Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Do i need to clean my primary mirror on my Dobsonian

  • Please log in to reply
13 replies to this topic

#1 bluesilver

bluesilver

    Vostok 1

  • -----
  • topic starter
  • Posts: 127
  • Joined: 03 Jul 2020

Posted 13 July 2020 - 06:50 AM

Hi,  I had a strange issue tonight when observing the planets with my 16" Skywatcher dobsonian with goto.

Rolled the scope out about and hour and a half or so before nightfall to get it acclimatised  to the air as it cools.

As the night went on, i found that right in the centre of the eyepieces i was getting what looked like a slight fog issue,  kind of like when your glasses fog up.
But it was right dead centre of the eye piece, Only noticed this when viewing the bright planets.

I thought i might have some issues with the eye piece internally fogging up,  So swapped it out for another and same issue.

Got me thinking a bit,  so i got a torch and had a look at the primary mirror only to see that there was condensation / fog right around the centre of the mirror and then also right around the outside of the mirror,
The rest of the mirror appeared to be fine,    forgot to take a look at the secondary.

 

The night was very cold and there was condensation all over the outside tube.

So i am thinking,  do i need to clean my primary,   as i was getting that odd due/ fog on the primary but only around the outside and centre.
And if so,  what is the best way to clean them?

Or is it just one of these things that happen?
I have fans for it,  but in my error,  didn't have them on.

Any advice is appreciated.
Thanks.



#2 tjschultz2011

tjschultz2011

    Ranger 4

  • *****
  • Posts: 373
  • Joined: 02 Feb 2019
  • Loc: Eldorado, WI

Posted 13 July 2020 - 07:08 AM

Your mirror is probably fine as is. They have to be pretty dirty before it makes sense to try and clean them. Often, you can do more damage by cleaning. Sounds like it was just a good night for condensation to happen. Usually the secondary mirror will fog first so many install a dew heater for that one to use on the worst nights. Is this a truss tube dob or solid tube? Most of the time the primary mirrors are fine unless it's a truss design and you don't have a shroud over the trusses. The fan likely would have helped some too. 



#3 cuzimthedad

cuzimthedad

    Just Be Cuz

  • *****
  • Moderators
  • Posts: 7,558
  • Joined: 09 Apr 2006
  • Loc: Nampa, ID

Posted 13 July 2020 - 11:32 AM

When I lived in Northern California, dew was always an issue. The couple times I had problems as you described were due to the secondary fogging up after I had forgotten to replace the battery on the dew heater. My best guess is that your secondary probably fogged up.



#4 Napp

Napp

    Mercury-Atlas

  • *****
  • Posts: 2,642
  • Joined: 26 Jul 2015
  • Loc: Northeast Florida, USA

Posted 13 July 2020 - 11:52 AM

Yep, could be the secondary. Could also be the eyepiece. The human eye is very damp.
  • Codbear likes this

#5 Codbear

Codbear

    Apollo

  • *****
  • Posts: 1,038
  • Joined: 23 Jan 2016
  • Loc: Novato, CA

Posted 13 July 2020 - 01:57 PM

Yep, could be the secondary. Could also be the eyepiece. The human eye is very damp.

I thought I was slowly going blind a couple of years ago. I looked at a planet (where the effect was the most noticeable) it would fade a bit after 10 seconds or so.

 

This happened two sessions in a row, so I pondered what was different about those sessions compared to previous ones. Then I realized the only thing that had changed was that I had purchased and attached Dioptrx to my eyepieces. Next time I tried an eyepiece that didn't have a Dioptrx...no fading.

 

A small part of my exhaled breath got into the tiny gap on the side of the Dioptrx and was slightly fogging the inside glass of the Dioptrx. It only happens in winter and I now hold my breath at the eyepiece and exhale away from it.


  • cuzimthedad and Napp like this

#6 cuzimthedad

cuzimthedad

    Just Be Cuz

  • *****
  • Moderators
  • Posts: 7,558
  • Joined: 09 Apr 2006
  • Loc: Nampa, ID

Posted 13 July 2020 - 02:30 PM

Usually for me, fuzzy views were due to unremoved bottom ep caps. The other night, I forgot to take the dust cover off my refractor. Duh.



#7 bobruben

bobruben

    Explorer 1

  • *****
  • Posts: 93
  • Joined: 03 Mar 2017
  • Loc: Urbana, IL

Posted 13 July 2020 - 03:06 PM

Try looking using your other eye.

 

My cataracts are worse in my right, dominant eye, and give the whole visual fields a foggy appearance. I had to switch to my left eye years ago.



#8 bluesilver

bluesilver

    Vostok 1

  • -----
  • topic starter
  • Posts: 127
  • Joined: 03 Jul 2020

Posted 14 July 2020 - 02:10 AM

Thanks for the replies,  Yes it is the colapsiable 16" kywatcher Dobsonian.

I use an Astrozap shroud over the trusses.

Never thought of a dew heater,  let alone know where to get one from.

There was a bit of fog i guess you could call it on the primary just around the centre and around the edges  when i shone a torch in there to see what was going on.

 

I like the sound of the dew heaters,  will have to see where they are available from.

 

The night was a cool night also,  tempretures were around 32F - 30F,  so was getting cool.



#9 Napp

Napp

    Mercury-Atlas

  • *****
  • Posts: 2,642
  • Joined: 26 Jul 2015
  • Loc: Northeast Florida, USA

Posted 14 July 2020 - 09:34 AM

I have a 16 inch ES.  Here in Florida condensation can appear on the primary. I just leave the fans running and that usually keeps the primary clear.  I haven’t built a heater for the secondary yet but I will.  For now I have been using a 12 volt hair dryer to remove condensation from the secondary, finder and eyepieces.



#10 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 48,202
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 14 July 2020 - 12:52 PM

Increase the air flow up the tube and the secondary won't dew.

Add a tube extension like an SCT dewshield to the upper and the secondary is far less likely to dew.

And ALWAYS use a shroud.

And clean the mirror if it looks dirty (at least once a year--more if you observe a lot):

Use this technique and it's very very easy:

https://www.youtube....h?v=9Y8xFnXFVGQ


Edited by Starman1, 14 July 2020 - 12:53 PM.

  • cuzimthedad likes this

#11 kb58

kb58

    Messenger

  • *****
  • Posts: 415
  • Joined: 09 Nov 2018
  • Loc: San Diego

Posted 14 July 2020 - 01:47 PM

That process surprises me; I thought it was a no-no to ever touch the mirror with a bare hand. He even mentions how if your finger catches a piece of dirt, it'll just roll off. That seems ripe for scratches. That said, I've never washed my mirror so what do I know; I guess if this way works, that's pretty sweet, and simple.



#12 cuzimthedad

cuzimthedad

    Just Be Cuz

  • *****
  • Moderators
  • Posts: 7,558
  • Joined: 09 Apr 2006
  • Loc: Nampa, ID

Posted 14 July 2020 - 01:52 PM

That process surprises me; I thought it was a no-no to ever touch the mirror with a bare hand. He even mentions how if your finger catches a piece of dirt, it'll just roll off. That seems ripe for scratches. That said, I've never washed my mirror so what do I know; I guess if this way works, that's pretty sweet, and simple.

Tried and true process and has worked every time for me. I used to take my 20" mirror on the deck and use an old wash pan to do it as it wouldn't fit in the kitchen sink.



#13 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 48,202
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 14 July 2020 - 02:51 PM

That process surprises me; I thought it was a no-no to ever touch the mirror with a bare hand. He even mentions how if your finger catches a piece of dirt, it'll just roll off. That seems ripe for scratches. That said, I've never washed my mirror so what do I know; I guess if this way works, that's pretty sweet, and simple.

For many years, the standard technique was to submerge the mirror and drag a cotton ball across the mirror.

That did not seem to eliminate the haze on the mirror, and it could catch a piece of grit and carry it across the entire mirror without feeling it.

Clean finger tips can feel a piece of grit immediately and stop, allowing you to re-rinse the mirror so the grit doesn't get dragged across the mirror.

After all, the surface is lubricated by water, and the coating is essentially quartz, which is pretty tough stuff, so your fingers aren't going to do any damage.

If I encounter anything gritty on the mirror, I stop and rinse the mirror thoroughly, then submerge it and resume.

You never want to touch a dry mirror with a dry anything, but a submerged mirror is wet.

 

The same goes for eyepieces--a wetted lens can be cleaned.  A dry lens will be scratched.


  • cuzimthedad, Illinois and sopticals like this

#14 RLK1

RLK1

    Viking 1

  • -----
  • Posts: 643
  • Joined: 19 Apr 2020

Posted 14 July 2020 - 03:29 PM

What Don said plus I'd add a few tips: Wash your hands first to remove skin oils/clip your nails to avoid scratches/nestle the mirror in a towel if you wash it it a tub or sink so the edges don't impact anything hard and cause chips/ remove the washed mirror carefully and deliberately with firm support because you're dealing with slick sides. Other than that, you should be good to go...


  • Starman1 and cuzimthedad like this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics