Thermal Issues and Fans Successes

Mike,
You can use what they call a "hole saw". It's like a cup with teeth on the rim and it attaches to a drill. There's a center bit to guide it in straight.
You can get these saws up to fairly large sizes on-line, but most larger hardware centers typically have them up to 3".
I did this to create a focuser hole in a tube I had and it's how carpenters put holes in doors for door handles.

Mike,
You can use what they call a "hole saw". It's like a cup with teeth on the rim and it attaches to a drill. There's a center bit to guide it in straight.
You can get these saws up to fairly large sizes on-line, but most larger hardware centers typically have them up to 3".
.

I did this to create a focuser hole in a tube I had and it's how carpenters put holes in doors for door handles.

Be sure to get one that cuts metal as most are designed for cutting wood

Don,

You can use what they call a "hole saw". It's like a cup with teeth on the rim and it attaches to a drill. There's a center bit to guide it in straight.
You can get these saws up to fairly large sizes on-line, but most larger hardware centers typically have them up to 3".
I did this to create a focuser hole in a tube I had and it's how carpenters put holes in doors for door handles.

Thank you. I had seen these, but always thought they were meant for wood, not metal. They have no problem cutting a hole in a metal tube?

Mike

Thanks, Nevy, you answered my second question.


Mike

Thanks for the tips. Did you ever try it without the holes on the opposite side of the tube having the rear fan also blowing up the tube?

Alan Adler did and it was not as effective ad the heat hits the wall of the tube on the other side and basically goes all over the place. It needs a short direct path immediately out.

Pete

It would probably be better to have the vent holes at the top edge of the primary without the opposing fan, than to have the fan without the holes. At least the holes will allow some of the warm air from the primary to escape from the OTA without going all the way up the tube and further disturbing the image. Having all the warm air eventually vent out the top end of the OTA - rather than out a quicker route - is probably not a good thing if you can avoid it. In fact, I'm certain that it is bad.

Mike

Has anyone else drilled vent holes at the primary without using an opposing fan? I really ought to take my 8" Dob out at the next opportunity to see the effects that it has on the image.

Mike

I have for the time the fan was adjusted wrong it was turned off. The holes in and of themselves didn't seem to do anything though I'm sure some good was had.

Pete

I thought the vent holes would have some benefit, if nothing else than to more quickly acclimate the primary and possibly diminish tube thermals. They probably are not enough to substantially remove the boundary layer.

I keep thinking of those vent holes along the correcting lenses of SCTs and Maks. They must be doing some good.

Mike

I don't think it would be worthwhile to drill small holes as it would still take very long. Just run a fan at the back of the mirror, that should be fine. I've been doing that for years with good results.

Sure, the boundary layer is the culprit, I just run a computer fan on the back of my mirror for an hour or two and I am good to go. I know I won't be drilling holes all over my scope tube seeking perfection.

You'd be hard pressed unless you have a perfect sky to really notice anyways and you're in excellent skies all of the time. I know in my neck of the woods the skies are rarely perfect, or close to it. I'll stick with my fan on the back of my mirror, thanks.

Whoever is the first to do it here, let me know , LOL!

I don't think it would be worthwhile to drill small holes as it would still take very long. Just run a fan at the back of the mirror, that should be fine. I've been doing that for years with good results.

Sure, the boundary layer is the culprit, I just run a computer fan on the back of my mirror for an hour or two and I am good to go. I know I won't be drilling holes all over my scope tube seeking perfection.

You'd be hard pressed unless you have a perfect sky to really notice anyways and you're in excellent skies all of the time. I know in my neck of the woods the skies are rarely perfect, or close to it. I'll stick with my fan on the back of my mirror, thanks.

Whoever is the first to do it here, let me know , LOL!

From the articles I have read the big benifit of the side mounted fan is it clears up the views almost immeadiately upon turning the fan on even before the mirror has cooled.

From the articles I have read the big benifit of the side mounted fan is it clears up the views almost immeadiately upon turning the fan on even before the mirror has cooled.

I can attest to that. Scrubbing the boundary layer has an immediate effect. Leaving low vibration fans on at 1/2 speed or lower during an observing session, prevents the layer from forming at all while the mirror is still in cool-down.

Has anyone else drilled vent holes at the primary without using an opposing fan?

Several years ago I tried drilling a ring of vent holes even with the mirror surface, without using a fan. It did not seem to have any significant effect. This was a 10" mirror in a solid tube.

If you want to get rid of the boundary layer, I believe it is best to have the mirror totally out in the open, as in my post Naked Mirrors.

From the articles I have read the big benifit of the side mounted fan is it clears up the views almost immeadiately upon turning the fan on even before the mirror has cooled.

I can attest to that. Scrubbing the boundary layer has an immediate effect. Leaving low vibration fans on at 1/2 speed or lower during an observing session, prevents the layer from forming at all while the mirror is still in cool-down.

This is what makes it a winner for me as I don't usually have an hour or two the let the mirror fully cool. I get home from work at 1am and if the skies are good I like to grab n go and not wait over an hour to get good high power views. I'm going to try a few things before I do the side mounted fan though.

I don't think it would be worthwhile to drill small holes as it would still take very long. Just run a fan at the back of the mirror, that should be fine. I've been doing that for years with good results.

Sure, the boundary layer is the culprit, I just run a computer fan on the back of my mirror for an hour or two and I am good to go. I know I won't be drilling holes all over my scope tube seeking perfection.

You'd be hard pressed unless you have a perfect sky to really notice anyways and you're in excellent skies all of the time. I know in my neck of the woods the skies are rarely perfect, or close to it. I'll stick with my fan on the back of my mirror, thanks.

Whoever is the first to do it here, let me know , LOL!

Hi Markus,

Unfortunastely I disagree with that entire post - mutual respects here, but the results of a boundary fan while less than the rear fan is a very real and measurable improvement even in the rotten seeing/transparency lastnight that started at 5 then went way south. Infact because the thermals removed are within the scope it self, itd be measurable even in 1/10 seeing. Too its not a matter of drilling holes all over the tube, its a fairly resttricted lower end affair and frankly it looks better with the mods.

The boundary fan might not be for everyone. If the observer is enjoying typicaly low power wide field deepsky on the order o 15x per inch or less, its not evgen worth it. At typical planetary magnifications of 25x per inch its another story. The enlargement is enough at that point that the gallilean moons for example showed reduced flare after a minute or so of turning the boundary fan back on. Normally it should be virtually instantaneous the moment the fan blows off the layer, but the heat accumulates the moment its off and needs to be removed - too the surface of the mirror chills back down to ambient -or wonderfully close to it.

You dont HAVE to have a boundary layer fan and if it were impossible to have Id begrudgingly get on without it, but its a simple thing to install afterall and again... medium and especially high power truly favor its use.

If you had one installed today Id say look at the moons of jupiter at 250x. Get used to how they appear without the fan scrubbing the boundary layer - then turn it on, keep the image of what youve been seeing on Io for example, give it a couple minutes and then look.

another nice thing - those AWFUL bubbling thermal blobs in the out of focus stars and such are history. The ones that form like dark shadowy amoebas and fade then reform anew. Gone.

Lol its a beautiful thing Marcus, but look you have a rear fan already so most of the benefit is realized. still...

Pete

From the articles I have read the big benifit of the side mounted fan is it clears up the views almost immeadiately upon turning the fan on even before the mirror has cooled.

I can attest to that. Scrubbing the boundary layer has an immediate effect. Leaving low vibration fans on at 1/2 speed or lower during an observing session, prevents the layer from forming at all while the mirror is still in cool-down.

This is what makes it a winner for me as I don't usually have an hour or two the let the mirror fully cool. I get home from work at 1am and if the skies are good I like to grab n go and not wait over an hour to get good high power views. I'm going to try a few things before I do the side mounted fan though.

It shouldnt require more than 60 minutes cool time and if i wanted to stir the pot id bet i could have my full thickness 8" mirror equalized in 40 minutes. Its a guess but its getting win on both sides now so. At anyrate, with the rear fan i NEVER needed over an hour for cool down and Im talking from 70F to 30F or less. Brother, if it took that long... I dont know. Ive heard the horror stories of maks needing 2 to 2.5 hours to chill. I havent go the time or the patience.

Id be extraudinarly surpised if you needed a penny over 60 minutes.

Pete

From the articles I have read the big benifit of the side mounted fan is it clears up the views almost immeadiately upon turning the fan on even before the mirror has cooled.

I can attest to that. Scrubbing the boundary layer has an immediate effect. Leaving low vibration fans on at 1/2 speed or lower during an observing session, prevents the layer from forming at all while the mirror is still in cool-down.

This is what makes it a winner for me as I don't usually have an hour or two the let the mirror fully cool. I get home from work at 1am and if the skies are good I like to grab n go and not wait over an hour to get good high power views. I'm going to try a few things before I do the side mounted fan though.

It shouldnt require more than 60 minutes cool time and if i wanted to stir the pot id bet i could have my full thickness 8" mirror equalized in 40 minutes. Its a guess but its getting win on both sides now so. At anyrate, with the rear fan i NEVER needed over an hour for cool down and Im talking from 70F to 30F or less. Brother, if it took that long... I dont know. Ive heard the horror stories of maks needing 2 to 2.5 hours to chill. I havent go the time or the patience.

Id be extraudinarly surpised if you needed a penny over 60 minutes.

Pete

I have a similar temperature delta as you 72deg f to 40s f in the winter. So far with the stock 80mm fan and no baffle it seemed to never cool. My last outing I had my prototype rear baffle and it still took about an hour. I'll see what my new 120mm fan will do this weekend.

Mark,

I don't think it would be worthwhile to drill small holes as it would still take very long. Just run a fan at the back of the mirror, that should be fine. I've been doing that for years with good results.

Yes, a fan below the primary is the first thing to do to control thermal issues. But I'm looking to do everything possible to help the problem, as long as it doesn't have an exorbitant price or takes higher DIY skills than I possess.

Sure, the boundary layer is the culprit, I just run a computer fan on the back of my mirror for an hour or two and I am good to go. I know I won't be drilling holes all over my scope tube seeking perfection.

It doesn't take very long to drill seven or so holes for venting. And it doesn't take a lot of skill. I did it! Now that 3" hole at the other side will be a bit more difficult. You don't need to drill holes all over the OTA. Just where they'll do some good.

You'd be hard pressed unless you have a perfect sky to really notice anyways and you're in excellent skies all of the time. I know in my neck of the woods the skies are rarely perfect, or close to it. I'll stick with my fan on the back of my mirror, thanks.

I'm not in excellent skies. Most of the winter has poor seeing here ... those twinkling stars in a cold sky are not a good sign. Usually we only have really good seeing here in Maryland now and then in the summer and fall.

Whoever is the first to do it here, let me know , LOL!

Well, Pete and many others have already done it.

Mike

Dick,

Has anyone else drilled vent holes at the primary without using an opposing fan?

Several years ago I tried drilling a ring of vent holes even with the mirror surface, without using a fan. It did not seem to have any significant effect. This was a 10" mirror in a solid tube.

If you want to get rid of the boundary layer, I believe it is best to have the mirror totally out in the open, as in my post Naked Mirrors.

Well, I'm not doing anything like that to my 10" solid-tube Dob! As mentioned in the thread, this would also leave the mirror exposed to dewing. The dew in my area is substantial. I need to have dew strips at eyepiece, the finder eyepiece and my Telrad or they will dew quickly. I can't imagine having a completely exposed mirror in this neck of the woods. Not gonna' happen' here! I'd have to completely encircle the primary with a dew strip. What would that do to the thermal issues?

I think I'll continue to experiment with other solutions, including venting and scrubbing the boundary layer with a fan.


Mike

Pete,

The boundary layer fan is something I definitely want to do, and I've wanted to do it for sometime, first in my 8", then in my 10" Dob. The only thing that's stopping me is my lack of experience and confidence rigging up electric devices and using power tools to cut big holes in metal tubes. The theory is easy. The execution ... maybe not so much.

First, where do you get the potentiometers to control the speed of the fans and how do you set them up? Second, where do you get the rubber to mount the fans in and exactly how did you go about working with it? Maybe these are obvious things to most, but to me ... again, not so much.

Mike

Lol Mike,

Hell if I know! I just googled potentiameters and got a schematic image that looked kinda right, wired it by twisting copper. When it worked I went and soldered it and sealed with shrink tubing! Works tho by golly!

Same thing with the dew heater and led light for my finder - googled schematics and such online! Wiring a pot is simple but a little peculiar to me as I'm not an electrician at all. My crowning achievement to date is repairing/replacing blown capacitors on my flat screen.

Pete

Wiring is kind of my forte as I grew up racing electric R/C cars and the graduated to installing car audio. Try this link http://www.allaboutc...6/chpt_3/7.html
All the stuff you need can be found at your local Radio Shack. You can get one of their small project boxes to install the potentiometer into. Check out my photos of a remote focuser I wired up for my refractor. In the members galleries just put my screen name in the search box. Rob

I was thinking of going to a Radio Shack and seeing what they have there in the way of pots (that's what the wireheads call'em!) and free advice. Parallel, series, PIE ... I don't remember much from those electronics kits I put together back in the '70's. But I was able to listen to Russian on that short wave I built.

Soldering I've done. Sealing with shrink tubing? I've seen that stuff but never used it. Don't know how to use it.

I hate climbing the learning curve ...


Mike

rlmxracer,

Thanks for the link and your advice. This boundary-layer fan is something I definitely want to do.

Mike

rlmxracer,

Thanks for the link and your advice. This boundary-layer fan is something I definitely want to do.

Mike

No problem feel free to pm me with any wiring ?s.