Hi guys, quick question for you lot who have big dobs with open mirrors. Do you find fans good enough to keep the glass at ambient or is it necessary to apply a little heat?
I have a small 5" newt. I do get dew on the primary after a whole night imaging especialy if I'm pointing up. It's not major but I'd like to keep the optics as dry as possible. So I peeled the backing of the primary to expose the glass and velcroed a small 100mm 12V fan.
Probably testing tonight. What are your thoughts? Waste of time or one step in the right direction?
From what I've read if you are going to run a fan during observation you need some isolation to prevent vibrations in your optics. I'm going through the build and test myself with a 10" with a rear mounted fan so I'd be interested to hear of your results.
To what are you velcroing the fan ..?
Marc,
I have a fan on my 12"LB. I let it run all night without any vibration or dew problems.
By heating it, you introduce another whole set of problems.
You would only heat a primary if you observed in extreme conditions (snow, ice)
With my 12" dob I leave the fan running all night and have NEVER had a fogged up mirror. Even in frost and -6c temps.
The few times I have had my mirror fog up is when I forgot to turn on the fan.
It's not so much that the fan is cooling the mirror to prevent fogging, but that there is an airflow around the mirror.
Fogging will not occur when there is airflow.
From what I've read if you are going to run a fan during observation you need some isolation to prevent vibrations in your optics. I'm going through the build and test myself with a 10" with a rear mounted fan so I'd be interested to hear of your results.
To what are you velcroing the fan ..?
It's quite small and very light. It's velcroed directly on the glass by the center axis.
Quote:
Originally Posted by astronut
Marc,
I have a fan on my 12"LB. I let it run all night without any vibration or dew problems.
By heating it, you introduce another whole set of problems.
You would only heat a primary if you observed in extreme conditions (snow, ice)
Yeah John. Makes sense. I have to admit I tried nichrome wire first in a spiral shape and as soon as I pumped some amps in there I could see the focus going south. Not good
Quote:
Originally Posted by bmitchell82
visual astronomy. no problems
astro photography problems.
Thanks Brendan. Might have to switch it off when imaging. Will test.
Quote:
Originally Posted by ballaratdragons
With my 12" dob I leave the fan running all night and have NEVER had a fogged up mirror. Even in frost and -6c temps.
The few times I have had my mirror fog up is when I forgot to turn on the fan.
It's not so much that the fan is cooling the mirror to prevent fogging, but that there is an airflow around the mirror.
Fogging will not occur when there is airflow.
Ha... so you reckon I should take the fan further from the glass so I get air-flow around the cell? At the moment it's in contact with the back of the primary glass. I'll post a picture in a sec.
Here it is. You can see the main cell with the push/pull collimation screws. The mirror cell is the small one inside. You can see its edge at the bottom. The back of the mirror has some black fabric and I peeled the center portion to expose the glass and velcro the fan. You can see the glass through the fan. It's a sleeve bearing fan so hopefully no vibrations. To give you an idea of the scale the OD of the OTA is 130mm.
I have been messing about with a fan that draws air through the top of the tube and out the cell end. 2 things I noticed, one was the air was drawn from further away from the ground (a tad less moisture) and that it really helped breakup any boundary layer of warmer air on the mirror surface.
I will probably end up with 2 fans, one axial fan blowing air across the face of the mirror (and will one day have the air drawn through a peilter cooled stainless steel tube filled with desiccant) ie: cooler than ambient and much drier.
The other fan will draw the used air out the back of the OTA.
The reason for the axial fan blowing across the mirror is because it is virtually closed to any light sources ,un like a typical blade fan.
The peilter cooler will of course be controlled by an ambient and mirror temp differentiator.
Marc, re the pic, I use 3 thick rubberbands from rings I put in the fan housing and then around the collimating screws. Easy to remove and ABSOLUTELY NO vibration.
Peltier cooling, temperature differential. Sound good David but out of my league I just hope one fan will keep the mirror at ambient temp (reasonably). Thanks for the info though.
Marc, re the pic, I use 3 thick rubberbands from rings I put in the fan housing and then around the collimating screws. Easy to remove and ABSOLUTELY NO vibration.
The center of the fan is velcroed so there's no direct contact between the fan and the glass. I'd assume the velcro would help dampening vibrations if any. I guess I'll have to see on first light.
What about building a simple foam dew protector to sit on the end of the scope too Marc? I've used one made from cheap camping bedding foam with a couple of bits of velco to hold it together - easily sits on the end of the tube (about 45cm long) and greatly assists with keeping dew down on 2'ndary and primary. If I can hear other people swearing about dew I just make sure I don't image within 20 degrees of overhead for a while.
Hi Rob, yep I got that too. Flocked and much longer (about 350mm). You're right it works really well but still over time the primary gets real cold especialy when imaging at the zenith. I know.. I'm nitpicking
Talk about cheap, quick easy and effective !! After I'd been fiddling round with foam rubber boxes, springs and what not. At 5 volts no vibration at all. The mirror rear is directly exposed to the airflow.
