Smudges on SCT mirror

KG8 · #1 27-04-2007, 05:10 PM

Hi there. I started looking around your forum a week ago and decided to join so I can ask a question, if I may. I recently purchased a C11 secondhand. It had the mirror mod done, ( packing up the focus lever with silicone ) I checked it out under a suburban sky and the optics looked great. Today though I had a close look at it in the daylight and there appears to be smudges or regions where something has been deposited on the main mirror. I was thinking perhaps some residure from the curing of the silicone has wafted around onto the surface? Anyway I didn't want to strip it down and clean it unless necessary and I thought a few opinions from others here might help me make up my mind.
The marks are hard to see without some illumination, here are a couple of photos taken under a dim white led and one under a flash. Thanks in advance for any opinions.

http://img.villagephotos.com/p/2006-1/1141581/s1.jpg
http://img.villagephotos.com/p/2006-1/1141581/s2.jpg
http://img.villagephotos.com/p/2006-1/1141581/s3.jpg

matt · #2 27-04-2007, 05:29 PM

If it were me, I'd be cleaning it. However, that's just me

For peace of mind, you could always let an expert clean your mirror. I know Bintel offers such a service.

If it doesn't affect the views and there's nothing corrosive about the 'deposit' perhaps it's not worth worrying about.

You might need to do some research on what those fumes may or may not be, and how corrosive they are.

jase (Jason) · #3 27-04-2007, 05:42 PM

Leave it, unless you decide to get it professionally done. In all seriousness, a smudge of this kind will have very little affect on optical performance. I've seen some absolute filthy correct plates on SCTs. I recall one of them, a C14 - this scope delivered the best views of Jupiter I've seen to date. The southern belts were extremely rich in detail.

DobDobDob (Ron) · #4 27-04-2007, 05:47 PM

Welcome to the forum KG8, you will get a lot of great advice here

Blue Skies (Jacquie) · #5 27-04-2007, 07:13 PM

They look a bit like residue from a cleaning attempt. They shouldn't noticably affect the performance of the scope, if at all, but if you want perfection get a pro to do it (unless you're feeling very gung-ho and confident or insatiably curious about how it all works.) There is some danger that you might actually make it worse of you attempt to clean it yourself. If it was a newtonian mirror I would say go for your life but perhaps not with a SCT.

mick pinner · #6 27-04-2007, 07:29 PM

if it is silicon residue remove it straight away, silicon has a very high acid content and will eat into glass or a protective coating.
if you are sure it is not silicon just leave it.

rumples riot · #7 27-04-2007, 07:36 PM

Do not worry. One thing you should never do, is shine a torch down an SCT. It will do stuff like that. My scope does the same and my previous SCT did too. Both scopes had remained unopened from the factory and one came from Meade and one from Celestron. Light on the coatings of these scopes show all types of imperfections and what look like defects. It is really normal.

So no need to clean the optics. If the star images look good, and you views of the heavens are sharp then you don't have any problem. :-)

KG8 · #8 28-04-2007, 04:07 AM

Thanks for all the input Guys, and for the welcome. Whatever it is it's been there for some time, Months at least, so I doubt if it will degrade further. I think you may have pegged it blue skies, residue from an attempt to clean off some finger marks perhaps? I will go with the cautious advice and just leave it (as long as I don't notice any serious abberations when viewing) and no doubt in six months I will have forgotton all about it.

astro_nutt · #9 28-04-2007, 06:07 PM

The primary mirror on my 10" dob looks good in normal light but shine a torch on it and you'll see a swirl pattern across the surface..it looks gaudy like someone has waved white spray paint over it..but..it still delivers a clear image!!..only when the image becomes seriously degraded when compared to a new mirror then consider a professional clean or recoating..otherwise leave it and enjoy!!
Cheers!

csb (Craig) · #10 05-05-2007, 01:42 AM

Hmm, why not shine a torch down the sct?

Does that mean it's ok for a newt?

Craig

rumples riot · #11 05-05-2007, 10:51 AM

If you read the literature it states that shining a torch down the scope is not a good test of the optics. It states that you will see what looks like defects in the mirror figure and surface and that those defects do not really exist. Those smudges are present on my scope and I have nice optics and have gained some great Jupiter, Saturn and Mars images. So shining a torch down the tube of an SCT will only play tricks with your mind.

I have not noticed this effect on newtonian mirrors. Perhaps the corrector plate is the caussative factor here.

casstony · #12 05-05-2007, 01:40 PM

I just pointed a torch into my sct and saw smudges, scratches, etc. If the scope performs well under the stars I think you should not worry about the mirror and enjoy using it. I've suffered from optics paranoia in the past though so I know how you feel.

KG8 · #13 05-05-2007, 11:23 PM

optics paranoia

that's what I'm suffering from alright. I'd tell you about the worries I have had over a new eyepiece but it would only make you queesy and want to turn away from anything further I post

I think I'm getting better, a few more months perhaps...

csb (Craig) · #14 13-05-2007, 10:30 PM

For me that paranoia can be translated as "I must be losing some image quality and now I'm not satisfied with what I see"

Of course there was satisfaction before.

20 years ago my first scope was a Pentax refractor of about 60mm. It cost $600 and I was very happy with what I saw through it.

Then I read that really you need at least a 4" to get a good look at things. Boy did that make me dissapointed in my scope.

But when I got a 6" newt (only 2 years ago), I thought that the images in the Pentax (as I remember it) were actually sharper and I could see just as much (and probably there was more contrast).

But I do tend to think that a good quality ep is essential to get the most out of a good scope.

Craig

Astroman (Andrew Wall) · #15 13-05-2007, 10:41 PM

I shone a torch down my tube and could see light through the other end

LOL

astro_nutt · #16 14-05-2007, 04:02 PM

Mine does the same thing if I shine the torch from behind the primary mirror..it's like looking through mirrored sunnies/two-way mirror..shiny on one side but you can see through the other..and unless you can see a lightbulb's filiment..it's fine!!
Cheers!

shredder · #17 20-05-2007, 06:58 PM

Actually the old celeston manuals state exactly what you are seeing, that if you shine a torch down a SCT you will always see marks, regardless of how well its been cleaned. It doesnt affect performance, and in truth you will try endlessly to get rid of it. I am speaking from experience here, my old scope didnt have coatings on the corrector so cleaning it wasnt such an issue, but still could never get rid of all the marks when shining a torch down it. In the end I decided to finally read the manuals, and promptly gave up. And it never affected performance.

But it does look like someone has cleaned your scope in the past, the marks look circular, as if someone has whiped a cloth (say clockwise) around the plate. This is a mistake in that if the cloth was dirty (heaven forbid) you leave a nice circular scratch right around it. A profesional always whipes it out in a straight line from centre to edge.

KG8 · #18 20-05-2007, 11:38 PM

I did the corrector plate the other day, it was a complete mess and still looks dusty on the inside. I took my time, flushed it with a spraypack full of distilled water with a little mild soap, then sprayed and sprayed with cold distilled water ( might warm the water if I ever do it again) and in the end it was clean but with streaks and some finger marks. I took to it with a bag of cottonwool balls and the mild soapy water, just dabing at it with gentle wipes, then more clean water and the hair dryer to blow it off. In the end there were still streaks all over it so I damped it down progressively with a moist wool ball, then lightly buffed it with a dry one while I held the hairdryer over it (on cool) That worked, perfectly clean, no streaks.

shredder · #19 21-05-2007, 04:37 PM

Sounds good!

Would be interested to hear how it now goes with the torch test? I can't quite recall but I believe doing it at night shows up more marks than in the day....

KG8 · #20 22-05-2007, 12:39 PM

I guess your talking about the primary mirror there shredder? Funny how they have all these taboos about the optical surfaces. I did a torch test down the tube of one the showroom floor, with the same torch, down on the coast last week. I saw some anomalies on the new mirror but nothing like the large smudges I have on mine. I'll get them off one day but I'm not in any rush. The corrector plate was another matter. It had about 3 years of atmospheric deposits on it, and I think a bit of cigarette smoke residue as well. I took a couple of photos of the corrector, before and after with the flash on, just for reference. There was a big BIG difference and I imagine all the crud would have been seriously scattering the incoming light.

before http://img.villagephotos.com/p/2006-...copedewS02.jpg
after http://img.villagephotos.com/p/2006-...ectorafter.jpg