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heliumstar

Science behind seeing (SCT C5 vs 80mm APO)

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Posted (edited)

I was completely taken aback yesterday. First a little context. I was observing Jupiter and Saturn a few days ago with C5 and all was well. Sharp view, really pleasing. Better than AA 80ED-R that I had side by side. Was happy camper so to speak. But yesterday the seeing was not so great and I had them both again side by side but this time C5 just didn't perfrom. Mushy views all over the place. Collimation is spot on so that's not an issue. The view through 80mm was not ideal but much much sharper at the same mag. There must be something going on with a telescope design here and I would like to understand science behind this. I am not imagining this. I got my lady to take a look through as well. I have to admit, I am kind of down because of this. Is SCT really usable only with decent seeing? That puts it out of commision for most clear nights. Mak for example has no such issues....

Edited by heliumstar
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Two things come to mind here:

1. thermals - did C5 get a chance to cool properly on both nights and was second night thermally unstable (like getting cold quickly so scope could not keep up)?

In any case this can cause issues for Cat vs refractor on second night

2. Type of seeing effects / aperture size. Effects of seeing change with aperture size. There is something called coherence length and coherence time. This is related to seeing cell size and especially to relationship between cell size and telescope aperture.

Small apertures usually "fit" inside seeing cells most of the time and as result main seeing aberration is tilt - image shimmers rather than blurs.

Larger apertures usually don't fit inside single seeing cell and this changes dominant aberration - it is no longer tilt and image starts to blur rather than only shimmer.

Blurring is responsible for detail loss much more than shimmer. For this reason it is said that smaller apertures better tolerate poor seeing.

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I can compare, virtually side by side a 8" F10 SCT and a 8.5" F12.5 refractor.  The refractor generally gives the better image.  Both telescopes are subject to the seeing cell issue, the difference in the main is the light path through the system.  The light in the SCT passes three times through the tube, the first pass hugging the tube wall, the second hugging the external surface of a radiating baffle and the third hugging its internal surface.  By contrast, the light through the refractor is converging, getting ever further away from the tube wall as it approaches the eyepiece during its one and only pass.  Which do you expect to have the steadier image on the average?.  Place your bets!       🙂

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2 hours ago, andrew s said:

@vlaiv has captured the core of the issue. If you want a fuller more detailed explanation see here https://telescope-optics.net/induced.htm and the sections that follow.

Regards Andrew

Thanks, very interesting link - especially the conclusion in section 5.1.3. I've always thought that the average cell size here is fine for telescopes up to around 12" ( can't remember where I read that) and I can't say I've ever changed to my Mak127 or TS80APO because of seeing. I've often had to wait one or two hours for my C8 to stabilise thermally though, and when I had an 8" Newton, I often had to swap to one of the others due to meteorological wind.

CONCLUSION

What all this implies is that it is possible for a significantly smaller aperture to outperform the larger one, but it requires them to be, and remain at a specific seeing error level, generally involving compromised seeing conditions. It is only possible when D/r0 in the larger aperture is ~4, or larger, in which case a smaller aperture with D/r0~2 will have - all else equal - better resolution and contrast transfer. In other words, smaller aperture could perform better while the seeing error is large enough, but if it lessens sufficiently due to seeing fluctuations, larger aperture would rebound and perform better. Typically, seeing fluctuations are wide enough for that to happen.

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As @Peter Drew points out tube seeing is also important (it's covered in later sections of the link). Excellent,  thermal management is needed in folded designs.

In addition focus and collimation are also very important and are more easily distributed in some configurations than others.

Regards Andrew 

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Posted (edited)

Thanks for answers guys. The scope was well 'cooled down'. It was out for more than 2hrs before I observed. Telescope-optics.net is a great resource even though I struggle with it somewhat. Thanks for the reminder that it exists.

Edited by heliumstar

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As I mentioned a couple of weeks ago on another thread, the veteran solar observer, Alan Heath of Nottingham, regularly sees solar granulation in his 80+ year old 3-inch uncoated Broadhurst Clarkson refractor, whereas it's a challenge in his C8. Extra heating compared to night time, of course, but I guess the principles are the same. 

(BTW, surprised @mikeDnight hasn't chimed in on this one yet 🙂 )

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Here goes:  There's another thing to consider also and that's the amplification factor of the secondary mirror. I seem to remember from years back that an F2 primary with a 5X amplifying secondary, creating a F10 system, doesn't amplify the heat errors by 5X, but by 25X. (5X^2). So thermal equilibrium is much more difficult to achieve in a catadioptric system using an amplifying secondary, or even with an open Cassegrain design. What puzzles me though, is that a Maksutov always seems to perform better, presenting sharper stars and higher definition than a SCT. Why? I have no idea! Perhaps it's that awful Schmidt corrector plate that's responsible for those cotton wool star images and drunken planetary views?? 

I'm working on being more sensitive. How am I doing?👍 👎

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42 minutes ago, mikeDnight said:

 I seem to remember from years back that an F2 primary with a 5X amplifying secondary, creating a F10 system, doesn't amplify the heat errors by 5X, but by 25X. (5X^2). 

I'm working on being more sensitive. How am I doing?👍👎

I have never seen anything like that can you or anyone provide a reference to that memory (about amplification not his sensitivity)? 😀

Are you referring to tube currents or atmospheric seeing? I understand the tube current amplification on multiple passes in the tube but find it hard to see how it is anything but linear on wave front errors from the atmosphere or elsewhere.

If it were the case, then Cassigrain primaries would need very high tolerance as their  wave front errors would be amplified similarly.

Off to see what I can find.

Regards Andrew

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It's the residual internal heat that I was referring to Andrew. It's a long time ago now and I could be wrong, but I think it was in Mackentosh's book on Optics. Cant remember the exact title!

I've no doubt the accuracy  of the optics needs to be unimaginably high with every component in any mirror system using such short focal length primary and amplifying secondary, including the corrector. I suppose the amplification of the secondary could also have an effect on how sensitive the scope would be to the seeing conditions, as it would amplify atmospheric error  ( in the image produced by the primary ) compared to zero amplification in a Newtonian for example. At least thats how I understand it - I think! 

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11 hours ago, mikeDnight said:

Here goes:  There's another thing to consider also and that's the amplification factor of the secondary mirror. I seem to remember from years back that an F2 primary with a 5X amplifying secondary, creating a F10 system, doesn't amplify the heat errors by 5X, but by 25X. (5X^2). So thermal equilibrium is much more difficult to achieve in a catadioptric system using an amplifying secondary, or even with an open Cassegrain design. What puzzles me though, is that a Maksutov always seems to perform better, presenting sharper stars and higher definition than a SCT. Why? I have no idea! Perhaps it's that awful Schmidt corrector plate that's responsible for those cotton wool star images and drunken planetary views?? 

I'm working on being more sensitive. How am I doing?👍👎

The reason I've read is that, while in an ideal world there's no reason a Mak/Cass should outperform a SCT, it's easier to accurately mass produce the Mak corrector than it is the SCT corrector so the customer is more likely to get better quality optics.

 

(Disclaimer: I am not a telescope manufacturer) 😉

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Posted (edited)
12 hours ago, steveex2003 said:

Everyday really is a School day on this site 🙂

Well its taught me a lesson, and I stil spend much of my time outside the headmasterz office! :rolleyes2:

Edited by mikeDnight
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Posted (edited)

Presentation heat transfer.pdfWell the weather was poor this morning so I have searched high and low using Google and found papers and articles from the 1800s to 2019 on the topic of tube currents. Many I had seen before but many were new. 

Interestingly the older ones comparing refractors and reflectors often considered the need to reduce them in large refractors as well as reflectors.

My main conclusions was that total light path length in air of differing refractive index was key so reducing thermal gardiants or mixing the air were cures. In addition having the largest thermal mass at the bottom to the tube was worse that having it near the top!

There seem to be two related issues. Initially, reaching or getting close to thermal equilibrium with the air causing bulk convection currents or unstable air columns with cool air above warm. Secondly, over cooling, wrt to the air, of some parts due to radiation to the sky which creates boundary layers. 

Differing strategies to overcome this were discussed but amount to reducing temperature gradients as much as possible and mixing the air to normalise the refractive index. 

Eventually I can across this discussion https://www.cloudynights.com/topic/624178-heat-transfer-in-and-around-a-telescope/ the second drop box link to the full presentation is live and well worth a read. It the most modern and comprehensive discussion I found.

I am going to follow up some of the ideas including the low emissivity transparent film - even on a Takahashi.

Regards Andrew

PS Here it is for ease of access

Presentation heat transfer.pdf

Edited by andrew s
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13 minutes ago, andrew s said:

Well the weather was poor this morning so I have searched high and low using Google and found papers and articles from the 1800s to 2019 on the topic of tube currents. Many I had seen before but many were new. 

Interestingly the older ones comparing refractors and reflectors often considered the need to reduce them in large refractors as well as reflectors.

My main conclusions was that total light path length in air of differing refractive index was key so reducing thermal gardiants or mixing the air were cures. In addition having the largest thermal mass at the bottom to the tube was worse that having it near the top!

There seem to be two related issues. Initially, reaching or getting close to thermal equilibrium with the air causing bulk convection currents or unstable air columns with cool air above warm. Secondly, over cooling, wrt to the air, of some parts due to radiation to the sky which creates boundary layers. 

Differing strategies to overcome this were discussed but amount to reducing temperature gradients as much as possible and mixing the air to normalise the refractive index. 

Eventually I can across this discussion https://www.cloudynights.com/topic/624178-heat-transfer-in-and-around-a-telescope/ the second drop box link to the full presentation is live and well worth a read. It the most modern and comprehensive discussion I found.

I am going to follow up some of the ideas including the low emissivity transparent film - even on a Takahashi.

Regards Andrew

Did you come across that classic paper by Kitaro Takahashi and Mike Hezzlewood in the Journal "Optical Telescope Design", Vol 512, pages 3 to 503? The conclusion was "refractors are better than reflectors"?

🙂

 

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2 minutes ago, JeremyS said:

Did you come across that classic paper by Kitaro Takahashi and Mike Hezzlewood in the Journal "Optical Telescope Design", Vol 512, pages 3 to 503? The conclusion was "refractors are better than reflectors"?

🙂

 

No google does not seem to find it. If you have a copy or a link please send it.

The most common conclusion was that refractors were better than reflectors of the same aperture but that with good thermal management reflectors could equal refractors. Howverer, they were all subjective not based on measurements.

Regards Andrew

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11 minutes ago, JeremyS said:

Did you come across that classic paper by Kitaro Takahashi and Mike Hezzlewood in the Journal "Optical Telescope Design", Vol 512, pages 3 to 503? The conclusion was "refractors are better than reflectors"?

🙂

 

What a surprise finding 🤪

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Posted (edited)
24 minutes ago, Alan White said:

What a surprise finding 🤪

It might be worth noting that Pickering commented that "refractors below 5" were only useful for variable star work and that 8" to 10" was required for serious planetary observations". You pays your money and picks your quote!😉

Regards Andrew

PS @JeremyS I can't even find the Journal!

Edited by andrew s
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37 minutes ago, andrew s said:

Pickering commented that "refractors below 5" were only useful for variable star work and that 8" to 10" was required for serious planetary observations".

Would that include "During the favourable opposition [of Mars] of 1892, W. H. Pickering observed numerous small circular black spots occurring at every intersection or starting-point of the "canals"? 😁

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47 minutes ago, andrew s said:

It might be worth noting that Pickering commented that "refractors below 5" were only useful for variable star work and that 8" to 10" was required for serious planetary observations". You pays your money and picks your quote!😉

Regards Andrew

PS @JeremyS I can't even find the Journal!

Andrew, it's normally to be found between the "Takahashi Journal on Reassuringly Expensive Optics" and "The Burnley Proceedings on Impossibly High Magnifications"

🙂

 

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50 minutes ago, andrew s said:

It might be worth noting that Pickering commented that "refractors below 5" were only useful for variable star work

Nothing wrong with that, Andrew 🙂

 

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43 minutes ago, JeremyS said:

Andrew, it's normally to be found between the "Takahashi Journal on Reassuringly Expensive Optics" and "The Burnley Proceedings on Impossibly High Magnifications"

🙂

 

Your a very naughty boy. Regards Andrew 

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Laurel and Hardy had nothing on you two 👍👍👍😂😂😂😂

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1 hour ago, andrew s said:

Your a very naughty boy. Regards Andrew 

I felt for you Andrew, I could see you were taking Jeremy seriously.
Easily done, he has that serious look about him 😉

 

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