Adding a fan to Bresser Dob

[Stardaze]

It looks like I need to clean the mirror of my dob and so whilst I'm at it I was going to add a fan to the bottom. I noticed they do an option with a fan and so it's a 100mm aperture which looks to seat a 4" nicely. There's just a couple of questions I have if anyone can assist please:

Is there any recommendations on a specific brand of fan. They all seem to be Chinese unbranded from £2-£20?

The cable looks to be tucked into the tube still. My dew heater has a 12v out supply so was going to hook up to this. It sits at the front so probably was going to leave the lead trailing from the back of the OTA but sounds a bit untidy, any ideas?

I haven't disassembled yet and so presume the black plate to attach the fan to is part of the mirror cell and is easy enough to drill?

Thanks in advance for any input. 

IMG_0070.HEIC

Edited October 26, 2021 by Stardaze

[johninderby]

There are a lot differences in quality of fans. Get one with ball bearings and that is a low vibration one. If you ever run the fan while observing this is important. 

Good place to look is one of the places that deals in computer modding. And yes the price does vary quite a bit, I've found good fans are nearer 10 quid or so but worth checking the cearance sections.

And you can get anti vibration fan mountings. Just rubber bits where the fan is screwed down.

Edited October 26, 2021 by johninderby

[Stardaze]

7 minutes ago, johninderby said:

There are a lot differences in quality of fans. Get one with ball bearings and that is a low vibration one. If you ever run the fan while observing this is important. 

Good place to look is one of the places that deals in computer modding. And yes the price does varryquite a bit, I've found good fans are nearer 10 quid or so but worth checking the cearance sections.

Thanks John. Looking at a dual ball bearing fan for around £16, single bearing seems to be £10. Thanks for the input. 

[Dave scutt]

Hi stardaze 

I had a tray with 2 fans in it , for cooling down my laptop I attached hair bungees for no vibrations  and use a power pack to keep it running 

[Piero]

I use a noctua fan and found it really effective.

I agree with other members that you should install it with some elastic material, or pads at least, to avoid vibrations.

I used an abs plastic panel which is screwed to the back of my mirror cell. Pads were added between the panel and the rungs where the bolts are. My fan is almost always on when I observe and there is no vibration whatever mag is used. 

 

[Stardaze]

3 hours ago, Dave scutt said:

Hi stardaze 

I had a tray with 2 fans in it , for cooling down my laptop I attached hair bungees for no vibrations  and use a power pack to keep it running 

Thanks Dave. I had seen @Ricochetdo something similar but thought I'd try to drill it in place but I hadn't thought out the dampening properly. It does make sense to bungee it to stop the vibrations. 🤔

I love seeing these big old dobs - that's a proper job @PieroJust looking at those fans and seem to do a 92mm or 120mm option. The latter is too big but cheaper than the smaller one for some reason?

[Merlin]

Some years ago there was a definitive Sky & Telescope article showing that the most effective way to use a fan is to mount it on the side of the main tube. The fan should be blowing air across the mirror surface and exiting through three or four inch diameter, side-by-side, vent holes on the other side of the tube and on a level with the fan.

This arrangement removes the blob of cold air that settles on the mirror, causing poor definition. We should forget about “tube currents” and go for the blob.

[Merlin]

An important point I’ve missed out on the previous post is that the fan should be mounted on a block of foam to absorb vibration.

[Stardaze]

42 minutes ago, Merlin said:

Some years ago there was a definitive Sky & Telescope article showing that the most effective way to use a fan is to mount it on the side of the main tube. The fan should be blowing air across the mirror surface and exiting through three or four inch diameter, side-by-side, vent holes on the other side of the tube and on a level with the fan.

This arrangement removes the blob of cold air that settles on the mirror, causing poor definition. We should forget about “tube currents” and go for the blob.

Thanks for this @MerlinI presume that you mean to remove the boundary layer for truss dobs. Mine is a solid tube and so really from underneath just helps cool the mirror itself. 

[JeremyS]

The biggest Bressar fan around is our very own John Handleby 

[Stardaze]

35 minutes ago, JeremyS said:

The biggest Bressar fan around is our very own John Handleby 

He does like a Bresser with a good ‘andle. It’s all John’s fault that I bought one. Just need to add another sibling now though. 

[Piero]

49 minutes ago, Stardaze said:

Thanks for this @MerlinI presume that you mean to remove the boundary layer for truss dobs. Mine is a solid tube and so really from underneath just helps cool the mirror itself. 

Well.. there are two distinct problems: 

1. cooling down the primary mirror
2. removing the boundary layer above the mirror surface

To tackle the former issue, a fan installed behind the mirror and pushing air to the back of the mirror is the most effective way. Lateral fans pushing air on the mirror surface (not the mirror edge!) can be used for tackling the latter issue.

 

cooling down

Leaving the telescope outside to cool down for 1 or 2h before observing matters but it is not sufficient. The temperature continues to drop over night. The issue is that a mirror takes more time to cool than the surrounding environment. Astigmatism and spherical overcorrection will appear as soon as this temperature discrepancy is 1-2C. A fan will help the mirror cool down faster, approaching the ambient temperature effectively. 

 

 

boundary layer

It does not matter whether the dobson is a truss or solid tube. The boundary layer can be present in both. I believe the S&T articles mentioned by Merlin are these two:

• https://skyandtelescope.org/astronomy-equipment/thermal-boundary-layers-in-newtonian-reflectors/
• https://skyandtelescope.org/wp-content/uploads/Adler-NewtThermal.pdf 

As you can see in the second one (by Adler), a solid tube is used. 

The mirror box of my dobson is quite shallow (11" deep). If the light shroud is lifted up 4-5 inches from the bottom, the flow of incoming air is sufficient for breaking the boundary layer and preventing its reformation. For deeper mirror boxes, the installation of fans would do the same.

In my opinion, the boundary layer 1 inch above the primary mirror surface is one of the unconscious reason why people state over and over that refractors are better than reflectors and that larger apertures suffer from seeing conditions more than smaller apertures. It's also the same reason why people say that they cannot push the magnification much with their reflector (despite the fact that they say they can with their refractors). 

 

 

BTW, my dob is only few months old. 

[Stardaze]

21 minutes ago, Piero said:

Well.. there are two distinct problems: 

1. cooling down the primary mirror
2. removing the boundary layer above the mirror surface

To tackle the former issue, a fan installed behind the mirror and pushing air to the back of the mirror is the most effective way. Lateral fans pushing air on the mirror surface (not the mirror edge!) can be used for tackling the latter issue.

 

cooling down

Leaving the telescope outside to cool down for 1 or 2h before observing matters but it is not sufficient. The temperature continues to drop over night. The issue is that a mirror takes more time to cool than the surrounding environment. Astigmatism and spherical overcorrection will appear as soon as this temperature discrepancy is 1-2C. A fan will help the mirror cool down faster, approaching the ambient temperature effectively. 

 

 

boundary layer

It does not matter whether the dobson is a truss or solid tube. The boundary layer can be present in both. I believe the S&T articles mentioned by Merlin are these two:

• https://skyandtelescope.org/astronomy-equipment/thermal-boundary-layers-in-newtonian-reflectors/
• https://skyandtelescope.org/wp-content/uploads/Adler-NewtThermal.pdf 

As you can see in the second one (by Adler), a solid tube is used. 

The mirror box of my dobson is quite shallow (11" deep). If the light shroud is lifted up 4-5 inches from the bottom, the flow of incoming air is sufficient for breaking the boundary layer and preventing its reformation. For deeper mirror boxes, the installation of fans would do the same.

In my opinion, the boundary layer 1 inch above the primary mirror surface is one of the unconscious reason why people state over and over that refractors are better than reflectors and that larger apertures suffer from seeing conditions more than smaller apertures. It's also the same reason why people say that they cannot push the magnification much with their reflector (despite the fact that they say they can with their refractors). 

 

 

BTW, my dob is only few months old. 

Great post @Piero

I think drilling holes into the side is probably a step too far for me. Eventually I’d like a nice truss which would make it easier or even a secondhand OO tube. Did think about just butchering mine and adding a better base and mirror in time. Far too many options with dobs 🙂

Edited October 26, 2021 by Stardaze

[Ricochet]

11 hours ago, Stardaze said:

The cable looks to be tucked into the tube still. My dew heater has a 12v out supply so was going to hook up to this. It sits at the front so probably was going to leave the lead trailing from the back of the OTA but sounds a bit untidy, any ideas?

My plan was to run a very small length of mini trunking inside the tube with 12V ports out of the tube between the rings (power in from the rocker box) and out at the bottom of the tube. However, five years in and the test cable that was just looped around the ring screws is still sat there. 

The fan I've got on mine is an 80mm Coolink with a fan filter fitted to the incoming side to prevent dirt being blown up into the tube. I did also buy a Noctua for something and that was also a very nice fan that would work well. 

[Stardaze]

10 minutes ago, Ricochet said:

My plan was to run a very small length of mini trunking inside the tube with 12V ports out of the tube between the rings (power in from the rocker box) and out at the bottom of the tube. However, five years in and the test cable that was just looped around the ring screws is still sat there. 

The fan I've got on mine is an 80mm Coolink with a fan filter fitted to the incoming side to prevent dirt being blown up into the tube. I did also buy a Noctua for something and that was also a very nice fan that would work well. 

A fan filter isn’t a bad idea either. I’m thinking that 2 core cable up the side isn’t the end of the world.

[Pixies]

How about something to match the Bresser white?

https://www.overclockers.co.uk/be-quiet-shadow-wings-2-120mm-fan-white-fg-03v-bq.html

[Merlin]

The Sky & Telescope article I saw was indeed the Alan Adler article. I’d forgotten to mention that it deals with closed tube Newtonians.

[Stardaze]

The article is a really interesting view, certainly have read a fair bit about the boundary layer and I'd love to be able to implement something for that, but I'll concede that my handcraft skills are average and so cutting a hole to mount the fan on the side, with corresponding holes opposite is probably a little beyond my comfort zone. Though maybe a downlighter hole punch could do the job well enough with some insulating tape on the tube to mitigate slip? Maybe I'll give it some thought..

Thanks @Pixiesfor the link to a white fan. I don't think the look of it all concerns me that much tbh but was thinking it screwed into the bottom would make it look a bit tidier for my OCD but certainly see the benefit above to bungee chord mounting to mitigate vibration, which is probably even easier to implement. Have you considered adding a fan to yours? 

PS. I think my primary mirror may have had some moisture on it the following morning still from a very wet hour I had out Monday evening - everything was sodden/damp around me. Odd though as I kept the OTA horizontal for half hour for cooling beforehand and had it back in an hour later. There's a bit of dust/small particle all over the mirror when shining a light onto it, even a couple of very fine strings of cobweb in the tube itself. I always store it with the top cap and shower cap on the bottom too but it's not as bad as I first thought. I left the cap off the following day for a couple of hours just in case it was moisture. I'm still tempted to clean it all out one evening regardless. 

[Stardaze]

On 26/10/2021 at 13:17, Piero said:

I use a noctua fan and found it really effective.

I agree with other members that you should install it with some elastic material, or pads at least, to avoid vibrations.

I used an abs plastic panel which is screwed to the back of my mirror cell. Pads were added between the panel and the rungs where the bolts are. My fan is almost always on when I observe and there is no vibration whatever mag is used. 

 

Quick question, what is your fan powered by? Have been looking at 4 pin fans but typically the converter plugs I've found are 2 pin like this one: https://uk.rs-online.com/web/p/dc-power-connectors/8104595/?cm_mmc=UK-PLA-DS3A-_-google-_-CSS_UK_EN_Connectors_Whoop-_-DC+Power+Connectors_Whoop+(2)-_-8104595&matchtype=&pla-362265064714&gclid=EAIaIQobChMIwoulkofw8wIVze3mCh3xBwNpEAQYBSABEgLflfD_BwE&gclsrc=aw.ds

The additional wiring looks to be needed if you add a controller, which I assume isn't really necessary for mounting behind the mirror. Need to get something ordered really. 

[Piero]

57 minutes ago, Stardaze said:

Quick question, what is your fan powered by? Have been looking at 4 pin fans but typically the converter plugs I've found are 2 pin like this one: https://uk.rs-online.com/web/p/dc-power-connectors/8104595/?cm_mmc=UK-PLA-DS3A-_-google-_-CSS_UK_EN_Connectors_Whoop-_-DC+Power+Connectors_Whoop+(2)-_-8104595&matchtype=&pla-362265064714&gclid=EAIaIQobChMIwoulkofw8wIVze3mCh3xBwNpEAQYBSABEgLflfD_BwE&gclsrc=aw.ds

The additional wiring looks to be needed if you add a controller, which I assume isn't really necessary for mounting behind the mirror. Need to get something ordered really. 

 

Just checked my data.

The fan I mounted is a Noctua NF-R8 redux-1800, High Performance Cooling Fan, 3-Pin, 1800 RPM (80mm, Grey) and was bought on Amazon. The specifications regarding the 3-pin is available online. I cut the small cable which is redundant for us and connected the two small cables to a plug.

I use a Tracer 12V 7Ah LiFePO4 battery. This connects to a Lynx Astro 4 Port Dew Controller. The channels power: 1) the fan, 2) secondary mirror dew heater, 3) Rigel quick finder dew heater, and 4) a 2" dew heater which I use with the finder, eyepiece, etc whenever needed. While in use, I reduce the speed of the fan to half using the dew controller.

Edited October 29, 2021 by Piero

[Stardaze]

16 minutes ago, Piero said:

 

Just checked my data.

The fan I mounted is a Noctua NF-R8 redux-1800, High Performance Cooling Fan, 3-Pin, 1800 RPM (80mm, Grey) and was bought on Amazon. The specifications regarding the 3-pin is available online. I cut the small cable which is redundant for us and connected the two small cables to a plug.

I use a Tracer 12V 7Ah LiFePO4 battery. This connects to a Lynx Astro 4 Port Dew Controller. The channels power: 1) the fan, 2) secondary mirror dew heater, 3) Rigel quick finder dew heater, and 4) a 2" dew heater which I use with the finder, eyepiece, etc whenever needed. While in use, I reduce the speed of the fan to half using the dew controller.

Thank you, that’s great. I have a 4 way controller from Dew Control so I’ll look at that configuration. Can’t seem to find grilles to go on the back now.. 

 

[Stardaze]

So I can lose the molex plug and add some thing like the below to plug into the controller to power?

https://www.switchelectronics.co.uk/male-rca-phono-plug-screw-terminals?gclid=Cj0KCQjwt-6LBhDlARIsAIPRQcJ4aJzLmKOyeV4r65ih2Zw84rIjzU7iSC84boHhMOQ2A3DfHlJ1FCgaAnFIEALw_wcB

[Ricochet]

Yes, you only need the red and black wires to run a 3 pin fan. I also have a basic speed controller in my setup but if the fan is properly isolated this isn't really needed. 

[Stardaze]

6 minutes ago, Ricochet said:

Yes, you only need the red and black wires to run a 3 pin fan. I also have a basic speed controller in my setup but if the fan is properly isolated this isn't really needed. 

I did start to read up on that and saw a couple of cheap controllers but if I can plug it into one of the dew controller channels and control from that, then even better. I was thinking that it might need its own supply. Already have a 12v 7ah battery powering the dew kit.

[Piero]

2 hours ago, Stardaze said:

So I can lose the molex plug and add some thing like the below to plug into the controller to power?

https://www.switchelectronics.co.uk/male-rca-phono-plug-screw-terminals?gclid=Cj0KCQjwt-6LBhDlARIsAIPRQcJ4aJzLmKOyeV4r65ih2Zw84rIjzU7iSC84boHhMOQ2A3DfHlJ1FCgaAnFIEALw_wcB

Yes, those work fine. I used them too. There might be better options and I would be interested in knowing more. 🙂