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Ignoring the Flashlight Test

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#26 DesertRat

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Posted 17 August 2014 - 02:38 PM

Every amateur uses a flashlight.  Don't we all use one when dusting off a corrector plate?

 

Of course a new scope delivered with 'grunge' on the mirror is not acceptable.  I see nothing controversial about that and we should have universal agreement on it, even if this is CN!  :flowerred:

 

Determining the quality of the Al coat however is a different subject than originally asked about.  A flashlight shown down an OTA has some value there.  Non reflective portions and some discolorations indicate possible damage.  Some new mirrors will show traces of contaminants left over from manufacture as color interference effects.  Not a happy thing but nothing to be alarmed about.

 

It is possible the original owner of the M809 had angst over a flashlight test and decided to clean the mirror, performed a poor job and hurt the finish.  Possible, I am not saying that happened.  It is also possible the maker did a poor job in coating.  For that maker it would be a little surprising but it is possible.

 

These closed tube scopes are pretty good at protecting the primary.  Normally the mirror will remain fine for many years.  The seal is far from perfect however and with time some dust, pollen and even insect droppings and trails will eventually show up in some environments.   If somehow it gets a covering of moisture with any frequency, damage will be accelerated.

 

I once worked at an observatory where I had to chase bats away from the scope with a broom.  And yes I used a flashlight to check the mirror.  Puts a new 'light' on the concept of dome seeing.  :lol:

 

Glenn


Edited by DesertRat, 17 August 2014 - 02:40 PM.


#27 tclehman1969

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Posted 17 August 2014 - 05:39 PM

This is indeed a contentious topic given the arguments on both sides. It would be nice if all amateurs could conduct tests on the quality of their individual scopes, but that is completely unrealistic. For those of you who do, great. I know for myself that taking my new scope apart to shine a bright light at it and check to see if it is dark as it should be, well, seems as though once that screwdriver removes one of the retaining screws on my corrector, it may just void the warranty. Kind of something I'd like to avoid. 

 

Ideally, sure, I'd like to know the quality of the scope I have, but I don't have the skills/knowledge to even try. The only thing I can check on my SCT is it's collimation, really. Maybe we could start a thread on how to check the quality of a mirror. This is something I would like to learn about, but ultimately, my biggest concern is getting the scope outside and looking up.

 

i will add another item, the thumb print on the mirror of a new scope is completely unacceptable, that is for certain. And it seems that those who do test their scopes have found a continuous problem that manufacturers are not meeting stated quality. Is anybody dojng anything about it?


Edited by tclehman1969, 17 August 2014 - 05:42 PM.


#28 elwaine

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Posted 17 August 2014 - 07:24 PM


I know for myself that taking my new scope apart to shine a bright light at it...

 

:confused: Taking the scope apart? That is not required. That is not necessary. And that is not how one conducts a flashlight test. You just shine the light down the OTA. Furthermore, it is not done to see how dark the inside of the OTA is. It's done to look at the primary mirror.

 

 


I'd like to know the quality of the scope I have...

 

The flashlight test is not used to test the quality of the scope. It's done only to check the mirror to see if it's dirty. Nothing more. 

 

 

 Maybe we could start a thread on how to check the quality of a mirror.

 

 

There are already far too many dozens of threads on that topic. But if you are really interested in that, I'd suggest you read Star Testing Astronomical Telescopes: A Manual for Optical Evaluation and Adjustment (2nd ed.), by Harold Suiter. It's sort of the Bible around here on that very interesting topic.


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#29 tclehman1969

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Posted 18 August 2014 - 03:22 AM

Larry,

 

Thank you so much for adding bold to your comments. It adds context to your comments, context not required!:-)

 

Point by point:

 

Taking the scope apart? That is not required. That is not necessary. And that is not how one conducts a flashlight test. You just shine the light down the OTA. Furthermore, it is not done to see how dark the inside of the OTA is. It's done to look at the primary mirror.

 

My comments are in reference to a couple of the comments within this thread that apparently you are not familiar with, not specific to your flashlight test. For example:

 

Post #10 -- 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.

 

I believe the only way to test a mirror on a table would be to remove it.

 

Post # 22 -- 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.

 

Please view this post and note the mirror sitting on a table with a light on one side and a pic taken from the opposite to determine how much light is coming through the mirror's old coatings. An interesting test, but in order to accomplish this test, I have to dig the mirror out of my SCT. If you buy a new scope, you aren't likely going to be taking the scope apart to check its coatings. My comment, "I know for myself that taking my new scope apart to shine a bright light at it and check to see if it is dark as it should be..." I am not referring to how dark the inside of the optical tube is, but rather referencing how dark the back side of a mirror is when a light is sitting on the other side of it in relation to this post (#22).

 

Next:

The flashlight test is not used to test the quality of the scope. It's done only to check the mirror to see if it's dirty. Nothing more.

 

I am aware of this, but, again, it is in relation to other posts within this same thread. For example:

 

Post #18 -- 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.

 

Also...

 

Post #24 -- 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.

 

I'd like to know if the scope I have is "putting out the best image" that can be expected, but for me, I'd have to have someone check this for me, and this is to what I was referring. 

 

Hope this clarifies the remarks. 

 

Ultimately, do the tests if you want to. If you don't want to, then don't. 

 

I wonder, do you suppose on any of the Hubble servicing missions any astronaut shined a light down the front end to see how pitted the mirror had become from micro meteorite impacts? It would be a pretty amazing shot given the mirror is located down at the end of the shroud, but after 24 years in orbit, it could be possible.



#30 Asbytec

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Posted 18 August 2014 - 03:56 AM

Is this thread confusing or what? :)

 

My only point is, hey, don't fear or ignore it. Use it. It gives you information you can use should you choose to do so. Well, except for sparse tiny dust specks, leave them be. 



#31 rmollise

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Posted 18 August 2014 - 08:51 AM

 

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.

 

 

Who is this "Al" you speak of? :lol:

 

As for an SCT mirror not showing dust? It most assuredly will show some under that dadgummed flashlight beam. An SCT's tube ain't airtight, you know. ;)


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#32 Ed Holland

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Posted 18 August 2014 - 11:46 AM

There is one instance where I have found it useful to shine a torch (flashlight) at an instrument, and that is in order to deteremine it's working aperture, in conjunction with a short focal length eyepiece.

 

It was this that led me to discover that my 127mm Maksutov was really only 119mm. And you lot are worried about dust :lol:


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#33 DesertRat

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Posted 18 August 2014 - 02:19 PM

Oh no Ed, a flashlight test from the other end. :p

 

Just don't go shining that light through your very expensive telephoto lens. :grin:    

 

The best use of a flashlight for me is navigating the backyard around presents my dogs have left behind.  :lol:

 

Almost everything else I can do with ambient light.  But not that!

 

In addition to water, the great enemy of optics is over cleaning.  Excessive use of a flashlight can lead to that practice with great frequency.  All it takes is one piece of grit to scratch, and over the years your glass will take on a pearly appearance.  One thing that cleaning is beneficial for is the removal of pollen, which can be very nasty.  In some areas users avoid taking their big scopes out entirely when trees are releasing that stuff.

 

Its amazing how much dust it takes to hurt things.  The amount of light scattered is very small.  Of more importance is the polish of a mirror, and before a mirror is aluminized a bright light source may be used to confirm a good polish when the front surface becomes almost invisible.

 

Glenn


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#34 Ed Holland

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Posted 18 August 2014 - 03:29 PM

:lol:

 

Well put

 

That's OK, I haven't got a very expensive telephoto lens. My small selection were used bargains :)



#35 RodgerHouTex

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Posted 18 August 2014 - 10:24 PM

:bow:  Sorry Uncle ROD.  I'm old and my hair is blond.  Those are my excuses and I'm sticking with them.  



#36 azure1961p

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Posted 18 August 2014 - 10:51 PM

I personally use the daylight test or lighted room test. If an oblique angle reveals no blue haze residue - I could care less about the flashlight test.  Its nothing but a hyper amplification of something that has no practical or meaningful value.  I can see the concern on one hand but the overwhelmingly low level of scatter or what have you is strictly in the realm of neurosis.

 

Relax.

 

Pete


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#37 elwaine

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Posted 23 August 2014 - 01:19 PM

Another (recent) example of how the flashlight test proves to be valuable.  :lol:



#38 Pinbout

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Posted 23 August 2014 - 01:37 PM

Celestron Schmidt Cassegrain Telescope: http://youtu.be/BqnP3a3X07s

 

You think a flashlight is scary...do not do a knife edge test against an oil flat.

 

Robert piekiel has a book on it.

 

That can be very scary.


Edited by Pinbout, 23 August 2014 - 01:37 PM.


#39 Glen A W

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Posted 27 August 2014 - 02:35 PM

The most perfect flashlight test I have ever seen was on my Vixen VMC-260L.  That is no small feat for a 10.5 inch mirror.  The test was absolute perfection when I bought it, and has only gone downhill a little because I have never cleaned it.  When new, it was so clean, you could eat off of it!  No haze or patches or anything else, just perfect reflection.

 

Might this have something to do with the fact that the scope is the best planetary scope I have ever used, especially on Jupiter?  It has a large obstruction yet it performs amazingly well.  Sharpness and contrast are refractor-like.

 

So, I am a believer in the test.  One of the worst tests I have seen was on a Hardin 10" Dob from about fifteen years ago.  I think they simply did not finish the polishing of the mirror.  It still gave good deep sky views, and planetary views were fair but seriously lacking in contrast.  The Moon was a waste, however.  Totally washed out at all phases!

 

Glen


Edited by Glen A W, 27 August 2014 - 02:37 PM.


#40 RodgerHouTex

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Posted 27 August 2014 - 10:59 PM

Celestron Schmidt Cassegrain Telescope: http://youtu.be/BqnP3a3X07s

 

You think a flashlight is scary...do not do a knife edge test against an oil flat.

 

Robert piekiel has a book on it.

 

That can be very scary.

 

As a person who has designed, fabricated, and tested telescope optics I would say it's hard to say what you're seeing in your "test".  First of all it doesn't look like things are in thermal equilibrium.  You have thermal air currents all over the place which means that the optics are probably not in thermal equilibrium.  Until an optic is at thermal equilibrium, you will see all kind of defects as things cool or heat up.  I would let things set for a while and retest when the thermal currents are gone.  All good opticians do that.  I would also find a way to mount the Ronchi grating.  It is moving in and out of focus thus making hard to interpret what's actually going on.  Either that or go star test this scope.  That will really tell you what the optical quality is.








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