17 replies to this topic

[TayM57]

After a few outings with the Swayze 24" that has a f/3.17 Swayze-Lockwood primary (I attribute credit to both Swayze and Lockwood for the primary work- Swayze started it, and Lockwood finished it), I noticed the star test showed the mirror wasn't completely cooling, despite 2+ hours of cooling prior to observing.

 

1-2 hours is plenty for my other scopes, which are much smaller. But for the 24", the star test was showing it wasn't enough. Outside of focus, the diagonal shadow was indistinct, indicating undercorrection. The star test also showed the mirror was holding heat (I can tell the difference between a warm mirror and seeing). 

 

It just so happened that I came across Mike Lockwood's talk at WSP, seen HERE. I watched it in it's entirely and I highly recommend watching it. It's informative and the claims are backed by empirical evidence.

 

In Mike's talk, he demonstrates seven different cooling schemes. Basically, what he found was that a rear fan arrangement was only as half as effective as it could be. This is the cooling scheme the 24" had. Mike found that a combination of rear fans + mirror box corner fans was much more effective.

 

So I undertook a project to add four more fans, all 120mm, to the 24". They're all controlled by an in-line voltage controller. The star test shows the result pretty clearly- much much better. The 24" reaches equilibrium in about 1 to 1 1/2 hours. This will vary according to nighttime conditions but the point is, I can reach equilibrium in a reasonable amount of time now and get nice tight stars.

 

All fans were mounted on mini ball head mounts, and secured using neoprene tape + 8" hose clamps. I got the idea from ad701xx's thread. Where he used couplers, I used nuts. The hose clamps are secured by a nut threaded onto the threaded stud on the ball head mounts. I can run all 7 fans on high and not notice any vibration whatsoever, even at high magnifications. The bases on all four mini-ball head mounts have rubber padding- so any fan vibrations are dampened by both the rubber bases AND the neoprene tape around the fans themselves.

 

The result is simply magnificent. A nice tight snap focus can be attained, and the star test is excellent. The Swayze-Lockwood primary can now attain best possible performance, especially at high magnifications.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The last image was a test to make adjustments to the fans- I used the mirror cover for the 24" with a tear in the middle to tell me whether the air flow was directed properly.

Edited by TayM57, 24 May 2024 - 07:38 PM.

[Tangerman]

Have you tried using the fans while the mirror is still cooling? Do the front fans improve the image before the mirror has fully cooled? In any case, it's good to see you can speed up conquering the thermals, thanks for passing on your knowledge!

[Neanderthal]

Thanks for the video link! 

[Jethro7]

Hello tayM58,

Thanks for the link. I have enjoyed talking with Mike and have learned a good bit from him

 

 

HAPPY SKIES AND KEEP LOOKING UP Jethro

[Mike Lockwood]

Thanks for posting the video link, I had posted it in my Vendors thread, and shared it on my Groups.io group.  A lot of work went into video editing by Sergio from SCAS.

 

Many don't realize that the star test is dependent on multiple variables besides the actual shape of the mirror.  Therefore, when doing the star test you are seeing the effect of the mirror's shape, the temperature distribution in the glass warping the shape of the glass, the mirror cell support effect (rear and edge supports), and also the same things from the secondary mirror.

 

Uneven temperature distribution in the glass is caused by temperature changes, and this changes the shape of the mirror in a time-varying way.  Therefore, I recommend experimentation to determine what results for the best performance with your particular mirror, telescope, and fan arrangement.

[ninelives]

Mike was kind enough to talk about star tests and mirror defects and mirror cooling at our club this past Tuesday. It was very informative! My 20" F/3.3 dob should be arriving soon and I feel more prepared to identify and deal with any collimation problems. Thanks Mike! Oh, and the discussion about night vision viewing was fascinating. Going to have to get into that.

[Aperturefever]

Thanks for the write-up mate. How far up from the primary surface are your fans sitting? I can't quite pick from the first photo - you have them firing in at a slight angle? If this is the case, did you have to play around with the angle, or how did you determine it?

[TayM57]

Have you tried using the fans while the mirror is still cooling? Do the front fans improve the image before the mirror has fully cooled? In any case, it's good to see you can speed up conquering the thermals, thanks for passing on your knowledge!

An interesting question. I’m fussy when it comes to my star images so I’ve gotten into the habit of letting my scopes cool before I do any type of observing. Rarely will I view within an hour after setting up and it’s usually to get a quick look at an object, like Venus or Mercury. Outside of that, I don’t bother. I really don’t like bloated stars. When the mirror is warm, nothing is really sharp and I do like sharp views in the EP.

 

Thanks for posting the video link, I had posted it in my Vendors thread, and shared it on my Groups.io group.  A lot of work went into video editing by Sergio from SCAS.

 

Many don't realize that the star test is dependent on multiple variables besides the actual shape of the mirror.  Therefore, when doing the star test you are seeing the effect of the mirror's shape, the temperature distribution in the glass warping the shape of the glass, the mirror cell support effect (rear and edge supports), and also the same things from the secondary mirror.

 

Uneven temperature distribution in the glass is caused by temperature changes, and this changes the shape of the mirror in a time-varying way.  Therefore, I recommend experimentation to determine what results for the best performance with your particular mirror, telescope, and fan arrangement.

I agree the star test is dependent on variables such as cell support, secondary mirror quality + mounting + conditions it is exposed to, and seeing. Generally speaking, I don’t need seeing to be perfect to be able to discern optical anomalies in the star test. A few sessions ago, right after I replaced the spider + diag holder, I was seeing some significant stig which I traced to the secondary not being seated properly all the way around the lip of the astrosystems holder. It was binding where the two ends of the diag holder shroud meets which is the end near the focuser.

 

It was also packed extremely tightly with the batting, to the point where the backing plate popped out as soon as I unscrewed the last fastner. I fixed this by trimming 2” off the back of the foam plug and reducing the amount of batting in it. I also experimented with different ways to secure the shroud without binding the mirror. I ended up shimming half of the circumference of the backing plate that the shroud attaches to with electrical tape. Doing that enabled the shroud to wrap around the backing plate and the secondary mirror without the end nearest the focuser shifting upwards due to binding. The secondary is 5.5” and ¾ thick, so about ¼” thinner than most secondaries of this size. The shroud was made just a little too oversize relative to the backing plate. Shimming fixed this.

 

When I star tested tested after implementing the aforementioned interventions to address the stig, the seeing was terrible AND the mirror was not cooled at all (I didn’t bother even turning the fans on nor waiting for it to cool as it was a quick roll out to star test), I could see very clearly the stig was gone.

 

I do plan to experiment with the cooling fans if the star test tells me I need to. Right now, I have the fans angled so that the outer 75-100% zone + mirror edge is cooled by the fans, since the back 3 fans cool the center out to about the ~50% zone from the back. I plan to dial the fans back to see what happens to the star test. The sessions I’ve had it out, I’ve had all the fans all on maximum voltage.

 

Thanks for the write-up mate. How far up from the primary surface are your fans sitting? I can't quite pick from the first photo - you have them firing in at a slight angle? If this is the case, did you have to play around with the angle, or how did you determine it?

The distance between the mirror surface and the center of the fans as they are angled currently, is 10”. This seems like a large distance but I believe it works to my advantage. The shape of the air flow from the fans is shaped like an alluvial fan or river delta, so the air disperses over the surface of the mirror instead of a narrow stream of air being directed at a specific region on the mirror. For this reason I chose static pressure fans. When deliberating on which type of fan to install, one of the concerns I had about airflow fans was directing a narrow stream of air at a localized region of the mirror. I also used airflow fans in my son’s computer that I built, and they vibrate like nobody’s business. Generally, I find static pressure fans vibrate less than airflow fans.

 

I had to experiment to find the angle I thought would work most efficiently. I did by looking how quick and how often the tear in the middle of my mirror cover lifted when the fans were on. If it flagged like a flag in high winds, I decreased the angle of the fans towards 0° (lowered the fans) until the tear whiffed about, instead of standing on end. I also put my hands around the perimeter of the mirror area to make sure I wasn’t wasting air by directing the fans at too steep of an angle (towards the floor of the rocker).

 

Hope this makes sense.

 

I realized I didn't really break down how I wired the fans. I used a spool of 22 gauge wire and wired each of the four fans to a single female USB connector. I thought about daisy chaining them, but individually wiring each fan to wires that all meet where the USB female connector is, was a bit easier to do. The longest run of wire for the furthermost fan of the usb female connector is, if I recall correctly, 66".

 

All four fans are soldered to a single female USB connector that remains in the mirror box. I then made my own 6" male to male USB cord with an in-line voltage controller that connects to a 26,800mAh battery which also powers the 3 rear 120mm fans, which are also soldered to their own female USB connector. I also made a 6" male to male with an in-line voltage controller for the rear fans. 

 

I plan to evidently make a switch board, so I can simply connect the battery to the board, and just use the switches on the board to control the fans. This would be on top of the mirror box next to the finderscope.

Edited by TayM57, 24 May 2024 - 09:08 PM.

[CrazyPanda]

I tried a similar approach with my 15" build. There are four 60mm fans mounted in the corners with baffles.

 

 

I've found that they don't really do anything to clean up star images out of the gate and cooling takes about the same amount of time as just a rear fan blowing at the back. When you turn them on and look at the boundary layer on a defocused star, you just see the air turbulence circulating around on the mirror very similar to the video posted. The problem is, the air has nowhere to go and the air streams from the four mirrors collide and just jumble up in the light path. Star images remain bloated and lousy.

 

Basically you're just blowing warm air trying to rise out of the mirror box, right back at the mirror.

 

A couple things I've considered:

 

1. Drilling holes in the corners of the mirror box to allow cool air to flow into the corner fans
2. Placing a series of exhaust fans around the mirror perimeter so that air hitting the back of the mirror and air hitting the front of the mirror gets evacuated away from the mirror

But ultimately I don't think this will work great. With such an open box, exhaust fans will do little to pull air away from the mirror, and even if you have holes drilled in the corners to help feed cooler air to the front fans, you still have the issue of those air streams colliding and the air having no place to go.

 

IMO traditional boundary scrubbing fans blowing across the face of the mirror and exhausting at the opposite side will work better to rid of the boundary layer faster, even though it risks cooling it asymmetrically.

Edited by CrazyPanda, 24 May 2024 - 11:19 PM.

[TayM57]

I tried a similar approach with my 15" build. There are four 60mm fans mounted in the corners with baffles.

 

IMG_9901.JPG

 

I've found that they don't really do anything to clean up star images out of the gate and cooling takes about the same amount of time as just a rear fan blowing at the back. When you turn them on and look at the boundary layer on a defocused star, you just see the air turbulence circulating around on the mirror very similar to the video posted. The problem is, the air has nowhere to go and the air streams from the four mirrors collide and just jumble up in the light path. Star images remain bloated and lousy.

 

Basically you're just blowing warm air trying to rise out of the mirror box, right back at the mirror.

 

A couple things I've considered:

 

1. Drilling holes in the corners of the mirror box to allow cool air to flow into the corner fans
2. Placing a series of exhaust fans around the mirror perimeter so that air hitting the back of the mirror and air hitting the front of the mirror gets evacuated away from the mirror

But ultimately I don't think this will work great. With such an open box, exhaust fans will do little to pull air away from the mirror, and even if you have holes drilled in the corners to help feed cooler air to the front fans, you still have the issue of those air streams colliding and the air having no place to go.

 

IMO traditional boundary scrubbing fans blowing across the face of the mirror and exhausting at the opposite side will work better to rid of the boundary layer faster, even though it risks cooling it asymmetrically.

 

There is some validity to what you are saying here. I do like your first idea and It's something I'd consider if I had some nice grilles to put over the holes. Drilling into the beautiful veneer though- that'd be hard. I wouldn't do it for the cooler air though, I'd do it to increase the air pressure behind the fans so the fans have more available air to pull from.

 

Sort of like in bed, when you pull the blanket over your head, it's a bit harder to breathe. When you remove the blanket, it's much easier to breathe.

 

I get what you are saying about blowing warm air back onto the mirror. It's not so much the boundary layer / warm air that I'm trying to scrub off the surface of the mirror. It's that big hunk of glass being above ambient temperature and holding heat. With the four fans blowing onto the surface, the goal is to get the glass to ambient temperature within a reasonable amount of time. Once the mirror reaches ambient, the boundary layer isn't an issue anymore.

 

Besides, the 2 miles of atmosphere above the telescope is a bigger problem than the warm air in the mirror box.

 

Your 15" I assume, is the 3/4 quartz in your signature? If so, it's a much simplier task to cool a piece of 3/4" thick 15" quartz glass than a 2" thick 24" pyrex mirror like the one I'm cooling. So, two different pieces of glass, that both require different cooling schemes.

 

My 10" does fine with 3 cooling fans on the rear. Ditto for my 8" with just the one fan in the rear.

 

The star test doesn't lie though. The corner fans do make a difference- nice tight stars and the star test doesn't have caustic lines, nor does it shimmer like a pot of boiling water.

 

Keith Rivich has boundary layer fans on his 25" and I believe, he has commented that he doesn't see a difference with them on. The problem with boundary layer fans is that one side of the mirror is cooler than the opposite side, depending on how the boundary layer fans are positioned. If they are positioned that the edge of the mirror gets exposed to the air flow, than that could be a problem.

 

The four corner fans is probably a better implementation of a cooling scheme when it comes to larger mirrors since it avoids cooling just one side of the mirror.

Edited by TayM57, 25 May 2024 - 11:06 AM.

[Tinker]

Thanks for posting this! I've been looking for ideas to do something similar after watching Mike's video.

[CrazyPanda]

I get what you are saying about blowing warm air back onto the mirror. It's not so much the boundary layer / warm air that I'm trying to scrub off the surface of the mirror. It's that big hunk of glass being above ambient temperature and holding heat. With the four fans blowing onto the surface, the goal is to get the glass to ambient temperature within a reasonable amount of time. Once the mirror reaches ambient, the boundary layer isn't an issue anymore.

 

I think what's most crucial no matter which way you go, is to evacuate the warm air from the mirror box.

 

I like Ed's CVS approach and it's what I'll be designing into my next telescope build. A pressurized plenum blowing air across the back of the mirror, and an annular nozzle around the mirror edge at the front. As the air collides in the center, it's thrown up and out of the mirror box.

[Aperturefever]

Thanks for the explanation; it makes perfect sense. Keen to see your switchboard when you get to it too. Me, I just follow Mike's advice on cooling theory. He does this stuff for a living... I don't. At the end of the day as long as you are happy with your image - and who wouldn't be with a stonking big dob?!

[Tinker]

I think what's most crucial no matter which way you go, is to evacuate the warm air from the mirror box.

 

I like Ed's CVS approach and it's what I'll be designing into my next telescope build. A pressurized plenum blowing air across the back of the mirror, and an annular nozzle around the mirror edge at the front. As the air collides in the center, it's thrown up and out of the mirror box.

What I've done in the past is intermittently run a small clip on fan attached to the mirror box. Not a full time solution obviously but an efficient way to quickly flush the mirror box with ambient air.

 

[Peter Natscher]

I think what's most crucial no matter which way you go, is to evacuate the warm air from the mirror box.

 

I like Ed's CVS approach and it's what I'll be designing into my next telescope build. A pressurized plenum blowing air across the back of the mirror, and an annular nozzle around the mirror edge at the front. As the air collides in the center, it's thrown up and out of the mirror box.

If the mirror box walls are tall, then the air being blown around the mirror will still be trapped inside then box. The warmer air rising off the mirror and cell will remain under the top wall of the mirror box if the telecope is angled lower than vertically. The warmer air won't evacuate quickly but swirl around remaining partly over the mirror as a boiling mess, as observed at the eyepiece of a defocused bright star. I saw this negative effect with a 24" Starmaster I used to own. After experiencing no cool down with its 24" 1-3.4" thick Pyrex mirror well into the night especially after hot daytime temperatures, experimented and added two fans behind the mirror and four fans placed even with the mirror's sides inside the mirror box at its interior corners.  With these six fans runnig full bore, plenty of fresh cooler air was being pulled into the mirror box from the backside and recirculating around the mirror's sides inside the mirror box and then passing along the mirrors side and out in front of the mirror. A lot of air entered the mirror box but it didn't evacuate very well. The traped air created a huge mass of boiling currents that remained mostly in front of the mirror.  The warmer air didn't leave the mirror box. In a later and smaller 16" Dob, I had John Pratte of JP Astrocraft build into my ordered Sweet 16" Dob one backside mirror fan and two three-fan banks of fans on opposite sides on the walls of the mirror box.  The two banks of mirror fans are placed alined to the front surface of the mirror.  They blow air in tandem through the mirror box from one side to the other, air in and air out. With the Dob angled 45° and lower, the lower bank of fans pulls the cooler air into the mirror box, te air crosses the front surface of the mirror, and then the upper bank of mirror fans pulls the warmer air out through the top of the mirror box.  The moving air within the mirror box is warmer than the ambient air surrounding the two boxes so the air rises also on its own along with the moving air that the fans are blowing. This is a combined upward motion and direction for the warmer air.  Warmer air rises on its own so this setup isn't fighting the natural movement of the warmer air. You don't want to blow warmer air downward, that's counter productive. I am experiencing positive mirror cooling results with this cooling system, hastening the mirror cool down on the warmer months of the year.  All fans are powered with variable speed control for fan speed adjustment. I can run the fans earl in the evening and not notice any vibrations observing at the eyepiece.

 

 

Attached Thumbnails

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[Bob4BVM]

Newt cooling, here we go, again.... This needs to be a sticky !

https://www.cloudyni...or-a-fast-newt/

 

Here is a system that covers all the bases- Pri cooling, Boundary layer, Sec dew control, sanitary pre-cooling, , pri edge-temp equalization, dust control, and more. All with a bottom-only fan(s).

 

I have two of these systems on my BT, and turning one off makes for a pretty interesting side-by-side comparison of its effectiveness

 

Did I mention Dust control ? This system uses HEPA air filtration. When i see scope sucking in the dusty air in the desert places i observe it makes me cringe. Amazing we so rarely see any kind of filter on most scope fans.

 

Where are you Ed ?

 

-Bob

Edited by Bob4BVM, 26 May 2024 - 01:16 PM.

[GeneT]

After a few outings with the Swayze 24" that has a f/3.17 Swayze-Lockwood primary (I attribute credit to both Swayze and Lockwood for the primary work- Swayze started it, and Lockwood finished it), I noticed the star test showed the mirror wasn't completely cooling, despite 2+ hours of cooling prior to observing.

 

1-2 hours is plenty for my other scopes, which are much smaller. But for the 24", the star test was showing it wasn't enough. Outside of focus, the diagonal shadow was indistinct, indicating undercorrection. The star test also showed the mirror was holding heat (I can tell the difference between a warm mirror and seeing). 

 

It just so happened that I came across Mike Lockwood's talk at WSP, seen HERE. I watched it in it's entirely and I highly recommend watching it. It's informative and the claims are backed by empirical evidence.

 

In Mike's talk, he demonstrates seven different cooling schemes. Basically, what he found was that a rear fan arrangement was only as half as effective as it could be. This is the cooling scheme the 24" had. Mike found that a combination of rear fans + mirror box corner fans was much more effective.

 

So I undertook a project to add four more fans, all 120mm, to the 24". They're all controlled by an in-line voltage controller. The star test shows the result pretty clearly- much much better. The 24" reaches equilibrium in about 1 to 1 1/2 hours. This will vary according to nighttime conditions but the point is, I can reach equilibrium in a reasonable amount of time now and get nice tight stars.

 

All fans were mounted on mini ball head mounts, and secured using neoprene tape + 8" hose clamps. I got the idea from ad701xx's thread. Where he used couplers, I used nuts. The hose clamps are secured by a nut threaded onto the threaded stud on the ball head mounts. I can run all 7 fans on high and not notice any vibration whatsoever, even at high magnifications. The bases on all four mini-ball head mounts have rubber padding- so any fan vibrations are dampened by both the rubber bases AND the neoprene tape around the fans themselves.

 

The result is simply magnificent. A nice tight snap focus can be attained, and the star test is excellent. The Swayze-Lockwood primary can now attain best possible performance, especially at high magnifications.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The last image was a test to make adjustments to the fans- I used the mirror cover for the 24" with a tear in the middle to tell me whether the air flow was directed properly.

Great detective work!

[TayM57]

Great detective work!

Thanks. This was a back breaking project, literally, because of all the awkward positions my body was in to install the wiring. I was hitting all kinds of muscles I didn't know I had. I'm happy with the star test results as well. I may tweak the set-up a little at some point in the future. Whether that is drilling holes in the mirror box to increase available pressure, I'm not sure. If the star test results continue to show good results across a large sample of observation sessions, I may just  leave it the way it's currently set up.

 

But I do plan to put in a control board.