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Equipment Discussions >> Reflectors

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pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: Seeing rules! No more fan for me? new [Re: DocFinance]
      #6490396 - 04/26/14 05:42 PM Attachment (5 downloads)









Quote:




I can't imagine having ANY fan running on my scope while trying to use it. There is no fan out there that would be smooth enough...




If the scope and mount are beefy enough you don't have to worry about fan vibration.


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azure1961p
Postmaster
*****

Reged: 01/17/09

Loc: USA
Re: Seeing rules! No more fan for me? new [Re: pstarr]
      #6490524 - 04/26/14 07:04 PM

I've had my 8" at well well over 1000x - the atmospheric dispersion was godawful but there was no vibration at all.

Pete


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nevy
professor emeritus
*****

Reged: 02/07/12

Loc: UK
Re: Seeing rules! No more fan for me? new [Re: azure1961p]
      #6490853 - 04/26/14 09:50 PM

I don't get any vibration.

Noise blocker with its own rubber suspension for vibration free blowing.


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azure1961p
Postmaster
*****

Reged: 01/17/09

Loc: USA
Re: Seeing rules! No more fan for me? [Re: nevy]
      #6490864 - 04/26/14 09:57 PM

Really nice job on that Nevy.

Pete


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beatlejuice
Pooh-Bah
*****

Reged: 04/05/11

Loc: Hamilton, ON,Canada
Re: Seeing rules! No more fan for me? new [Re: nevy]
      #6491026 - 04/26/14 11:47 PM

Again, no vibrations:



Eric


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Cotts
Just Wondering
*****

Reged: 10/10/05

Loc: Toronto, Ontario
Re: Seeing rules! No more fan for me? new [Re: beatlejuice]
      #6491064 - 04/27/14 12:19 AM

I've seen a very clever fan idea for larger Newts, probably not possible with scopes smaller than 12 inches... There's a fellow named Joe Wambow (sp.?) who goes to WSP and OkieTex with a 32 inch dob. He has a fan suspended above the center of the mirror with wires (guitar strings?) blowing down onto the face of the mirror. The fan is in the 'shadow' of the secondary mirror so there's no blockage or extra diffraction issues. There don't seem to be vibration problems - the views at above 400x on Jupiter defy description.

The smallest fan of this sort that I've seen is about 2 inches on a side so if your secondary is larger than this you could try it! Difficulty: You'll have to line up the wires with the spider vanes so as not to increase diffraction spikes....

Dave


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okieav8rAdministrator
I'd rather be flying!
*****

Reged: 03/01/09

Loc: Oklahoma!
Re: Seeing rules! No more fan for me? new [Re: Cotts]
      #6491073 - 04/27/14 12:28 AM

Quote:

I've seen a very clever fan idea for larger Newts, probably not possible with scopes smaller than 12 inches... There's a fellow named Joe Wambow (sp.?) who goes to WSP and OkieTex with a 32 inch dob. He has a fan suspended above the center of the mirror with wires (guitar strings?) blowing down onto the face of the mirror. The fan is in the 'shadow' of the secondary mirror so there's no blockage or extra diffraction issues. There don't seem to be vibration problems - the views at above 400x on Jupiter defy description.

The smallest fan of this sort that I've seen is about 2 inches on a side so if your secondary is larger than this you could try it! Difficulty: You'll have to line up the wires with the spider vanes so as not to increase diffraction spikes....

Dave




I saw that fan set-up also Dave, it was on a Webster, I believe. The views through that telescope were indeed fantastic. Clever, the way the fan was mounted over the mirror. Such a setup would probably make me a little nervous though.


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Cotts
Just Wondering
*****

Reged: 10/10/05

Loc: Toronto, Ontario
Re: Seeing rules! No more fan for me? new [Re: Cotts]
      #6491077 - 04/27/14 12:34 AM

What about sealing the tube of the newtonian entirely? Put the mirror in a solid tube, seal the back and get an optical quality window at the front end. It seems to me I've read that refractors have much reduced internal tube currents because of their sealed OTA's... Also, it has been said that Dob mirrors being near the ground are overly affected by ground temperature issues. Then there is the observer's body heat going through the truss tubes and porous shrouds... Seal the tube and these issues can go away..

Can't get an optical window for the front? How about stretching some clear mylar over the front of the tube. The coated mylar works for solar filtration without degrading resolution....

Ambitious folks could glue the diagonal to the center of a glass optical window and lose the spikes from the spider...

Wacky ideas are fun....

Dave


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schang
scholastic sledgehammer


Reged: 04/24/13

Loc: columbia, sc
Re: Seeing rules! No more fan for me? new [Re: Cotts]
      #6491426 - 04/27/14 08:49 AM

The main issue is the mass of the glass, not closed tube. Large refractors, especially those air spaced triplet APOs, also take a long time to cool down.

Edited by schang (04/27/14 08:50 AM)


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Jeff Morgan
Postmaster
*****

Reged: 09/28/03

Loc: Prescott, AZ
Re: Seeing rules! No more fan for me? new [Re: Cotts]
      #6491490 - 04/27/14 09:51 AM

Quote:

Can't get an optical window for the front? How about stretching some clear mylar over the front of the tube. The coated mylar works for solar filtration without degrading resolution....




This idea has appeared in Sky & Telescope several times. Specifically July 1978, September 1978, and August 1991. Probably not the best solution for high definition work, but fine for DSO's.

There was another interesting article I was trying to track down. The author used external ducts and fans to recirculate the air from top to bottom within a closed tube. Basically, the idea was that even within the closed tube there were temperature gradients. IIRC, he reported excellent results from the moment the scope was set up (although the scope didn't look particularly pretty).


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choran
Carpal Tunnel
*****

Reged: 12/28/12

Re: Seeing rules! No more fan for me? new [Re: Jeff Morgan]
      #6491597 - 04/27/14 11:01 AM

I'm sure there are many reasons why this wouldn't work, but I wonder if anyone has considered initially cooling a large mirror with a cold pack placed against the back of the mirror, the sort of cold pack used to treat an athletic injury. Wouldn't have to be frozen, just fairly cold. (I'm ducking as the answers to this come in).

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charles genovese
professor emeritus
*****

Reged: 02/04/06

Loc: Madisonville Louisiana
Re: Seeing rules! No more fan for me? new [Re: choran]
      #6491647 - 04/27/14 11:23 AM

Anthony Westley has peltier cooling for his scope

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Adam Taylor
insignificant bystander
*****

Reged: 11/11/10

Loc: Arizona
Re: Seeing rules! No more fan for me? new [Re: beatlejuice]
      #6491756 - 04/27/14 12:44 PM

Eric, is the Velcro at 3 points, or all the way round? And how is the fan mounted to the disc?

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Eric63
scholastic sledgehammer


Reged: 06/16/12

Loc: Ottawa, Ontario
Re: Seeing rules! No more fan for me? new [Re: choran]
      #6491764 - 04/27/14 12:48 PM

Actually, that's what I do with my 127Mak. I put an ice pack in my case an hour before I go out. But for my 6" Newt I use a computer fan for 30 minutes or so.

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beatlejuice
Pooh-Bah
*****

Reged: 04/05/11

Loc: Hamilton, ON,Canada
Re: Seeing rules! No more fan for me? new [Re: Adam Taylor]
      #6491865 - 04/27/14 01:48 PM

Quote:

Eric, is the Velcro at 3 points, or all the way round? And how is the fan mounted to the disc?




Between each bolt - 6 points. 4 small bolts hold the fan frame to the baffle which had a hole cut out of it to match the size of the fan blades.

Eric


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choran
Carpal Tunnel
*****

Reged: 12/28/12

Re: Seeing rules! No more fan for me? new [Re: Eric63]
      #6492215 - 04/27/14 05:00 PM

Great! Seems like there wouldn't be any problem with currents and so forth.
Maybe an adjustable cooling coil would be a possibility.


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svdwal
Vendor (mBrain Software)


Reged: 03/10/13

Loc: Leiden, The Netherlands
Re: Seeing rules! No more fan for me? new [Re: Cotts]
      #6492233 - 04/27/14 05:08 PM

Texereau already mentions this in his 1957 "How to make a Telescope". He also points out that the tube must be insulated.

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Jeff Morgan
Postmaster
*****

Reged: 09/28/03

Loc: Prescott, AZ
Re: Seeing rules! No more fan for me? new [Re: choran]
      #6493051 - 04/28/14 12:17 AM

Quote:

I'm sure there are many reasons why this wouldn't work, but I wonder if anyone has considered initially cooling a large mirror with a cold pack placed against the back of the mirror, the sort of cold pack used to treat an athletic injury. Wouldn't have to be frozen, just fairly cold. (I'm ducking as the answers to this come in).




I do this now. When I know I will be using the telescope, I place a cold pack in the mirror box. There is a lot of thermal inertia in there (so it can not possibly get cold), but I would like to think the cold pack gets things trending down a few degrees. If you live in a humid climate condensation may be an issue though.

There is another possibility. I have often used a 12" household fan placed on the ground behind my mirror box. I suppose one could put a block of ice (or even dry ice) behind the fan to chill the air being blown onto the mirror.

Or better yet, buy an old refrigerator large enough to hold your primary mirror. On the day of a session, place the mirror in the 'fridge and crank it down to the expected evening temperature.


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schang
scholastic sledgehammer


Reged: 04/24/13

Loc: columbia, sc
Re: Seeing rules! No more fan for me? new [Re: Jeff Morgan]
      #6493394 - 04/28/14 08:51 AM

Sound like a lot of "work" to me to observe. Had I read this before I buy a dob, I might be turned away from this thing.

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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Seeing rules! No more fan for me? new [Re: schang]
      #6493438 - 04/28/14 09:19 AM

Quote:

Sound like a lot of "work" to me to observe. Had I read this before I buy a dob, I might be turned away from this thing.




+1


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