Being the proud owner of a 12” Lightbridge and being fearful of dust, last weekend I looked into the mirror and was horrified to see specks of dust and blemishes all over the mirror. I immediately marked it down for another clean this next weekend.
However, the sky here in Brissy was so magnificent last night, that out came the scope which I trained on Eta Carina. What magnificence I thought, all those countless bright stars that I could not fit into the EP field of view. But Oh dear, how much better would have been the sight if only my mirror was perfectly clean. ..
Then I came up with a plan – lets test the mirror with a large simulated “blemish”.
So I stuck out my index finger and held it in the middle of the light path, and then looked through the EP. No difference. Stuck out 2 fingers. No difference. Then I stuck into the light path my whole hand – perpendicular to the light path. All the stars were still there – just a tad dimmer this time. That was all – a whole hand …. So what does this prove????
DON’T WASTE YOUR TIME WORRYING ABOUT HOW DIRTY YOUR MIRROR LOOKS, do the JimmyH dust N blemish test (pat.pending) and prove to yourself that a few blemishes and dust on your mirror make not a SCRAP OF DIFFERENCE!!
Jimmy,
I looked at my mirror and thought the same - it needs a clean. Then searched the web for advice etc. The actual process is not complicated, provided you take care and time. But the main advice I picked up was - don't do it unless you really have to.
So your "JimmyH dust N blemish test" has a real market.
But we will have to wait till you get the copyright sorted before we can use it!
Jimmy, I have just given mine a wash,both Primary and Secondary,It looks very nice, you are right when you say don't do it unless you have to , but eventually there will come a day when it really has to be done, Then you will notice the difference
You figured it out! Welcome to the Oscar Madison school of astronomy (Oscar Madison was the "slob" component of the "Odd Couple" that used to be on TV Felix UNger was the neat freak)...
While a mirror does need to ahve a lot of dust to impact the view, it is important to keep "gunk" off of the mirror that might etch or eat through the coatings over time...
I'm with you Scott. I'd be starting to worry about some of the nastier things in the dirt on my mirror. They might cause permanent damage to the mirror surface if left there too long (sort of like not brushing your teeth for a long time).
John...it might be time to give your mirror a bath
The reality is the human eye would struggle to pick up a slight decrease - lets say 1% due to dust on the mirror - at the eye piece. The human eye is not a wonderful quantitative measuring system. Its designed better to compare and see movement - we were hunters once! So though no difference may be seen at the eye piece dont mean there is no difference! If its a little dust I wouldn't worry - if its a lot I would clean it.
Last edited by Vanda; 17-05-2010 at 10:06 PM.
Reason: amended text.
I agree with all the previous comments, BUT..............I have washed the mirror a couple of times using different techniques explained here and elsewhere. The result was very little change.
I also used the spray on (peel off) cleaner used by professional observatories, again very little difference between dirty and clean.
Whilst the views with this mirror have always been very good, I feel that the original coatings (the scope is just over 4 years old) are deteriorating.
This could explain the lack of visual improvement, when cleaning.
I'm open to suggestions.
As a mirror maker, what is your opinion Mark?
Keep your mirror nice and clean . You wouldn't let your refractor lens get like that so why let the mirror get dusty? Mirrors already scatter more light due to the aluminium coatings. Dirt and dust will also lower the life of the coating as it will absorb harmful airborne pollutants and really activate whenever there is moisture around.
I cleaned my Mirror last week and the difference between before and after was chalk and cheese.
I know which state of cleanliness I would rather have.
Cheers
Last edited by astroron; 18-05-2010 at 11:02 AM.
Reason: Clarety
Don't forget that usually both the primary AND the secondary are equally dirty, so you loose light twice through the system. The hand test is effectively just mimicking a single dirty surface.
I cleaned my mirrors recently and it did make a noticeable difference.
, you are right when you say don't do it unless you have to , but eventually there will come a day when it really has to be done, Then you will notice the difference
Would you let the windscreen on your car get that bad ?
