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Do I need to clean the primary mirror on my Dob?


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Looking for some advice again please folks. 

I bought a second hand Skywatcher Skyliner 250 over the summer and got first light with it a few days ago. Very pleased with it, best views of Jupiter and Saturn I have ever had with a telescope. With a 10mm eyepiece, banding was clearly visible in Jupiter's atmosphere and I could clearly see Saturn's shadow on the rings. At one point, I thought the optics were fogging up but this turned out to be due to Jupiter just grazing the top of some trees in the next garden. While investigating the cause however, I shone a torch down the tube to check the primary and was horrified to see how dirty it looked.  I checked it again in daylight the next day just to make sure it wasn't dew. If I look at it in natural daylight (as I did when inspecting it prior to purchase) it seems fine, but if I shine the torch at it the dirt is there. I took a couple of pics to illustrate this, see below:

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I've read up on how to go about cleaning the mirror so I think I'm up to speed on that, but I have seen a lot of sources saying not to clean if it isn't necessary due to the potential risks associated with the process. Given the performance doesn't seem to be impaired, should I leave well alone or am I likely to see a big improvement if I do clean the mirror?

Thanks

Mike

  

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I don't think you'll see a big improvement- it's surprising how dirty a newt mirror can get without apparent impact visually. That said, if you don't know what the source of the muck is, it might be better to get rid of it than risk damage to the mirror . A very careful wash (primarily using the gentle sloshing of water over the mirror) and rinsing with de-ionised water afterwards can assure you that you're getting the very best out of your instrument.

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This is right up my street ! 

I bought a 12" dob from another SGL member and the only thing "wrong" was that the primary was really dusty . I actually had to get the primary recoated in the end because it was quite badly stained. BUT... when my mirror was caked in dust etc i still got acceptable views through it  maybe just losing a bit of contrast . I took the mirror out and cleaned it which made a difference. The views became sharp again , despite the staining 

The point i am trying to make is that the primary on my scope was in a very poor state but still produced fine viewing . If you are getting great views then why clean it although i bet you are like me ,can't stand seeing dust on the primary :) . But  cleaning a mirror isn't the perilous procedure that we are led to believe . 

 

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Rule No.1 for Newtonian Owners:

NEVER shine a light down the tube at the mirror - not that it will endanger your eye sight, just your peace of mind - as it will ALWAYS look dirty. Even if it isn't.

If it really bugs you however cleaning a mirror is very straightforwards - as noted above it is not a dark art, and there are plenty of on line video guides on how to do it - but to be honest your mirror looks fine and if it ain't broke don't fix it.

 

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Personally, I would clean it. With patience and gentle method, no harm is done.

It can be quite shocking to hear how much dirt would people accept on their mirror before considering cleaning it.. One time I was in a public observatory and the 20 f4 dobson had a large seagull poo on the primary mirror in addition to an awful lot of dust, dirt, fungi, etc. Nearly every guest noticed the status of that mirror and some, including me, asked the guy offering the tour, why that mirror was so dirt. His answer was "it does not matter, it does not affect the views".  A classic example of how much people can neglect issues.

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5 hours ago, Stu1smartcookie said:

This is right up my street ! 

I bought a 12" dob from another SGL member and the only thing "wrong" was that the primary was really dusty . I actually had to get the primary recoated in the end because it was quite badly stained. BUT... when my mirror was caked in dust etc i still got acceptable views through it  maybe just losing a bit of contrast . I took the mirror out and cleaned it which made a difference. The views became sharp again , despite the staining 

The point i am trying to make is that the primary on my scope was in a very poor state but still produced fine viewing . If you are getting great views then why clean it although i bet you are like me ,can't stand seeing dust on the primary :) . But  cleaning a mirror isn't the perilous procedure that we are led to believe . 

 

I am a bit OCD when it comes to dust on optics! I use camel hair brushes on my camera lenses and other optics to keep them dust free but this is not an option for a newt!

5 hours ago, Laurieast said:

Rule No. 2 for Newtonion Owners:

Always have the open tube horizontal when doing anything. Exception being collimation of primary.

Remember gravity and falling objects.

Good advice, though I did make sure the wrist straps were in use when using the torch. 😁

As for cleaning, I am a bit nervous about this despite the reassuring words above. I'll research this further but for now, as it seems perfectly usable I think I'll put it through its paces as is and monitor it with a view to cleaning it in the spring or sooner it it deteriorates further.

Thanks all.

Mike

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4 hours ago, Piero said:

Personally, I would clean it. With patience and gentle method, no harm is done.

It can be quite shocking to hear how much dirt would people accept on their mirror before considering cleaning it.. One time I was in a public observatory and the 20 f4 dobson had a large seagull poo on the primary mirror in addition to an awful lot of dust, dirt, fungi, etc. Nearly every guest noticed the status of that mirror and some, including me, asked the guy offering the tour, why that mirror was so dirt. His answer was "it does not matter, it does not affect the views".  A classic example of how much people can neglect issues.

So you don’t think he was right?

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19 minutes ago, Spile said:

So you don’t think he was right?

I don't recall saying anyone was wrong. I canvassed opinion, weighed up the responses and came to a decision based on the comments made. I *will* clean it, it's just a question of when.

If by chance a seagull does poo on it, rest assured I'll do it sooner. 

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