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Ignoring the Flashlight Test

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#1 elwaine

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Posted 13 August 2014 - 09:35 PM

I've often wondered about the many admonitions to ignore the temptation of shining a flashlight into the OTA of an CAT. I'm told that the sleeks, or small amounts of grunge that sometimes show on the primary mirror in the flashlight test do not impede functionality. And yet, I've read countless posts about the importance of diffraction limited optics, the importance of having a mirror ground to near perfection with minimal P-V fluctuations, etc. Some posters take great pride in announcing that their CAT has a mirror certified at better than 1/4 wave... and yet we pooh-pooh the defects seen so many times using the flashlight test. If you can see the dirt, it has to effect light transmission and scatter. Otherwise, you wouldn't be able to see it.

 

I'm trying to understand why, on the one hand, exceptional mirror quality makes a positive difference in what we can see through the telescope, yet dirty mirrors fresh from the factory really don't matter.

 

Thanks in advance for your thoughts.



#2 Steven

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Posted 13 August 2014 - 10:38 PM

In the early days when quality control is a hit or miss, telescope dealers invented this myths to prevent new owner from discovering their imprefct optics so they do not have to deal with high exchange or returns.

"imperfect" means some dust or sleeks. Just a though and no proof.



#3 GlennLeDrew

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Posted 14 August 2014 - 04:38 AM

Much depends on the fractional area occupied by the defect(s). Most sleeks, dust bits, etc., on reasonably clean/good optics cover just a tiny fraction of one per cent in area. The diffraction induced is pretty much 'swamped' by the otherwise unaffected wavefront. Consider what it does take in order to impair the image when looking at the central obstruction's contribution.


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#4 Brian Carter

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Posted 14 August 2014 - 08:24 AM

Consider what it does take in order to impair the image when looking at the central obstruction's contribution.

 

+1

 

I hate to tell this story:

 

My first real telescope was an old 10" Discovery dob.  The mirror was pretty old and when I got it the mirror needed recoating. There was a dime-sized portion where the coating had come off, and shining a light behind the mirror showed lots of tiny pinpricks of light in the front.  Then one day I was removing the secondary mirror and accidentally dropped it down the tube, it landed on the mirror and took out a little chunk.  I felt devastated because Discovery made such nice mirrors and this was a really good one that I had just ruined.  That night I took the scope out to look at something and I couldn't even tell that there was damage, even though there was clearly a shallow 1/2 cm chunk taken out of it.  Furthermore, years later when I decided to recoat the mirror, I could barely tell a difference with the brand new coatings, except it was a smidge brighter.

 

The thing is, these isolated optical issues and blemishes don't contribute much to the final optical quality.  The things to worry about are large scale figuring issues that affect the entire mirror (overall correction or smoothness) or specific zones.  A single blemish is almost impossible to detect, and that includes the bit of thumbprint and dust currently on my corrector (I draw the line at the ubiquitous dog hair though...).  What is detectable are the full diameter tiny scratches that eventually develop when one repeatedly cleans and rubs (however lightly) an optical surface.  I'm not one to discourage an occasional cleaning, but it shouldn't be done obsessively or frequently because eventually the damage will add up.

 

By the way, I don't drop things down telescopes anymore, I learned my lesson.


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#5 GJJim

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Posted 14 August 2014 - 08:32 AM

The flashlight test appeals to the OCD side of this hobby. Outside of a class 10 clean room, no optic in the real world will "pass" this test. Just don't do it.  :cool:


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#6 RodgerHouTex

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Posted 14 August 2014 - 08:52 AM

I always do the flashlight test and if it shows sleeks, hazes, uneven coatings, etc. I send the OTA back.  About 4 years ago I bought a 14" Celestron SCT and there was a thumb print on the outside edge of the mirror.  Would it have affected the images, probably not.  But I expect when I spend my money on a new OTA it will be pristine.  If it's not, it goes back.


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#7 rmollise

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Posted 14 August 2014 - 09:05 AM

The reason is that that DADGUMMED FLASHLIGHT of yorn throws the tiniest, tiniest details on the surface into stark relief. Any optics will look horrible under those conditions. Leading the owner to clean and clean and do more harm than good. That's why.

 

Also, I am assuming that you do not use your telescope in a clean room while wearing a fraking bunny suit... Even if you could get your optics to look good under strong oblique light, they wouldn't stay that way for long. Not hardly. ;)


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#8 Asbytec

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Posted 14 August 2014 - 09:12 AM

The flashlight test appeals to the OCD side of this hobby. Outside of a class 10 clean room, no optic in the real world will "pass" this test. Just don't do it.  :cool:

 Hi Jim,

 

You know, Glenn hit the topic pretty well above, IMO. Still, it's nice to clean them from time to time. Mine might not pass entirely the flashlight test once a year, and fails as the year rolls on. It passed nicely when new, too bad it didn't stay that way.

 

And it does come close when you can stumble onto a good cleaning technique. I finally tried hot breath and a soft cloth to finalize areas, after initial cleaning, seen against a black background with the right lighting. You gotta see it before you can "finalize" it. Otherwise you'll see it in the scope's darkened OTA. It's amazing, after all these years of fearing the flashlight test, that grime or sleek actually will come off.

 

If you can see and get all of it...it'll pass. Mostly. But, it's mostly a feel good measure. So, I feel good once a year. :)

 

Sincerely, 

OCD (Cuz, you're probably right.)



#9 elwaine

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Posted 14 August 2014 - 10:07 AM

Outside of a class 10 clean room, no optic in the real world will "pass" this test.

 

Not true. The mirror that came with my RCOS 12.5" RC certainly passed the flashlight test, as have at least two SCTs I owned in the past: one from Meade and one from Celestron. In addition, I had to send a C9.25 Edge back for repair and I asked the service tech to clean the mirror (which came, brand new, with a coating of grunge on it). When the C9.25 came back from being repaired, the mirror was clean as a whistle and passed the test with flying colors. It should have come that way in the first place.

 

 

Consider what it does take in order to impair the image when looking at the central obstruction's contribution.

Any optics will look horrible under those conditions. Leading the owner to clean and clean and do more harm than good.

 

 

I may be many things, but a clean freak just ain't one of 'em. Just ask my CEO.  :)

 

I know that discreet, or focal, defects do not matter. We've all seen or read about a large chip in the primary mirror that, when painted black, had no apparent effect on the scope's usability (although, depending on the size of the defect, it could reduce the effective aperture). And a few sleeks, or a few specks of 'whatever' is not my concern.

 

The central obstruction is not a flaw involving the primary mirror, so it's effects on contrast may not the same as light scatter off of the primary mirror due to a coating of dirt on the surface of the primary mirror. Or if it is similar, the effects of image degradation are additive - and that can't be good. 

 

What I have discovered from doing many flashlight tests over the past 30 years or so is that most of the time they uncover meaningless, extremely minor defects. (In them thar cases, Unk, ya'll are right on. Clean freaks will do more damage trying' to rid the mirror of them inconsequential flaws than what the dang defects cause.)

 

But I have certainly had CATs whose mirror surface - or at least half of the mirror's surface - was coated with a very pale gray-to-yellow cast caused by who knows what. That coating of crud only showed up during the flashlight test, so the owner of such scopes would not know their mirror was dirty unless they employed the flashlight. It's those instances that concern me.

 

We were recently treated to an Excellent paper "MIRROR VS. DIELECTRIC VS. PRISM DIAGONAL COMPARISON," by our friend Bill Paolini. I think is clear to say that Bill proved rather decisively that light scatter off of dielectric coatings can degrade contrast enough to hinder planetary detail... and them were clean diagonals he tested, Unk. So if light scatter from dielectric coatings can degrade planetary detail enough to make many of us buy new prism diagonals, are you guys who are dissin' flashlight testing of the opinion that mirror grunge has no such deleterious effect? 

 

I would encourage the use of flashlight testing - with this caveat: I'd ignore the minor stuff, but when the flashlight beam reveals a large area of grunge on the mirror (grunge that shows only during the flashlight test), I'd send the scope back (under warrantee) for a cleaning. 



#10 AhBok

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Posted 14 August 2014 - 10:43 AM

This is why learning to star test is important. I would never send an optic back for smudges or sleeks if it star tested well. I'd rather have a 1/6 P-V mirror that failed the flashlight test than a 1/4 wave mirror that was pristine. The only "light" test I've found useful is to set the mirror on a table with indirect light. Step away a few feet so that you are viewing the mirror from a 45 degree angle. If the mirror looks clean, it is and leave it alone. Then star test it. If it proves to be diffraction limited, be happy. If it is better than diffraction limited, celebrate! This has worked for me since 1960.



#11 elwaine

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Posted 14 August 2014 - 11:08 AM

Randy, I understand your point,  but it is not really relevant because you are comparing apples and oranges. So let me ask you this: would you rather have a dirty 1/6 P-V mirror, or a clean 1/6 P-V mirror? In essence, that is the question I was asking.



#12 DesertRat

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Posted 14 August 2014 - 11:26 AM

Optical quality is the point, that is - its more important by a large margin.

 

Any optic that 'passes' the flashlight test will fail with a brighter flashlight.  And so on.  Finally you reach a point that the light source will burn off your coatings.

 

Cleaner is better, mostly for applications like detecting faint objects close to bright ones.  But an optic has to be pretty dirty, a light scattering of dust will not hinder much.  In a coronagraph it would matter.  And microripple and roughness in the optical surfaces will impact even more, something you cannot see in a flashlight test.

 

Naturally we want the optics to look good.  However it appears the test is really for those who don't have the ability or desire to determine how well the scope performs where it really matters.

 

Glenn



#13 rmollise

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Posted 14 August 2014 - 12:56 PM

Larry, I guarantee I could make that beautiful mirror look horrible with a flashlight. And I don't mean by bashing it, either. :lol:

 

The flashlight business is a loser's game. A mirror that is used will always have dust on it. PERIOD. And that will not affect its performance. PERIOD. Enjoy your telescopes, but don't obsess over them. THEY ARE TOOLS! ;)



#14 elwaine

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Posted 14 August 2014 - 03:09 PM

Larry, I guarantee I could make that beautiful mirror look horrible with a flashlight. And I don't mean by bashing it, either. :lol:

 

The flashlight business is a loser's game. A mirror that is used will always have dust on it. PERIOD. And that will not affect its performance. PERIOD. Enjoy your telescopes, but don't obsess over them. THEY ARE TOOLS! ;)

 

 

OK I'm going to cry "Uncle." I promise not to do the flashlight test anymore - but before I buy my next SCT, I may first have to join Flashlight's Anonymous.

 

Thanks guys. I appreciate the info.



#15 PowellAstro

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Posted 14 August 2014 - 05:09 PM

The flashlight test is useful. I had a friend that made two 6 inch newts and one always had a brighter image even though they star tested the same and were the same f ratio, etc. The mirrors were also coated at the same time by the same vendor. The bottom line was the flashlight test showed the problem. The dim unit had a lot of pits from grinding that were not completely removed and those pits were not reflective. He estimated there was a 30 percent difference because of this. He said without the flashlight the mirrors looked the same. 



#16 dotnet

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Posted 14 August 2014 - 10:57 PM

Don't know about the flashlight test, but you should clean the corrector/meniscus before the dirt throws the OTA out of balance ;)

 

That said, I find that a dirty meniscus attracts good seeing, and vice versa :D

 

Cheers

Steffen.



#17 rmollise

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Posted 15 August 2014 - 12:50 PM

Don't know about the flashlight test, but you should clean the corrector/meniscus before the dirt throws the OTA out of balance ;)

 

That said, I find that a dirty meniscus attracts good seeing, and vice versa :D

 

Cheers

Steffen.

:waytogo:  :lol:



#18 Eddgie

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Posted 16 August 2014 - 11:53 AM

And yet, I've read countless posts about the importance of diffraction limited optics, the importance of having a mirror ground to near perfection with minimal P-V fluctuations, etc. Some posters take great pride in announcing that their CAT has a mirror certified at better than 1/4 wave...

 

Glen has given the best answer.  

 

Think if it this way.   Suppose you had a C8.  The secondary obstruction is about 5 square inches.

 

Now, each speck of dust is like a tiny obstruction, but suppose you scraped all of the dust on the mirror together and once concentrated, it made a square one inch on a side (one square inch).  This would increse the contrast loss only to the same point as if you made the secondary obstruction only a few millimeters larger, and the difference would be so small that it would be impossible for anyone to see.

 

As for the qualiity issue, I do think that most people rather overstate the small effects that this or that optical issue can have on telescope performance.

 

But it is far more complicated than this.

 

To have a high Strehl, an optic must be very precisely made.

 

Just having 1/4th wave of Spherical Aberration by itself is not a fatal flaw, and someone looking though two telescopes, one with zero SA, and one with 1/4 wave, most people would struggle to tell them apart.

 

Ah, but here is the very sad reality.  A manufacturer that is content to pass an optic with 1/4th wave SA will also tolerate many other small errors.   These errors accumulate.

 

And when you couple some SA along with a slight turned edge, and a bit of a zone, and a bit of astigmatism, and then, you throw in a big obstruction (which by itself is not at all a fatal flaw) you can get a telescope that is a very poor performer fo the apeture.

 

So, this or that small error by itself is almost always of little consequence, but if there are enough of them, it does add up, especially coupled with some SA (which is one of the most serious errors) and big obstruction. 

 

Dust, by comparison, is a tiny problem.


Edited by Eddgie, 16 August 2014 - 01:12 PM.


#19 elwaine

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Posted 16 August 2014 - 09:52 PM

Thanks Ed. I do understand what you say, and I do appreciate, Glenn's point. But I am not concerned about dust. I've mentioned that a few times in this thread, but folks keep discussing dust, clean rooms, etc., Dust has nothing to do with what I am trying to discuss.

 

What I am addressing, specifically, are coatings of "stuff" - I don't know what it is or what it is due to - that I have seen on at least two mirrors. I already mentioned the saga of my C9.25. Here's another:

 

I bought an Intes-Alter M809 from a friend. The views were sharp, but it seemed to me that they were not any brighter than views through my C6. I couldn't understand why the views, at very similar magnifications, looked as bright in the C6 as in the 8" Mak. The mirror in the M809 looked good, until I looked at it by shining a flashlight onto the mirror's surface. There was a yellowish coating covering more than 3/4 of the primary mirror's surface. Instead of trying to clean the mirror, I removed it and sent it to Spectrum to have it stripped and re-coated. When the mirror came back, it was essentially pristine. (Yes, after re-installing it, there were a few flecks of dust on the primary - but there was no yellow coating.)

 

The M809, after re-coating the mirror, threw up views that were noticeably brighter than the C6. 

 

If I did not use the flashlight test, I would not have known about the defective coatings on the M809.



#20 PowellAstro

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Posted 16 August 2014 - 10:07 PM

Same issue my friend had as I stated in post #15.



#21 Asbytec

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Posted 17 August 2014 - 04:41 AM

You know, it makes sense not to shine a flashlight down you're OTA and scare yourself with the horrid view: dust, grime, streaks, or coatings maybe. Instead of dreading it, I've come to trust and use the (moderately bright) flashlight test to tell whether the optics are clean and in good shape. Nothing more.

 

After years of just giving up getting that trace of grime from the surface, I stumbled onto a pretty effective method to clean that grime and now it's much cleaner and basically passes the flashlight test like new. At least at the beginning of the observing season it does. Then endeavor to keep them that way until next season.

 

It has essentially nothing to do with optical performance, but it does make us feel better when the optics are pristine, shiny and giving us all they can. The flashlight test reveals them to be so. If one discovers a poor coating (knock on weed), hey...have it re-coated, if possible.  

 

For me, the primary offender of the flashlight test appears to be the meniscus thus far (with a few dust specs on the enclosed, yet to be cleaned primary.) With light bouncing through the lens from both sides, a slight grime is easily seen after basic cleaning when it looks pristine to the eye. A bit of touch up and it approaches like-new condition.

 

Shining a flashlight down a pristine OTA is a much nicer experience than shining it down a dirty one and having to fight off those ill feelings. I cannot swear to any performance improvement (say, less scatter and improved throughput), but I can attest to having a better gut feeling thinking there might be. Until it get's dirty, again, anyway.


Edited by Asbytec, 17 August 2014 - 04:59 AM.


#22 garret

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Posted 17 August 2014 - 07:20 AM

A few weeks ago the 16" mirror in my 3 years old ASA newton astrograph received a new coat by Orion Optics UK.

Prior and after the new coat I made a image from behind the mirror with a light bulb in front of the mirror to see the difference in light shining through the mirror.

The ASA coating is a 97% coat (according to the ASA website) and the new Orion UK coat is also 97%.

The results shocked me, in the image the new coat is on the left, almost no light leaking through, right the old coated mirror: grey, streaks and smudges (ignore the black adhesive in the middle).

The images were taken with exact the same fl lenght and  f# number of the lens, and also ISO , conditions, etc.

 

I conclude:

1) It is always the best to give a mirror a new coat.

2) Was the old coat always this bad? then don't trust telescope makers with their claim about quality.

3) Or a coating on a mirror can deteriorate over time, due to weather, moist, gently cleaning as I have done..., then recoat a mirror every 2 year.

4) Always check mirrors with light bulbs in front and from behind.

 

Garrett van der Veen, The Netherlands

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#23 PowellAstro

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Posted 17 August 2014 - 09:15 AM

@ garret

 

Very good info and demonstration, thanks. I believe the coating is one place the manufactures do skimp on and its hard for a user to know this without a test like you have shown. 



#24 RodgerHouTex

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Posted 17 August 2014 - 12:07 PM

Larry, I guarantee I could make that beautiful mirror look horrible with a flashlight. And I don't mean by bashing it, either. :lol:

 

The flashlight business is a loser's game. A mirror that is used will always have dust on it. PERIOD. And that will not affect its performance. PERIOD. Enjoy your telescopes, but don't obsess over them. THEY ARE TOOLS! ;)

 

I have to say I typically respect your opinions Uncle Al, but not this one.  Of course, after use, an open optic such as a Newt will show dust.  But a closed optic, like a SC should not.  And when I said I always test a new OTA when it arrives, I do.  Both optically and mechanically.  Every OTA I have meets the manufacturers advertised specs.  Diffraction limited (1/4 wave), no aberrations visible that aren't inherent in the design, etc.  If people tolerate shoddy workmanship they deserve what they get.  I do not.



#25 elwaine

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Posted 17 August 2014 - 01:05 PM

Hey Rodger, Thanks. I appreciate your support.

 

But again, I am not addressing dust. A little dust is good for every optic.  :grin:

 

I'm talking about GRUNGE that only showed up on a flashlight test. GRUNGE that resulted in degraded performance.

 

It is difficult to apply a rule to every situation. I agree with Uncle Rod. Most of the time it is not helpful at all... and in some instances it can lead one to be disappointed in a perfectly good mirror. Or worse, it could lead one to fix, and then ruin, a mirror that didn't need fixin' in the first place. 

 

But we are all very astute here. We know when to use the flashlight test and how to interpret the results... don't we?  :lol:








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