cooling 8 inch skywatcher newtonian

[buddyjunior]

i intend to fit a 150 cm fan on the back of the mirror of my telescope --any advice would be good

[brantuk]

Lots of interesting info about cooling and fans here: http://www.fpi-protostar.com/bgreer/fanselect.htm

I have a fan on the back of my dob mirror and it does make a difference. A lot of folks fit one behind 8" primaries especially when imaging.

[craigg]

Yes, easy enough to do.  Added one to my SW 200P last year.  I used one of these: http://www.ebay.co.uk/itm/Arctic-F8-80mm-2000RPM-Silent-High-Performance-PC-3-Pin-Case-Cooling-Fan/182288768003?_trksid=p2047675.c100011.m1850&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D39839%26meid%3Dbb9e20fdd65b4b24bc0a7fb8a45a8a4f%26pid%3D100011%26rk%3D5%26rkt%3D18%26sd%3D171678430988 

and one of these to alter the speed:

http://www.ebay.co.uk/itm/Gelid-Solutions-Fan-Speed-Controller-with-Manual-RPM-Control-for-you-Case-Fans-/311047221642?hash=item486bdbf18a

However, if you have a 12v PWM based dew controller you could use that to power and control the speed of the fan instead (assuming you have a spare port), which is what I do now.

You'll need some cables, connectors, etc also.

I then bought some cheap perspex and got my son to laser cut it to size at school (good old design & technology lessons!!).  I put it behind the primary and get a good airflow coming out of the focuser when on full power and with the tube cap on, just to test.  I normally run it full steam while getting it to ambient, about 1/3 to half power when imaging/observing.  Noticeable difference in surface thermals on the primary using out of focus viewing of stars, but it can't do much about the high altitude winds (but at least you can see them with the fan).  I tried fixing it on with velcro since I thought that might help as a vibration buffer, but kept falling off in the cold, so now I just have some elastic bands around the primary adjustment bolts to hold it in place.  It's very slightly loose which helps reduce vibration, but that fan is very smooth and quiet anyway.

[CraigT82]

I'm assuming you meant 150mm not 150cm?  Or did you mean 150cfm?  Either way you don't need a fan that big for an 8" mirror, an 80mm fan (probably 25cfm?) will be more than enough.

[John]

I have a small (60mm) fan on the rear of the mirror cell of my 12" dob but I don't find that I need it. I can do low power observing almost straight out of the house and higher power after 30 minutes or so.

You won't do any harm fitting a fan or a larger fan to a smaller scope but it might not be a top priority ?

 

[James Vincent]

i have a 20 inch dob with a fan and have also had various other telescopes with fans fitted, i have not really found any advantage to having them. The image actually degrades in the 20 inch when the fan is on.Ii find it is much better to store the telescope in a cool place so that it dos not take so long to reach the ambient temperature.

[Charic]

I have a PC fan, 9V  PP3 powered, mounted inside a flexible hose, hung from my focuser tube, It simply draws air out of the tube,  I feel It works for my needs, however I think the best solution is to have air blowing across the face of the mirror, as  described in this article. http://media.skyandtelescope.com/documents/Adler-NewtThermal.pdf