Boundary layer fans for a dob.

[Mutley]

I'm thinking of adding boundary layer fans on a 20" dob, maybe 3 in a line on one side of the mirror box. I'm sure many people have already done this , so I'm looking for recommendations for a neat solution - fan/wiring/connector/battery case. Could anyone who's been down this route offer any advice? Many thanks. Mut.

[Merlin]

Type in thermal management in Newtonian reflectors and click on the Sky&Telescope article.

[Piero]

I assume you have a 20" truss dobson and that the reason you want to do this is for gaining steady views at higher powers.

I made 3 holes at the front and 3 at the back of the mirror box of my 12" f6 dobson, added fan grids, but not the fans. The height of the holes are right above the mirror edge so that the outside air coming from the ground flushes off the internal air above the mirror surface, without directly hitting the mirror edge, and eventually leaves from the back holes. The back of the mirror is fully exposed as this sits on a Kriege mirror cell. When testing, the views were very good in my opinion. Therefore, I decided not to install any fan. This mirror box is 15" deep. Also, the light shroud is generally lifted up at the bottom (approx 6"). 

 

I didn't make holes in my 16" f4 mirror box as this is quite shallow (11") and a bit wider. I found that pulling up the light shroud from the bottom for about 6" is sufficient for achieving the same effect.

In both cases the views become steadier and the magnification can be pushed an additional 100x, sometimes more.

Edited May 15, 2023 by Piero

[Merlin]

That’s clever. A passive ventilation system. Have you had this system working in periods of heavy dewing?

[Piero]

I've never had dew issues with the primary mirror, likely due to the fact that this is never really below ambient temperature because of size and thickness. I have experienced dew issues with the secondary mirror of my 12" dob as this is fully exposed, but this does not happen frequently. The cell of the secondary mirror of my 16" is an astrosystems and seems to shield the mirror better. With that telescope I also have an astrosystems dew heater installed behind the mirror, so the dew is never a problem with that telescope anyway. 

[Mutley]

Thanks for the suggestions, I'll have a read of the S&T article Merlin.  I have a David Lukehurst dob, circa 2008. Piero I read your thread about the extensive modifications you made to your Lukehurst. So much of it was familiar to me ( In a bad way). I've since made some similar modifications to the mirror cell, and converted to a sling support. My 2008 model used 2 posts for lateral support. The star tests were awful in the unmodified scope, and to be honest I had no idea why. I even contacted the mirror maker to confirm it was genuinely one of theirs (Oldham optical -it was genuine). Better now with the sling/ furniture pads/removal of those bolts for restraint of floating triangles (see Piero's thread). But still much room for improvement and I'm wondering if remaining problems are thermal in origin. Like the passive cooling idea for boundary layer. Certainly worth a try before fitting fans.

Piero if we ever meet I think I owe you a pint.

[AstroKeith]

I have an 18" home made ultra compact Dob. Its been through a number of iterations and I thought it was as good as it was going to get.

Then last year I considered thermal management more closely.

a) I added a big fan to the back

b) 4 x 80mm fans in a row blowing across the front surface https://www.amazon.co.uk/gp/product/B00IOIJ4AC/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

c) Space blanket insulation over the shroud to minimise tube currents.

a) just helps cool the basic mirror down. b) definitely helps - key was finding vibration free fans. The ones I found (in the link) are so smooth I cannot hear them running. c) helps a bit especially on really cold nights. It also stops the lycra shroud absorbing dew.

[Mutley]

Thanks AstroKeith. Yes I've been thinking more about potential vibration problems since reading around the subject - I've bookmarked that Amazon link for when I've done a bit more reading. What size was the rear fan btw? Was it the same brand?👍

Edited May 18, 2023 by Mutley
Addendum

[AstroKeith]

18 minutes ago, Mutley said:

Thanks AstroKeith. Yes I've been thinking more about potential vibration problems since reading around the subject - I've bookmarked that Amazon link for when I've done a bit more reading. What size was the rear fan btw? Was it the same brand?👍

It's a 120mm fan I had lying around, but it is also quiet/very low vibration.  https://akasa.co.uk/update.php?tpl=product/product.detail.tpl&no=181&type=Fans&type_sub=Ultra Quiet Fan&model=AK-183-L2B

[Piero]

21 hours ago, Mutley said:

Thanks for the suggestions, I'll have a read of the S&T article Merlin.  I have a David Lukehurst dob, circa 2008. Piero I read your thread about the extensive modifications you made to your Lukehurst. So much of it was familiar to me ( In a bad way). I've since made some similar modifications to the mirror cell, and converted to a sling support. My 2008 model used 2 posts for lateral support. The star tests were awful in the unmodified scope, and to be honest I had no idea why. I even contacted the mirror maker to confirm it was genuinely one of theirs (Oldham optical -it was genuine). Better now with the sling/ furniture pads/removal of those bolts for restraint of floating triangles (see Piero's thread). But still much room for improvement and I'm wondering if remaining problems are thermal in origin. Like the passive cooling idea for boundary layer. Certainly worth a try before fitting fans.

Piero if we ever meet I think I owe you a pint.

I see now..

Sorry to hear that you have been experiencing similar issues to mine. I really wonder how many of his telescopes share the same problems - my guess is many unfortunately.

Like you, I also contacted the maker of my mirror (John Nichol) to confirm that it was really made by him. It was before doing the large telescope redesign as I wanted to be sure that the mirrors were authentic. It is a shame that a customer has to go through so many issues. It isn't just about testing and some improvements. It is about real fixes needed due to poor and incorrect design choices. I feel a lot of people don't understand this.. these are the problems that can ruin a hobby because the whole time is invested dealing with tricky issues (a few of them unresolvable unless the component is redesigned), instead of enjoying observations.

I am not a telescope maker nor a wood/metal worker, but I spend quite a lot of time reading and understanding issues and solutions. The work I did on my two dobsons is nothing of innovative, but contains some of the best designs from talented engineers that I could implement with my basic skills and tools.

For unknown reasons (I raised the suggestion but no reply), SGL does not allow one to have more than one link in the signature. Here is the link to the album containing the redesign of my 12" Lukehurst dob (now called Phoenix as the bird reborn from its ashes..). I don't even called Lukehurst dob because a) it is significantly different, b) it works now, whilst the original telescope did not, and c) I don't want to remember the nightmare I experienced with it due to that maker. 

https://photos.app.goo.gl/qPdsWRQUiqxmm9ec9 

 

Whether in this thread or via PM, let me know if you would like me to share details about the changes. Happy to help.

 

From your original post, I didn't understand you had a Lukehurst telescope. I don't know in detail how far you have gone with improvements. Issues caused by boundary layer are very minor compared the rest. I would make sure that all the other issues are fixed before investing time and resources on improving the boundary layer.

[Mutley]

13 hours ago, Piero said:

Whether in this thread or via PM, let me know if you would like me to share details about the changes. Happy to help.

Thanks! Next time I get the mirror cell out I'll take some pictures of what I've done so far and post them here. I would be interested to see what you think. 

I read that sky and telescope article as recommended by Merlin, quite an eye-opener :

On 15/05/2023 at 21:21, Merlin said:

Type in thermal management in Newtonian reflectors and click on the Sky&Telescope article.

It's clear here the next few nights so I plan to do some more testing on thermal management. I was thinking of pointing a household AC fan at the mirror for a couple of hours - just to see if there is any improvement.

[Mutley]

Update. The AC fan worked quite well - star tests were nice and round, no sign of astigmatism. I was able to collimate using the star test for the first time ever. So I think there may be some benefit from fitting fans

Edited May 21, 2023 by Mutley

[Piero]

Did you direct the fan to the front of the mirror (e.g. from the top of the mirror box)? 

Would it be possible for you to take some photos of:

- mirror box

- mirror from the top of the mirror box

- mirror from the back of the mirror box (ideally showing the mirror cell)

- some photos of the mirror cell as fitted in the mirror box (no need to take it out)

 

Fans do help in my opinion, particularly the one fitted at the back of the mirror. The side fans are not for cooling the mirror down, but for avoiding the formation of 1-2" of slightly warmer air released by the mirror as this cools down. Wiping this out improves the views.  

[Mutley]

14 hours ago, Piero said:

Did you direct the fan to the front of the mirror (e.g. from the top of the mirror box)? 

Yes, pointed the dob up at about 45 degrees and hooked the fan over the front of the mirror box so the airflow was over the mirror face. Left it there running for about 2 hours.  It seemed to do some good - much better star test. Practically speaking, easily split Izar, which was just a mess before I came across your original thread, and copied some of your improvements (part of the reason I started down this route was discovering the dob was being outclassed by my 4" refractor on this particular double).

14 hours ago, Piero said:

Would it be possible for you to take some photos of:

- mirror box

- mirror from the top of the mirror box

- mirror from the back of the mirror box (ideally showing the mirror cell)

I could but you probably wouldn't see much. The 2008 Lukehurst dob was quite a bit different from your model. There were no fans anywhere. The mirror cell is surrounded by solid ply, apart from the slot at the front of the mirror box for getting the mirror in and out.  Also, there was no wire sling. Lateral support was from 2 posts at 45 degrees either side of the centre line. Cork lined. This was for a 20inch mirror, 31mm edge thickness. It didn't work great.

I'll probably take it to bits again after I've had a look at the 1st quarter moon this coming weekend. I'll take some photos of the mirror cell then so you can view my handywork. No laughing.

 

[Piero]

16 hours ago, Mutley said:

Yes, pointed the dob up at about 45 degrees and hooked the fan over the front of the mirror box so the airflow was over the mirror face. Left it there running for about 2 hours.  It seemed to do some good - much better star test. Practically speaking, easily split Izar, which was just a mess before I came across your original thread, and copied some of your improvements (part of the reason I started down this route was discovering the dob was being outclassed by my 4" refractor on this particular double).

I could but you probably wouldn't see much. The 2008 Lukehurst dob was quite a bit different from your model. There were no fans anywhere. The mirror cell is surrounded by solid ply, apart from the slot at the front of the mirror box for getting the mirror in and out.  Also, there was no wire sling. Lateral support was from 2 posts at 45 degrees either side of the centre line. Cork lined. This was for a 20inch mirror, 31mm edge thickness. It didn't work great.

I'll probably take it to bits again after I've had a look at the 1st quarter moon this coming weekend. I'll take some photos of the mirror cell then so you can view my handywork. No laughing.

 

So, if I understand this correctly, the bottom of your mirror box is fully closed with a plywood panel which forms the support of the mirror cell. How do you take the primary mirror out (e.g. for cleaning)?

Note that in the original MC/MB of my 12":

- the fan at the back of the mirror was ineffective as too distant from the back of the mirror and only blew on one part of it.

- the 3 additional ventilation holes were ineffective (I closed them for testing and there was no difference)

- the bottom/front opening, using his words "for taking out the MC" caused a lot of thermal issues and the side of the mirror was exposed.

- his sling is implemented incorrectly

- same for the 3 side mirror stops

- the triangles were nearly paralised and asymmetrical between each other.

 

Your mirror is particularly thin, meaning that it requires excellent support (back and size) to work well. What I don't understand is whether you have replaced his MC with your own, as I did, or, if not, what changes to the MC you have applied.

[Mutley]

23 minutes ago, Piero said:

How do you take the primary mirror out (e.g. for cleaning)?

The plywood "backplane" is secured with 4 Allen bolts. When these are removed the backplane, mirror cell and mirror slide out through the slot at the front/bottom of the mirror box. This slot is the only ventilation at the moment, apart from the open mirror face. 

28 minutes ago, Piero said:

What I don't understand is whether you have replaced his MC with your own, as I did, or, if not, what changes to the MC you have applied.

I have not replaced the MC, but made changes similar to you - before you replaced your MC. Namely :

1. Original MC used threaded bolts used to restrict the motion of support triangles. Often the triangles would stick on the threads, causing mirror distortion and some very odd looking star tests. I removed the bolts and use a milk-bottle plastic structure to restrict triangle rotation. It's an 18 point cell by the way.

2. Suspect triangle positioning not optimised using PLOP. I replaced nylon support points with 15mm diameter felt pads so support point positioning is not so critical. (Not sure wether this works)

3. Installed a 180 degree wire sling for lateral support, arranged so it moves with the mirror plane during collimation. Added Velcro guides to the mirror edge to keep the wire at the mirror COG. Removed cork lining from existing edge supports so the wire and mirror don't touch the supports (original supports only act as safety restraint now)

Next step is to improve mirror cooling, hence my original post. Pictures to follow...

I have to say thanks again for documenting on here the work you did on your mirror cell. I'm sure without that post I wouldn't have had the confidence or knowledge to make these changes. I'm grateful.

[Piero]

Some observations:

- nylon bolt vs felt pad. Felt pads can stick to the back of the mirror as well as get squashed. I used heavy duty felt pads on my 16" dob and they don't stick. Also the back of my 16" Lightholder mirror is smooth, so any friction is minimal. I used nylon bolts as back supports for my 12" Nichol mirror (which is not back smooth). In the original cell, I replaced the nylon bolts with pads and did not find any difference. Probably, where I live, it is not particularly humid. Dunno.

- the reason for the "cut" at the bottom / front of the mirror box is avoid that the box hits the central bolt connecting the rocker box to the ground board. Therefore that cut enables to reduce the height of the RB side panels, and therefore the telescope, by a couple of inches. This is well documented in telescope making books. This "cut" is not particularly high. Unfortunately, this other maker ignored the whole principle and used that opening for sliding out the mirror with its cell. The idea is crazy and a bad one too. It is crazy because it is much easier to access the mirror and its cell by rotation (2 locks, 2 pivots - see Astrosystems mirror cell). It is a bad idea because it exposes part of the mirror to colder air than the the internal side of the mirror. A minor breeze can cause astigmatism. Before redesigning my 12" (new MC, etc) I installed a panel of plywood closing that bottom / front cut on the MB. The astrosystems mirror cell does not suffer from this issue because the bottom front opening is still shallow (so air does not hit the mirror side) and it has two holes at the back. This enables a steady flow of air.

- side holes / fans are not added for cooling the mirror down, but to prevent the formation of warmer air above the mirror surface. To cool the mirror down faster, the most effective way is to install a fan to the back of the primary mirror. I would imagine that your mirror is F4 or similar, so the mirror box is not particularly deep. Therefore, lifting up the bottom of the light shroud might be sufficient to prevent the formation of air boundary layer (BTW my original light shroud was made by him and broke down within 6 months). I would recommend to get a copy of the book "the dobsonian telescope" by Kriege. There have been many advances to those ideas, but the design is simple and effective. It contains lots of way to redesign components. 

[Mutley]

@Piero I take your point about the felt pads - this is probably the least evidential change I've made but I still have the nylon support point to revert back to if the felt pads cause problems.

I've been moongazing the past couple of nights. Friday night was a real WOW! moment. Good seeing, seemed like I could pick out every micro-crater. Last night the seeing wasn't quite so good but I still managed to pick out the Hadley Rille , which I don't think I ever managed with this scope before the changes to the mirror cell.

I have dismantled it now for painting, so here are some pictures:

 

Edited May 28, 2023 by Mutley

[Mutley]

The unpaited plywood pier you see is fixed to the back of the moving plane of the mirror cell, just so I had somewhere to mount the brackets for the sling support. The sling itself I bought pre-fabricated from here : https://www.gsproducts.co.uk/

Thanks to all of you for all the helpful advice and information. Now off to do some painting and think about how to implement a cooling soultion.

[Piero]

On 28/05/2023 at 13:49, Mutley said:

The unpaited plywood pier you see is fixed to the back of the moving plane of the mirror cell, just so I had somewhere to mount the brackets for the sling support. The sling itself I bought pre-fabricated from here : https://www.gsproducts.co.uk/

Thanks to all of you for all the helpful advice and information. Now off to do some painting and think about how to implement a cooling soultion.

 

Thanks for sharing the photos of your telescope and mirror cell.

My thought:

• Looks like you need a spanner to collimate the primary mirror. In more recent versions, David uses metal wing nuts which work on that kind of cell. Of course this is not required but it would allow you not to use an additional tool.
• The first thing I would do to improve the cooling is to drill, using a hole saw, where you have that marking point at the centre of the triangle plane of the mirror cell. To make a clean cut at the back, I would clamp some wood to the other side so that the wood does not split. Following that, I would install a fan (80mm is fine if it runs at good speed - you could also opt for 100mm, depending on how much space you have, considering the collimation bolts). Having said that, this requires some thought on where to install this fan as it does not seem that you have much space between the triangle panel and the bottom panel of the mirror box. One way could be to install it to the back of the triangle panel, but this depends on whether the gap between the two panels is sufficient. A possibly neater way is to make a groove to the bottom panel of the mirror box (outside side) and install it there. It looks like that panel is quite tick. Doing so, the gap size between this panel and the triangle panel does not really matter. I have a feeling that your fan positioned at the front of the mirror box improves the views, by cooling down the mirror. As it is positioned to front of the mirror, it will blow away the boundary layer, but my feeling is that this is a secondary effect really. How long does it last? I haven't done the maths, but I would imagine a mirror of that thickness takes about 1h to cool down to ambient temperature. I keep the fan of my 16" on all night as the temperature decreases. However, I reduce the speed once the mirror is at ambient temperature, so that the views are not affected by the fan being on, whilst the mirror temperature continues dropping following the decrease in ambient temperature.
• If possible, you should consider painting the additional wood panel supporting the sling. The sling angle brackets should also be sanded coarsely, primed and painted matt black to avoid reflections coming back towards the secondary mirror. This will improve contrast a bit. I would do this later once everything is fixed though.
• was that black ring structure at the edge of the mirror installed to fix a turned down edge issue?
• Is your telescope f4 or even faster?  In the third photo, the centre of the sling cable seems quite low. Now, I would imagine it to be visibly lower than half mirror thickness as your primary mirror is fast, but from the photo it seems lower than what I would have imagined. Just to double check, is the centre of the cable really at the COG of the mirror? Unfortunately, that installation of the cable follows David's design and the biggest downside is that the height of the cable is fixed. Therefore, to regulate it to the COG of the mirror, the only way left (maintaining the existing installation) is to trim/raise the pads. This would allow you to decrease the mirror height and therefore raise the position of the cable - assuming that the cable is positioned a bit lower than the COG. If the cable is positioned below the COG of the primary mirror, the effective mirror figure will not be parabolic as it should and this will cause spherical aberration. Depending on whether it is position higher or lower than the COG, you will have an overcorrection or undercorrection effect. You might also see astigmatism if the cable is positioned at slightly different heights around the mirror from the back of the mirror.

[Mutley]

Yes, quite a challenge getting a fan on the back of the mirror with all that wood back there, and not much space to fit it There is also an encoder in the middle of the rocker box - providing clearance for that restricts options. I'm wondering whether I could fit fans to the side only, maybe arrange so airflow was over top and bottom surfaces of the mirror. Installation would be easier and I think I would get more uniform air flow. I may try that first.The scope is kept outside so initial cooling is not so onerous. 

The tests I did with the AC fan lasted about half an hour before I noticed degradation. Replacing the fan for a few minutes improved things - all a bit subjective though. All I can really conclude is that the fan helped a lot - it gave a much improved star test.

The mirror COG came out at 13.6 mm from back of mirror using that cruxis.com calculator (503mm dia, F4, 31mm thickness). The cable is 4mm dia so the top border of the wire should sit at 15.6 mm - almost exactly half mirror thickness. It doesn't look far off to me but I will re-measure. I intend to add a few more velcro guides so I'll check it then.

You're right - it is a TDE mask. I did some tests with a ronchi grating a good while ago which showed some hooks at the mirror edges. I didn't see much improvement at the time but I guess the TDE thing may have be swamped by other problems. I'll re-visit this when I've finished fan installation & painting. So ..  lots to do! I'll keep you posted on progress. Thanks for your input, I appreciate you sharing the knowledge you gained. 👍