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Dust on the glass (+ silly mistakes..)

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#1 betelgeuse91

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Posted 25 March 2019 - 03:57 PM

Hi, I just happened to see my refractor through the focuser. I pointed the scope on the wall and looked directly through the focuser (without filter, flattener or anything) with a bare eye and I saw few tiny black dots, which I suspected to be dust particles. So from the front of the glass, I used the lens dust blower to blow off the dust, and I looked again. I still saw the black spots, and I thought the dust was on the other side of the lens (inside the tube). So I made a mistake here and used the dust blower from the focuser, trying to blow off the dust on the glass from inside. And I probably made a swirl of dust sitting on the side of the tube and they got on the back of the lens. I see through telescope again, oh my god I now see so many white dust particles... 

 

I don't know if this is going to affect imaging but I know that I should not have done it... T_T Is there a way to fix this? I always hear that it's best to leave glasses untouched and I don't know why I happened to start blowing those things... Curiosity killed me. By the way, I only do imaging with this telescope. 

 

I am worried as I have been wrestling hard dealing with dust on the sensor, as they won't go away completely by taking flats... 

Thanks in advance..



#2 Peter Besenbruch

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Posted 25 March 2019 - 04:08 PM

With the proviso that I don't know what scope you are dealing with, I would recommend taking the lens cell off the tube, and blowing off the backside.

 

A lot of dust on the lens will reduce contrast, but won't be visible in your images, and it has to be a lot. Dust on the sensor is visible as dust, and it can drive you crazy.


Edited by Peter Besenbruch, 25 March 2019 - 04:10 PM.


#3 betelgeuse91

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Posted 25 March 2019 - 04:28 PM

With the proviso that I don't know what scope you are dealing with, I would recommend taking the lens cell off the tube, and blowing off the backside.

 

A lot of dust on the lens will reduce contrast, but won't be visible in your images, and it has to be a lot. Dust on the sensor is visible as dust, and it can drive you crazy.

I feel extremely nervous to take the lens off the tube myself, as it needs to be put back with perfect alignment again. Will telescope stores do it for me? And if they do, how much would they charge for the service? If you happen to know... 

 

Looking from outside, the amount of dust looks normal, but I think the dust on the back of the lens is not so visual from outside... I hope it doesn't affect much... 


Edited by betelgeuse91, 25 March 2019 - 04:29 PM.


#4 photoracer18

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Posted 25 March 2019 - 04:41 PM

Your objective should screw on and off the tube as one module. The only issue is the tube threads are very fine so depending on your scope you may just be able to thread it back on easily if its a smaller scope or you might have to put the objective module on the floor and screw the tube into it if its a bigger scope. Either way you just need to make sure the 2 parts are correctly aligned before turning them to go on. It will go on easily if its aligned correctly. If not stop and start over.

Also note that in the case of a bigger scope (over 100mm) once you have the threads loose to take it off you want to do the screw the tube off thing from the floor.


Edited by photoracer18, 25 March 2019 - 04:44 PM.

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#5 PirateMike

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Posted 25 March 2019 - 04:55 PM

One easy way to screw the lens cell back on without stripping the threads is to...

 

Start screwing the lens cell in the reverse direction (as if taking it off). You should hear (or feel) a slight "click" somewhere along the way. At that point start slowly turning the cell the other direction and the threads should screw in just fine. This should be done whenever screwing two things together!

 

I hope this helps.

 

 

Miguel   8-)


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#6 wrnchhead

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Posted 25 March 2019 - 05:32 PM

If you are having dust on your sensor problems and blew into the OTA, and now there's more dust on the inside of the lens, my conclusion would be there is dust inside your tube. If you want to fix the dusty sensor issue, cleaning all this may be the obvious solution. 

 

Refractors (generally) are pretty hard to screw up. I disassemble mine on my bed so if I drop something, it's unlikely to go crunch. But I've never owned a really nice one nor one with a collimatable lens cell, so it may be worth your time to mark the lens assembly and the tube assembly so they end up back in the same orientation. 

 

I don't have an unreasonable intolerance for dust on optics, but in KS, you will wind up cleaning it at least once a year, or you will be looking through a muddy window. 


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#7 Peter Besenbruch

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Posted 25 March 2019 - 06:03 PM

I feel extremely nervous to take the lens off the tube myself, as it needs to be put back with perfect alignment again.

What photoracer18 said. You leave the lens in its cell.

 

 

Will telescope stores do it for me? And if they do, how much would they charge for the service? If you happen to know...

Looking from outside, the amount of dust looks normal, but I think the dust on the back of the lens is not so visual from outside... I hope it doesn't affect much...

I don't have any telescope stores in my area. I can't tell what dust you have, as you haven't posted a picture. I don't know how to advise you on cell removal, because we don't know what scope you have.



#8 gnowellsct

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Posted 25 March 2019 - 06:05 PM

My grand daddy used to deal with this kind of situation by taking the focuser off and filling the tube half way with water, beer, and soap.  Then he would shake it vigorously like he was mixing a cocktail and pour it all out.  

 

A few hours to dry and good to go.  That was the way they did it in the old days.  And some of those giant f/15 12 inch refractors you can just clean out with a hose.

 

Greg N


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#9 stevew

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Posted 25 March 2019 - 08:11 PM

How about a vacuum cleaner hose up through the focuser?

Just don't get it too close to the lens...



#10 gnowellsct

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Posted 25 March 2019 - 08:41 PM

How about a vacuum cleaner hose up through the focuser?

Just don't get it too close to the lens...

Some cheese cloth over the tip of the vacuum hose, secured by a rubber band, would protect against an accidental brush against the optic.  But speaking for myself, I'd rather remove the lens cell.  

 

Greg N


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