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Strehl Numbers Question

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#1 Tom Duncan

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Posted 08 March 2021 - 02:27 PM

Do the viewing conditions help or hinder seeing the advantage of a high Strehl number in a Newtonian mirror? 

 

That is, is the high cost of a high Strehl number mirror wasted when the viewing conditions are poor?

 

Or put yet another way should I spend the money on a high Strehl number (say .999) if I live and view 99% of time under city skies? 

 

Thanks for your input.

 

Tom Duncan 

 

PS: I should add I need responses in layman terms, math is not my strong suit. 


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#2 eyeoftexas

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Posted 08 March 2021 - 02:36 PM

I have used my Orion 12" in my backyard a lot over the past 3 years; SQM =18.2 (more or less).  The bright LP is the limiting factor on my viewing, plain and simple.  I do not bother visually with anything that is Mv = 9+.  It can be the clearest, steadiest nights; makes no difference.  It is the light pollution.  This is not really the answer to your question, as I am certainly not saying my mirror is high Strehl number.  What I would say is I would not bother with getting such a mirror if I was only limited to my city skies.  Of course, I prefer viewing the faint fuzzies, and so YMMV if you prefer double stars, planets, solar viewing.


Edited by eyeoftexas, 08 March 2021 - 02:36 PM.


#3 SeattleScott

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Posted 08 March 2021 - 02:36 PM

Seeing conditions absolutely matter. High strehl optics are generally wasted in poor seeing conditions. You don’t need excellent seeing to get some benefit from a premium mirror, but you need decent seeing at least.

That being said, in my experience seeing is better in the city than in the darker mountains. That has to do with the local geography. Light pollution has nothing to do with seeing.

Now if you live on Long Island, there may be some question about how much you can realistically see, and how much it makes sense to spend on a telescope period. Although Ed might disagree. He has up to a 12” Dob. It could be that he sometimes goes to darker sites, but it seems like he mostly observes from his driveway on Long Island. So even for fairly extreme light pollution you could probably make a case for a premium mirror.

Scott

#4 helpwanted

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Posted 08 March 2021 - 02:42 PM

I feel it's not location that limits you, it's seeing conditions, such as the ability to view above 200x. Even in the city you can reach those magnifications of the seeing allows. So my point is you see the value of the more expensive optics once your skies allow you to get over 200x. If you are mostly limited to below, off the shelf may just work fine. 



#5 MitchAlsup

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Posted 08 March 2021 - 03:30 PM

Think about it like this::

 

Your scope has a strehl number

the atmosphere has a strehl number

 

If you scope is close to the diffraction limit (say 1/4.5 waves), and atmospheric turbulence will put the image outside of the 1/4 wave tolerance.

 

If your scope is well inside the diffraction limit (say 1/10 wave), you can tolerate more atmospheric turbulence before the image goes to pot.


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#6 Mike G.

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Posted 08 March 2021 - 03:36 PM

I live in bortle 7/8 conditions and generally have average to poor seeing.  some time ago I sent my 12" Lightbridge mirror off to be refigured and when it came back, I was sure that there was a noticeable improvement in contrast and double splitting.  and I'm not talking Zambuto quality here, just going from a 1/4 wave to 1/6 wave.  the mirror did not have TDE originally so that wasn't a problem.  but it is very nice now and at least to my astigmatic eyeballs, better than before.


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#7 Joe1950

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Posted 08 March 2021 - 03:38 PM

As far as light pollution is concerned, some feel that the pinpoint star images, and sharper DSOs, etc, you get with a high quality objective allows you to see those objects somewhat better. When the sky background is not black, a sharper image will be easier to see than one that is less sharp.

 

Now with regard to seeing conditions, it’s important to note that even if seeing is bad, there may be moments where you get a brief glimpse of good or very good seeing. If you’re looking at Jupiter, for example, during 1 minute of observation with poor seeing being the norm, you may get 3 seconds here, or 2 seconds there where it is excellent and the detail just pops in out briefly.

 

So I would say, no matter what your normal or average conditions are, having a higher quality optic is an advantage.



#8 TOMDEY

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Posted 08 March 2021 - 03:51 PM

My wife wants new countertops. I said, "But look how crummy the floor is --- they match!" Now I'm committed to replacing the counter tops, the flooring, and (yes, literally) the kitchen sink. A contractor will come over to present options and provide quotes. I hope to keep it within $20K. This will put a little dent in my secret ~telescope fund~   Tom


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#9 peleuba

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Posted 08 March 2021 - 04:09 PM

My wife wants new countertops. I said, "But look how crummy the floor is --- they match!" Now I'm committed to replacing the counter tops, the flooring, and (yes, literally) the kitchen sink. A contractor will come over to present options and provide quotes. I hope to keep it within $20K. This will put a little dent in my secret ~telescope fund~   Tom

 

I like the metaphor.

 

You're getting off light in only $20k for a kitchen remodel.



#10 Mike G.

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Posted 08 March 2021 - 04:12 PM

I like the metaphor.

 

You're getting off light in only $20k for a kitchen remodel.

I wonder how much extra it would be to just add in the floor of your observatory .....


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#11 MisterDan  Happy Birthday!

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Posted 08 March 2021 - 04:15 PM

Turbulence/seeing conditions often supersede optical wavefront accuracy.  "Poor" (general term) seeing conditions (i.e. turbulent skies, "swimmy" and/or "shimmery" star and planet images) will not be "offset" by superior optics.

 

On the other hand, seeing conditions don't have to be perfect (rock-solid stable star and planet images with no "smearing" of detail) before images will benefit from superior optics.

 

Think of a turbulent sky like a swimming pool on a calm day.  If there's a life-sized photo of Abe Lincoln at the bottom of the pool, and the water is dead calm, you'll see Lincoln's beard, heavy lids, facial mole/wart, and his trademark shock of hair.  If you introduce minor turbulence (say, by swishing your hand along the surface with one swipe), the photo will be impacted (distortion, "waves," and such), but you'll still know it's Lincoln, and you'll likely still be able to discern his beard and hair.  That swish introduced a minor wavefront error (relative to the earlier dead-calm "perfection"), but it didn't wash out all detail.  You can think of that scenario as good (but not perfect) seeing conditions impacting a perfect mirror.

 

Now, let's assume your best-case mirror is already distorted (equivalent to the distortion caused by your swishing hand).  If you again swish your hand through the water, you introduce the same "level" of distortion as you did the first time, but this time, it's affecting an imperfect medium (distortion already present).  The additional turbulence distorts the Lincoln image even moreso - perhaps "erasing" the shape of his hair and beard, and making it difficult to recognize Lincoln at all.  -Same "seeing conditions" (swishing distortion), but your baselines (one perfect mirror, the other one "average/typical") are different.  One distorted image still has fair detail, while the other image has much less detail.

 

Of course, if Andre the Giant did a cannonball or belly-flop into that pool, it wouldn't matter who was in the photo:  the sloshing, splashing pool water would eliminate any hope of discerning detail in the photo (perhaps even erasing/smearing the photo, itself).

 

Nutshell (my own - not speaking for anyone else):  a superior mirror under good-to- perfect seeing conditions can, indeed, yield more detail than a mediocre/average mirror under those same seeing conditions.  Under poor seeing conditions, almost any mirror (even a perfect one) will be effectively undermined and overridden, yielding little useful detail (beyond the shimmer and "wavy gravy" effect, itself).

 

Light pollution will not be "helped" or offset by superior optics.  If your skies are typically (often) swimmy and turbulent, you may never see the difference between a perfect mirror and one with a 1/4 wave error.  However, if you do experience good (or better) seeing conditions (at least sometimes), the extra expense may very well be worth it, to you.

 

My skies here in the Denver region are typically handicapped by poor seeing conditions due to two primary factors:  the nearby mountain front of the Rockies, and the many "appearances" of the northern polar jet's flow/route over or near Colorado.  Even so, there are enough nights of good seeing to "keep me going."

 

Best wishes, Tom.

Dan

 

Best wishes


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#12 MitchAlsup

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Posted 08 March 2021 - 04:26 PM

My wife wants new countertops. I said, "But look how crummy the floor is --- they match!" 

Did you learn your lesson about holding your tongue ?


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#13 photoracer18

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Posted 08 March 2021 - 05:11 PM

In those times of mediocre seeing when I found my SCTs could not reach 200X on Jupiter, I often pulled out one of my APO refractors just to be sure it was the seeing and the results were the same. Same with a Newt. As far as I can tell seeing trumps everything else. That's why you have to go to the mountains on occasion to see the good stuff.



#14 CHASLX200

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Posted 08 March 2021 - 07:20 PM

Do the viewing conditions help or hinder seeing the advantage of a high Strehl number in a Newtonian mirror? 

 

That is, is the high cost of a high Strehl number mirror wasted when the viewing conditions are poor?

 

Or put yet another way should I spend the money on a high Strehl number (say .999) if I live and view 99% of time under city skies? 

 

Thanks for your input.

 

Tom Duncan 

 

PS: I should add I need responses in layman terms, math is not my strong suit. 

The best optics in the best seeing make for the best images at high power for planets. So so optics will always be so so optics in the best seeing. And i can sure tell the diff in a flash. Bad optics are bad no matter what the seeing.. Always gets the best that you can afford for that one rare nite where everything comes together for a crazy image. I seem to have a good mirror in my 18" Dob, but i have a feeling it is going to the doctor to be much better.  Better really makes a world of diff as i have very steady seeing.  90% of my viewing is planets. Deep sky in bright city lights just is not the same vs a dark sky no matter how good the optics are.


Edited by CHASLX200, 08 March 2021 - 07:21 PM.

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#15 eyeoftexas

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Posted 08 March 2021 - 08:58 PM

Deep sky in bright city lights just is not the same vs a dark sky no matter how good the optics are.

 

Exactly.



#16 starman876

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Posted 09 March 2021 - 07:43 AM

The washington DC sky is not very cooperative.  Seeing most of the time is lousy.  However, there have been nights in where I thought I was in orbit over the planets.   Amazing detail.  I doubt if Zambuto had not made the mirror in my Porta ball I would have seen that detail.   My AP on those rare nights also performs wonderfully.  When I first got these scopes I thought they had bad optics.  Good thing I was paitent and waited for a good night.   


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#17 Tom Duncan

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Posted 09 March 2021 - 10:45 AM

Lots to consider, thanks for your input!

 

Tom


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#18 tommm

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Posted 09 March 2021 - 04:45 PM

To answer your "to put it another way...": If your main issue is light pollution then a better mirror will not help much. The image you get will be sharper, but it will be just as washed out, showing just as little detail. Those details you can see will just be a bit sharper.

 

Regarding seeing conditions, all things effecting the image sharpness and contrast add up.  There are a number, such as seeing, sky darkness, transparency, tube currents, boundary layer at the surface of the mirror...and the quality of the optics.  Improving any one of those will improve the image somewhat.  How much depends on how bad the others are, but it will improve it somewhat.  Make sure your secondary mirror is good before investing in a better primary.


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#19 Allan Wade

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Posted 09 March 2021 - 07:35 PM

Firstly I would ignore the Strehl propaganda that comes with some mirrors. A well known North American optician sent a mirror to Australia with a Strehl stating .998. When it was put in the scope, it was so bad it wouldn’t make a focused image. It went back north for the optician to fix and he said he had used the wrong test equipment to figure it in the first place.

 

If you are observing under city skies, then your primary targets such as the planets, lunar and double stars benefit even more from high quality optics than with deep sky observing under dark conditions. The key ingredient is your seeing conditions. If you have good seeing then investing in a high quality mirror will pay off significantly. Just don’t buy a mirror from someone who hands out those fake Strehl certificates.


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#20 CHASLX200

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Posted 09 March 2021 - 07:52 PM

Numbers never mean anything to me. Just a high power image of Jupiter on my best nites is all i need to know if i have a clunker or a winner.


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#21 Joe1950

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Posted 09 March 2021 - 09:20 PM

There was some clever quote about optical testing. Something like..

 

”Optical testing has ruined more good telescopes than any other cause.”

 

It has some merit. 


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#22 Feidb

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Posted 09 March 2021 - 10:09 PM

I have a Meade LightBridge and I have no illusions about the Strehl ration and could care less. It's no Zambuto and again, I could care less.

 

I also am no planetary observer or am a big fan of double stars, though I routinely catch both. In fact, I usually catch whatever planets are out every time I go observing just to test sky conditions.

 

My focus is on deep sky and while the best optics do give a slight edge on the extreme faint fuzzies, so far, with the two-thousand plus objects I've logged, I've not had any big issues or any obsessive envy other than the usual aperture fever most dreamers get that is almost always beyond practicality. That has nothing to do with the optical quality of my personal mirror.

 

I've seen plenty of impressive views of both the planets, double stars and deep sky objects mixed in with the usual blurry views given the most common sky conditions, even here in Las Vegas and at our dark sky sites when the club goes out of town. That's what it really boils down to. I have yet to observe through a high end optic and be impressed enough to want to trash my scope and spring three to four times the cost for a new mirror. The subtleties are not worth it to me for the "one night a year" for a view that frankly, to me, isn't wow enough of a difference. That is even comparing them side-by-side.

 

Sure, for some that are obsessed with perfection, if you can afford it, go for it from the start, but I'd rather spend my time enjoying observing and chasing faint fuzzies that chasing optical imperfections that may or may not exist.

 

My Chinese mirror is good enough for me.


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#23 Rollo

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Posted 10 March 2021 - 11:30 PM

I agree with Chas,, numbers don't always mean much of anything,, the true test is Jupiter on a good night, and how pinpoint the stars are,,,, how refractor like the stars are in a newtonian telescope.   Also how good the contrast is on a regular basis.   Good optics produce better contrast with less light scatter.



#24 CHASLX200

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Posted 11 March 2021 - 06:49 AM

I agree with Chas,, numbers don't always mean much of anything,, the true test is Jupiter on a good night, and how pinpoint the stars are,,,, how refractor like the stars are in a newtonian telescope.   Also how good the contrast is on a regular basis.   Good optics produce better contrast with less light scatter.

I use the moons of Jupiter as well to see how sharp the optics are. Get nice sharp disk at 500x+ then you know the optics are doing ok.
 


Edited by CHASLX200, 11 March 2021 - 07:31 PM.


#25 Asbytec

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Posted 11 March 2021 - 08:57 AM

In light pollution, better optics won't do much for dim extended DSO detail. The problem is not image quality or seeing, it's object contrast against the bright sky that matters most.

In the realm of high resolution lunar, planetary, and double star viewing at higher magnification, image quality matters more. In this case, atmospheric seeing trumps everything in any sky. Bright or dark.

In periods of near perfect seeing and depending on experience, one may be able to tell a descent optic from a premium one. A premium optic could be worth the money. Whether it is or not is a personal judgement.

I'm a believer in the old adage the best telescope is the one that gets used. In light pollution, pick high resolution lunar, planetary and doubles among other select objects. In poor seeing, one may favor DSO or lower magnification viewing.

Edited by Asbytec, 11 March 2021 - 10:33 AM.



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