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SCT corrector cleaning and "micro" scratches

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#1 ERHAD

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Posted 21 July 2019 - 06:30 AM

In the time I've had my Celestron C9.25 (a bit over 2 years) I must have cleaned it three or four times max. I live in a pretty humid area and eventually there is always a kind of light "fog"-"goo" on the glass as a consequence of this. Cleaning the corrector always makes me feel a bit anxious and I have been as careful as I possibly can. Have followed all the stantdard indications and procedures (clean with the recommended mix of isopropylic alcohol and distilled water, remove any debris previous to cleaning, never rub the surface, very light strokes, never in circles, etc, etc...).

 

Upon cleaning the glass looks flawless and even on close inspection I cannot appreciate any scratches. It just looks like new. Now, if I flash a light at the glass at night, well, that's a different story: I can see a myriad of very tiny micro-scratches that again, are only visible when flashing a bright light at them and not visible any other way.

 

I was wondering if this is normal or if they may be having an impact on the performance of my scope. If they are a result of me doing something wrong, well, I don't know ,for the life of me, how to be any more careful when cleaning...

 

I´d love to hear your experience, also, how often do you clean and how dirty should the corrector be before you consider cleaning it, in other words: what is the impact on image quality of all the humidity and junk that inevitably accumulates on the plate?

 

Thanks in advance for any kind replies, really interested to hear your take.

 

Best,

 

Erik.



#2 Asbytec

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Posted 21 July 2019 - 07:05 AM

Micro scratches? Wow good question. I've seen them, too. I use a flashlight at oblique angles to look for remaining sleeks after cleaning...about once or twice a year, by the way. I understand most modern coatings are baked on and fairly resilient. So, I have been unafraid of cleaning my corrector (meniscus) because of it, though still using many of the same safeguards as you. As I recall from a thread so many years ago, there is a very small amount of scatter caused by scratches. But, it's nothing to write home about. Another argument, as I recall, someone did an analysis of the total surface area of scratches and determined the scatter was a bit more severe. So, truthfully, I cannot tell you except to say I do not notice any change after cleaning mine about 10 times. 

 

We're often told never to use flashlights on our optics, they are misleading. There is some truth to that, they look dirtier than the really are and to the extent they affect performance. Knowing this, I still use a flashlight because I am AR about cleaning them back to original factory spec without a smudge anywhere. Enough to pass the flashlight test, which is not easy to do. I want every photon to make it to the glass. I dunno, I guess I clean mine before each observing season and as needed afterward. Sometimes I'll clean them, again, prior to stowing it for our monsoon season. But, cleaning is usually when I can visually see some smudges or whatever without a flashlight. I figure by then they are getting pretty dirty, even though they can probably take a bit more gunk before performance is noticeably affected.  Again, AR...you know. 

 

Yea, we hate to see scratches on our beloved optics despite our best efforts. However, I'd say micro scratches are not affecting your scope in any real way, and they are probably limited to the coating surface cutting down transmission by a very small fraction of a single iota. By that, I mean not at all. Same with any scatter that may be present. I clean my scope when it looks dirty, that's usually once a year and just for kicks at the beginning of the observing season. The gunk on the corrector really does not get my attention at the eyepiece, only when I happen to see it on the corrector. But, I clean my eyepieces if one of them blurs the image a little or exhibits a halo around bright objects.

 

I wonder if those micro scratches were there from the factory. I never checked, only checked after cleaning. So, I presumed it was my (our) fault. 


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#3 james7ca

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Posted 21 July 2019 - 07:10 AM

I see the same thing on all my scopes (SCTs and refractors) and I doubt it makes any significant difference. Is it as "good as new?" Probably not, but I suspect that you could never show any meaningful difference, but it obviously depends upon how many scratches, their clustering, and their size and depth. That said, you need to clean occasionally, since the film and dirt that builds up will over time have a greater impact than if you clean and leave a few minor and small scratches.


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#4 theApex

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Posted 21 July 2019 - 07:14 AM

This rather recent thread has it all: recommended cleaning procedure (including a mention to Rod Molise's own), frequency and even somebody's compilation of some manufacturers' take on it.

https://www.cloudyni...corrector-lens/

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Edited by theApex, 21 July 2019 - 07:36 AM.

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#5 theApex

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Posted 21 July 2019 - 07:32 AM

Also, I started looking at optical coatings less as fairy dust and more as "tough as nails" since an astrophotographer friend, who's well at easy with cleaning and even recoating his mirrors very occasionally, that:

1) His experience on recoating an 8" mirror is that removing the factory silver halide(?) took a great amount of acid (can't remember which), scrubbing and even sanding; and he still washes it (not just clean) occasionally!

2) after so much frequent cleaning, his rather great photographs (that's me and our astro club members saying so) are still equally as great as they were (probably better than, due to him honing in his skills) when his mirror still had the original coatings.

Okay, it's not the OP's SCT corrector's we're talking about here, but I very much doubt (some common sense may dictate it too) SCT manufacturers would be much less careful or perfeccionist with said plates' coatings as well, to the point of making something not similarly resistant to the former.

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Edited by theApex, 21 July 2019 - 07:43 AM.


#6 db2005

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Posted 21 July 2019 - 08:57 AM

I wouldn't worry too much about it. I mean, sure, we should be careful how we clean our optics, but... just shine a strong enough light source through any optical system arriving directly from the factory, and you will see seemingly horrifying sleeks and imperfections on more or less all of the optical surfaces. If you don't see any imperfections, you are (1) either not using a strong enough light source, (2) not pointing the light source at the correct angle or (3) not looking carefully enough. If you still don't see anything, just shine the torch through the telescope from the focuser end, and you will be convinced that I'm right.

 

I know, I know, we want to believe the precious optics we just bought with our hard-earned money will last for virtual eternity to be passed down through generations, but after I've owned maybe a dozen telescopes I'm realizing that we're not really buying telescopes, but we're really instead buying the enjoyment of owning a scope for however long time the telescope lasts or however long our interest in the scope lasts (which is incidentally usually much shorter than the scope's own useful lifetime). Our scope will get bumps, paint scratches and nicks from handling in the darkness under the stars, and it will eventually get polishing sleeks from cleaning or coating damage from not removing aggressive contaminants quickly enough. We're risking damaging the optics if we clean them and we are risking that harmful contaminant damage the optics if we don't clean them. We expose our optics to air pollution, pollen, falling dew and microscopic rain drops, all facilitating growth of lens fungus. We expose the paint on our telescopes, mounts and tripods to light (which eventually causes bleaching of paint) and dust particles if we decide to always keep a grab-and-go telescope ready for use instead of meticulously packing it down into storage boxes after each use. And if we keep our optics in a home or area with a frequently used fireplace or in a home with tobacco smoke, we expose all surfaces of all your belongings, including our precious telescopes, to substantial microscopic particle pollution (which causes about just as bad or even worse particle pollution than living near a large road intersection). For instance, mechanical clocks are mostly out of fashion today, but years ago it seemed to be common knowledge that the lifetime of mechanical clocks was substantially shorter in smoking homes or homes with fireplaces than in clean home environments.

 

My point is: I wouldn't worry too much about it. Nothing lasts forever, and I think it's better to consider the telescope as I would a new car. Sure, I don't want paint scratches and dents, but if the car stays parked in the garage under a tarp all the time, I'm are essentially wasting all the money I spent on it. So, I try to keep the eyepieces clean at all times, and I keep the main telescope optics reasonably clean, cleaning them about once or twice per year (living in a coastal climate with moderate pollen and air pollution). And I keep in mind my own eyes are slowly degrading too... I'm pretty sure my own eyes will degrade more in the next 25 years than a reasonably well-maintained telescope would in the same timeframe. Not being an alarmist, but just trying to keep things in perspective.

 

The first evidence of damage to our scope, self inflicted, accidental or through inevitable wear and tear, can actually be a bitter-sweet blessing: It allows us to stop worrying about keeping our scope in mint condition, and instead permits us to concentrate on enjoying what the scope can show us as a powerful tool rather than optical jewelry.

 

Enjoy your scope!

 

CS,

Daniel


Edited by db2005, 21 July 2019 - 01:22 PM.

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#7 ERHAD

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Posted 21 July 2019 - 12:19 PM

Thanks guys for commenting, your input is indeed refreshing and puts my anxiety to sleep... I will continue to enjoy my wonderful scope. If the sky ever clears, that is...

Best,
 

Erik



#8 rkelley8493

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Posted 21 July 2019 - 12:47 PM

The "flashlight test" is not a very good/accurate way to inspect the corrector plate. The coatings may have "spots" or blemishes, but it's simply a trick of the intense light. Here's the subject quoted from Meade's LX## Manual:

 

Note on the “Flashlight” Test: If a flashlight or other highintensity light source is pointed down the main telescope tube,
you may at first be shocked at the appearance of the optics. To
the uninitiated, the view (depending on your line of sight and
the angle the light is coming from) may reveal what would
appear to be scratches, dark or bright spots, or just generally
uneven coatings, giving the appearance of poor surface quality.
These effects are only seen when a high intensity light is
transmitted through lenses or reflected off the mirrors, and can
be seen on any high quality optical system, including the giant
research telescopes in use today. It should be pointed out,
however, that optical quality cannot be judged by this grossly
misleading “test”, but through careful star testing. The
Flashlight Test causes even the very best optics to look
“terrible”.

 

As the high intensity light passes through the Schmidt corrector
plate, most of it is transmitted through (about 98%+) while the
rest of the light scatters through the glass. As the light hits the
mirrored surfaces, most of it is reflected back (about 94%) while
the rest of it scatters across the coatings. The total amount of
scattered light will be significant, and its effects allow you to see
microscopic details that are normally invisible to the unaided
eye. These anomalous details are real, but their combined
effects will in no way impose limits on the optical performance,
even under the most demanding observing or imaging criteria.


Edited by rkelley8493, 21 July 2019 - 01:33 PM.

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#9 james7ca

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Posted 21 July 2019 - 02:09 PM

The "flashlight test" is not a very good/accurate way to inspect the corrector plate. The coatings may have "spots" or blemishes, but it's simply a trick of the intense light. Here's the subject quoted from Meade's LX## Manual:

 

Note on the “Flashlight” Test: If a flashlight or other highintensity light source is pointed down the main telescope tube,
you may at first be shocked at the appearance of the optics. To
the uninitiated, the view (depending on your line of sight and
the angle the light is coming from) may reveal what would
appear to be scratches, dark or bright spots, or just generally
uneven coatings, giving the appearance of poor surface quality....

I agree that some small scratches, dust, and other non-uniformities are probably nothing to be concerned about.

 

But, as for the "flashlight test," a vendor can say something like the above (from Meade) but that's kind of hard to accept when you shine a light down your brand new SCT and see something like in the following image (yes, that's the primary mirror as seen through the front corrector using a flashlight and the scope went back to the store that same day and was immediately replaced with no questions asked). Thus, the flashlight test does have use, it's just that you shouldn't go overboard on something that is fairly minor.

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  • Celestron EdgeHD Mirror.jpg

Edited by james7ca, 21 July 2019 - 02:23 PM.

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#10 Astrojedi

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Posted 21 July 2019 - 02:47 PM

I agree that some small scratches, dust, and other non-uniformities are probably nothing to be concerned about.

 

But, as for the "flashlight test," a vendor can say something like the above (from Meade) but that's kind of hard to accept when you shine a light down your brand new SCT and see something like in the following image (yes, that's the primary mirror as seen through the front corrector using a flashlight and the scope went back to the store that same day and was immediately replaced with no questions asked). Thus, the flashlight test does have use, it's just that you shouldn't go overboard on something that is fairly minor.

Looks like one of Chandra’s X-Ray images : )



#11 starman876

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Posted 21 July 2019 - 03:22 PM

I sent a C14 back to celestron when the flashlight test showed scratches all over the primary.  You could not see them without the flashlight.   They told me it was normal to see that.  I said bull.

They replaced the optic set and would you believe the replacement set passed the flashlight test.  Also, wonderful optics.  

 

If you see something that does not look right 9 times out of 10 it is not.  The other 1 time you have done something wrong.


Edited by starman876, 21 July 2019 - 03:23 PM.

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#12 Asbytec

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Posted 21 July 2019 - 06:15 PM

I agree that some small scratches, dust, and other non-uniformities are probably nothing to be concerned about.

 

But, as for the "flashlight test," a vendor can say something like the above (from Meade) but that's kind of hard to accept when you shine a light down your brand new SCT and see something like in the following image (yes, that's the primary mirror as seen through the front corrector using a flashlight and the scope went back to the store that same day and was immediately replaced with no questions asked). Thus, the flashlight test does have use, it's just that you shouldn't go overboard on something that is fairly minor.

I've never seen anything that disheartening. 



#13 ERHAD

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Posted 22 July 2019 - 09:23 AM

That is some scary stuff, james7ca, I would definitely be disheartened by that, but to be clear I was referring to imperfections on the corrector plate, not the mirror. The mirror, at least mine,  seems immaculate...

 

Best,

 

Erik.




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