 

Surface roughness is a product of optical technicians in a hurry. No need to re-engineer the entire telescope to address an elementary production problem!

 

A Stargate in 2016- lets see, that's pushing two years ago. Our Vendor rep says the print-through problem has been solved. Are you guys implying he's a bald faced liar?

 

Fini.


Edited by vhinze, 12 April 2018 - 11:42 AM.


#204 Pinbout

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Posted 12 April 2018 - 11:35 AM

Hi all.

 

I have been doing solar system imaging with the SW 16" flextube goto Dob for years and have been very happy with the results, so I upgraded into the 20" Stargate goto Dob little more than a year ago. I tried and tried and could not get a decent image. Actually after 10 Terabytes of imaging solar system videos, not even one sharp frame!

 

I still have the 16", so for several times I set the 16" and 20" side by side for Lunar imaging, using the same imaging train, and 16" gives sharp focus, 20" not. Realising there is something wrong, I started taking star test images, and the results were clear: The 20" mirror is having some sort of zonal error and a huge print through from the mirrors internal rib structure. And by huge I mean that you can actually see the mirrors structure in the defocused star when the seeing settles for a moment...

 

Below is Dubbhe with ir-pass filter and ASI183MM camera + 1,5x Barlow. The 12 ribs of the mirror or 12 areas between the ribs create 12 bright blobs to the defocused star. And the focused star has 12 blurred and distorted star images around and on top of it.

 

_full.jpg

 

 

If this was not enough, when the star is defocused more, you can see the actual mirror structure in the image! Below is defocused Dubbhe and for comparison image of the main mirror I loaded somewhere from this forum (I havent removed my mirror at any point, it's as it was delivered from the factory)

 

_full.jpg

 

The scope performs like a 8" scope resolutionwise. I can use it for low power visual. More focal length I use, the worse the image gets... Stargate got a good review on S&T recently, but mine is basically useless...

please post these in the photo gallery under star tests / reflectors...



#205 Pinbout

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Posted 12 April 2018 - 11:38 AM

This has been a problem with every brand of scope containing a layered or ribbed mirror structure.

I don't have it on my 12.5in hubble optics

 

https://www.youtube....D0mD0JM&index=5


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#206 Pinbout

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Posted 12 April 2018 - 11:39 AM

I see that the notion of "print-through" has thoroughly grabbed some respondents imaginations and they are unreceptive to other proposed explanations.  It also seems safe to conclude that few participants in the conversation own or have access to Star Testing Astronomical Telescopes.

 

Surface roughness is a product of optical technicians in a hurry. No need to re-engineer the entire telescope to address an elementary production problem!

 

A Stargate in 2016- lets see, that's pushing two years ago.

 

Fini.

the lines are not upper atmosphere, they are print thru.  turbulence doesn't rotate around the center of the mirror.

 

those images show a great deal of print thru.


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#207 Pinbout

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Posted 12 April 2018 - 11:45 AM

 

Below is Dubbhe with ir-pass filter and ASI183MM camera + 1,5x Barlow.

 

barlows add correction so you can't use those images to judge correction.

 

and with that it still looks undercorrected.


Edited by Pinbout, 12 April 2018 - 11:45 AM.


#208 vhinze

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Posted 12 April 2018 - 11:52 AM

"the lines are not upper atmosphere, they are print thru."

 

Where in my post you quote do you see word one about turbulence or upper atmosphere?

 

Kindly go look at the images I cite. You're right, they're not upper atmosphere. As stated before, THESE DON'T CHANGE! They are, as best I can tell, identical to the images Suiter includes as textbook images of surface roughness.

 

It does no good to fixate on print-through on the basis of zero evidence. You certainly can't address the problem if you don't define it correctly. It isn't as if I deny there's a problem. I just haven't seen a stitch of evidence that pins it as "print-through".

 

Show me a known image of print-through that looks close to the posters image and I'll shut up.


Edited by vhinze, 12 April 2018 - 12:00 PM.


#209 Starman1

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Posted 12 April 2018 - 12:00 PM

"So maybe when the scope is fully thermally acclimated it will not cause a problem"

 

This is where the thermal coefficient comes into play. The main stated purpose of the ribbed mirror is to accelerate cooling. Reduction of weight may be minimal over, say, a two inch thick mirror. This mirror is said to be 3/4" thick, and with the ribs and secondary ring all the same thickness, probably equals the mass of a mirror 1 1/2 inch thick. So, a little less mass and more surface area should result in quicker equalization. Weird stuff occurs with any mirror that's used before it has equalized. What's more, the complainant states that the distortion he's concerned with is static, not changing. That conclusively rules out thermals.

 

But, all this emphasis on print through and thermals may be a red herring. I'd submit that the original images you print-through advocates are hanging your hats on support surface roughness much more strongly. I humbly suggest finding a copy of Harold Suiters book ( Star Testing Astronomical Telescopes) and referencing  the illustrations cited in the above post before drawing your conclusions.

 

"the ribs act like cooling fins and they would always be a different temperature than the rest of the mirror."

 

Equilibrium means just that, the ribs and mirror structure reach the same temperature as the surrounding air, heat flow and dimensional change ceases.

 

"For all the owners I have spoken to here in the US no one has complained about the optics"

 

Not that there's that many happy owners of this scope yet. At least, if there are, they're a close-mouthed group. In fairness, I'd also reiterate that the scope in question is over a year old and hopefully not representative of current production.

Then how do you explain the 12 ribs in the mirror and 12 linear patterns in the star image in his last image?

I admit, it's possible these would go away when the mirror is at thermal equilibrium, but I'd like to note that I have never seen an interferogram of a multi-piece mirror that did NOT show print through.

When the mirror is being made, the surface heats differentially with such mirrors, which makes it extremely time consuming to make them.

That many such mirrors are actually less expensive than the top of the line mirror makers says something about that process.


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#210 mark379

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Posted 12 April 2018 - 12:12 PM

My 14" SW collapsible does not seem to have this problem either. Perhaps the ribs are designed differently on the 18/20 inch?

I don't have it on my 12.5in hubble optics

 

https://www.youtube....D0mD0JM&index=5



#211 Pinbout

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Posted 12 April 2018 - 12:14 PM

"the lines are not upper atmosphere, they are print thru."

 

Where in my post you quote do you see word one about turbulence or upper atmosphere?

 

Kindly go look at the images I cite. You're right, they're not upper atmosphere. As stated before, THESE DON'T CHANGE! They are, as best I can tell, identical to the images Suiter includes as textbook images of surface roughness.

 

It does no good to fixate on print-through on the basis of zero evidence. You certainly can't address the problem if you don't define it correctly. It isn't as if I deny there's a problem. I just haven't seen a stitch of evidence that pins it as "print-through".

 

Show me a known image of print-through that looks close to the posters image and I'll shut up.

not surface roughness, cause they clock around the mirror.



#212 vhinze

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Posted 12 April 2018 - 12:21 PM

"Then how do you explain the 12 ribs in the mirror and 12 linear patterns"

 

Indeed, I looked closely at the images he posted. I tried to count twelve ribs or lobes and counted the bright "globs" of light he mentioned. That's part of my problem with basing this print-through conclusion on those images. I simply don't see twelve clearly defined anythings (ribs, lobes or globs) in those images. The images are annular, seem to exhibit some sort of regularity, but I can't see a convincing representation of the mirror structure in the images.

The images do, however, strongly resemble the images in figure 13-5 of Suiter's book, annularity, regularity, "ribs" ,"lobes" and all. The man calls the condition primary ripple, also known as surface roughness or "dog biscuit".  I invite you to take a look for yourself. If I had access to the book right now, I'd gladly scan an image and show it to you right here. Perhaps I'll do just that when I get home tonight.


Edited by vhinze, 12 April 2018 - 12:24 PM.


#213 Starman1

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Posted 12 April 2018 - 12:25 PM

I don't have it on my 12.5in hubble optics

 

https://www.youtube....D0mD0JM&index=5

I'm not sure you could see the patterns created by the Hubble construction in seeing that bad.

I think you'd need to do a test bench image.

I did mention interferogram.

The issue with the Hubble mirrors, as I recall, isn't so much print-through as edge support-related issues due to the fact they cannot be supported at the COG.

I believe that is overcome by a support for both front and rear discs simultaneously.  If they are supported only on one disc, there is substantial print-through

at each contact point due to flexure.

I wonder what happened to the primary mirror in the prototype Sky Watcher?  It had front and rear discs joined by small tubes.  Wonder what happened as the air pressure

in those tubes changed with temperature?



#214 Starman1

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Posted 12 April 2018 - 12:28 PM

"Then how do you explain the 12 ribs in the mirror and 12 linear patterns"

 

Indeed, I looked closely at the images he posted. I tried to count twelve ribs or lobes and counted the bright "globs" of light he mentioned. That's part of my problem with basing this print-through conclusion on those images. I simply don't see twelve clearly defined anythings (ribs, lobes or globs) in those images. The images are annular, seem to exhibit some sort of regularity, but I can't see a convincing representation of the mirror structure in the images.

The images do, however, strongly resemble the images in figure 13-5 of Suiter's book, annularity, regularity, "ribs" ,"lobes" and all. The man calls the condition primary ripple, also known as surface roughness or "dog biscuit".  I invite you to take a look for yourself. If I had access to the book right now, I'd gladly scan an image and show it to you right here. Perhaps I'll do just that when I get home tonight.

I see the images in Suiter's book, and they do look like the images posted.

However, I count 12 linear lines in that last image and there are 12 ribs.

I'm just sayin'......


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#215 vhinze

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Posted 12 April 2018 - 12:32 PM

Percival Lowell had a penchant for counting things. The mind has a penchant for finding what it's looking for.

 

Just sayin...


Edited by vhinze, 12 April 2018 - 12:33 PM.


#216 vhinze

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Posted 12 April 2018 - 12:49 PM

Here's an idea!

 

Round up two or three people with no dog in this fight, show 'em the images and ask them to count the lines, ribs or whatever and see what the consensis is.


Edited by vhinze, 12 April 2018 - 12:50 PM.


#217 Arctic eye

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Posted 12 April 2018 - 01:21 PM

Well... Lets put more images so you can judge By yourselves. Remember, I was taking a lot of videos, not just some single image on one occasion, so I can stack frames. Lets take 100-200 frames and stack to get rid of random effects and turbulence effects and what you get? You get the top row of the image below and 12 blobs. You get always 12 blobs, like in the other rows of the image below. And they are at the same place. And when you defcus more, you start to get linear lines.

 

I know that there is an octagonal storm staying foot on Saturns polar area, but I have never heard of dodecaedronal turbulence, especially such that tracks Dubbhe for several days? 

 

_full.jpg

 

 

 

Actually I had it wrong on the original posting, it was not 1,5x Barlow I used. It was Televue Powermate 2,5x at that stage. Below is another set from another day. I was imaging low hanging Betelgeuse with no filter at all, so the camera saw everything from UV to IR. I was chasing possible astigmatism, but found none. With the large spectrum, you dont get proper star testing images, because you dont get proper diffraction rings without defocusing too much. But when I stacked the videos, those 12 linear lines start to appear again. There are also secondary mirror holders visible and enlarged By atmospheric dispersion.

 

_full.jpg

 

 

 

 

These markings actually appear when the turbulence calms down and are strongest at the calmest moments. And when the turbulence and seeing settles for a few seconds, I get this every time:

 

_full.jpg

 

There seems to be people with very strong opinions that this is not print-through. I would love it to be true, so please give me an alternative theory.

 

 

 

 


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#218 slavicek

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Posted 12 April 2018 - 04:49 PM

I will comment on S&T (and other) magazine reviews. In my opinion, they are kind of "informercials". There is very little critique of the product in review. Even thou some of the products are very "inexpensive", so they cannot be that good. When one reads the review, one has to kind of read between the lines. All products end up with 4, 4.5 or 5 stars. Wow! So,for example, one of my rules of thumb is that if it's 4 stars it's not very good. And all (or most) of the products reviewed are mass produced. Maybe I missed it but have you, for example, seen review of Obsession? My 2c.



#219 Starman1

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Posted 12 April 2018 - 05:46 PM

There is an "absolute" ranking in a review, and a "relative" ranking in a review:

 

--Relative: compared to other products at the same price or of the same type, how does this one stack up?

--Absolute: Compared to perfection in construction or quality, how does this one stack up?

 

I think reviews tend to use relative rankings.

If they really used absolute rankings, they'd need a 100 point scale or they'd go negative on most inexpensive scopes.  And only a few scopes would score in the high 90s.


Edited by Starman1, 12 April 2018 - 05:46 PM.


#220 vhinze

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Posted 12 April 2018 - 05:56 PM

"There seems to be people with very strong opinions that this is not print-through. I would love it to be true, so please give me an alternative theory."       
 

Arctic Eye

 

I seem to be the lone hold-out for an alternative theory.  You make a good argument, having looked at the additional jpegs, it's pretty easy to count twelve lines, zones or lobes in those images. I'm still not convinced you have print-through, however, because there is a lot of irregularity in the location and size of these features depending on how far from focus you are.   I accept that the defect is in the primary, but maintain that the appearance of twelve lobes may be a coincidence. That is not to say 12 lobes aren't there, just that the regularity is not sufficiently striking for me to conclude it represents an image of the mirror structure.

The jury is still out in my mind because the images you present still appear to be ringers for the ones Harold Suiter saw fit to publish as definitive for roughness. His commentary also emphasized the similarity between the signatures of roughness and turbulance. He states it is nearly impossible to detect roughness using the star test in the presence of turbulence. This mirrors your remarks.

 

Your last two images have a fine shadow of the spider, no controversy there. But, of course, no bearing on the subject of print-through. I see the bands, but they're not in the same location in both photos.

 

Also, in my mind, Suiter is "The Man". I don't discount his guidance lightly.

 

Finally, it seems moot what the defect is, you have optics that don't measure up and the manufacturer needs to make you whole. I reckon that means a replacement mirror at minimum.

 

Obsession doesn't advertise, they don't need to. Word of Mouth.  If you don't buy advertising space, don't hold your breath for a review of your product. Furthermore, if you can't say something nice, say nothing at all. Not rocket science.


Edited by vhinze, 12 April 2018 - 06:19 PM.


#221 Pinbout

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Posted 12 April 2018 - 07:20 PM

Well... Lets put more images so you can judge By yourselves. Remember, I was taking a lot of videos, not just some single image on one occasion, so I can stack frames. Lets take 100-200 frames and stack to get rid of random effects and turbulence effects and what you get? You get the top row of the image below and 12 blobs. You get always 12 blobs, like in the other rows of the image below. And they are at the same place. And when you defcus more, you start to get linear lines.

 

I know that there is an octagonal storm staying foot on Saturns polar area, but I have never heard of dodecaedronal turbulence, especially such that tracks Dubbhe for several days? 

 

_full.jpg

 

 

 

Actually I had it wrong on the original posting, it was not 1,5x Barlow I used. It was Televue Powermate 2,5x at that stage. Below is another set from another day. I was imaging low hanging Betelgeuse with no filter at all, so the camera saw everything from UV to IR. I was chasing possible astigmatism, but found none. With the large spectrum, you dont get proper star testing images, because you dont get proper diffraction rings without defocusing too much. But when I stacked the videos, those 12 linear lines start to appear again. There are also secondary mirror holders visible and enlarged By atmospheric dispersion.

 

_full.jpg

 

 

 

 

These markings actually appear when the turbulence calms down and are strongest at the calmest moments. And when the turbulence and seeing settles for a few seconds, I get this every time:

 

_full.jpg

 

There seems to be people with very strong opinions that this is not print-through. I would love it to be true, so please give me an alternative theory.

I’m assuming in the second image the left is inside?

your second image is showing a kink, a bright ring @ .8 zone, inside focus and a similar shadow same outside focus

 

the 1st set show undercorrection

 

The second set show overcorrection but still has the bright ring around the co on inside focus

 

just remember bright stuff inside focus are zones lower than the rest of the mirror

bright stuff outside focus are zones higher than the rest of the mirror

 

so that .8 zones is lower and around the co is low.

 

the 1st set of images are better for reading correction

your second set are less sensitive to correction but better for zones as I can see the kink which is a valley at the .8 zone

 


Edited by Pinbout, 12 April 2018 - 07:39 PM.


#222 Kunama

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Posted 12 April 2018 - 07:38 PM

Well... Lets put more images so you can judge By yourselves. Remember, I was taking a lot of videos, not just some single image on one occasion, so I can stack frames. Lets take 100-200 frames and stack to get rid of random effects and turbulence effects and what you get? You get the top row of the image below and 12 blobs. You get always 12 blobs, like in the other rows of the image below. And they are at the same place. And when you defcus more, you start to get linear lines.

 

I know that there is an octagonal storm staying foot on Saturns polar area, but I have never heard of dodecaedronal turbulence, especially such that tracks Dubbhe for several days? 

 

_full.jpg

 

 

 

Actually I had it wrong on the original posting, it was not 1,5x Barlow I used. It was Televue Powermate 2,5x at that stage. Below is another set from another day. I was imaging low hanging Betelgeuse with no filter at all, so the camera saw everything from UV to IR. I was chasing possible astigmatism, but found none. With the large spectrum, you dont get proper star testing images, because you dont get proper diffraction rings without defocusing too much. But when I stacked the videos, those 12 linear lines start to appear again. There are also secondary mirror holders visible and enlarged By atmospheric dispersion.

 

_full.jpg

 

 

 

 

These markings actually appear when the turbulence calms down and are strongest at the calmest moments. And when the turbulence and seeing settles for a few seconds, I get this every time:

 

_full.jpg

 

There seems to be people with very strong opinions that this is not print-through. I would love it to be true, so please give me an alternative theory.

All this mirror needs to be really useful is a 12mm hole drilled through the centre and a cheap chinese clock mechanism with gold toned hands as well as a hook on the back to hang it on a wall.

There is no question in my mind that the ribs are the culprit.


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#223 Pinbout

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Posted 12 April 2018 - 07:42 PM

 

 

All this mirror needs to be really useful is a 12mm hole drilled through the centre and a cheap chinese clock mechanism with gold toned hands as well as a hook on the back to hang it on a wall.
There is no question in my mind that the ribs are the culp

Oh stop.

 

its not the ribs, but too much pressure while polishing/figuring.

 

it could be corrected.



#224 Redbetter

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Posted 12 April 2018 - 07:42 PM

The simplest answer, print thru, seems the most likely based on the images.



#225 Kunama

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Posted 12 April 2018 - 07:48 PM

Oh stop.

 

its not the ribs, but too much pressure while polishing/figuring.

 

it could be corrected.

I agree, it is not the fact of having the ribs per se.... rather the pressure..... but don't you agree that it would make a fine clock... cool.gif




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