Is there a cooling fan for the SW 250px OTA?

[tooth_dr]

I bought a new OTA last week. I only noticed this when I set it up on the EQ6 in the dining room. The base of the scope has no cover like the 200p, for example. There are no screw holes around the side to attach a cover so I am assuming it is like this to aid cooling. What do you think? If there is a fan available, can someone link me to it, or could I modify a PC one and run it off a USB lead? I like DIY so if you can point me in the right direction

Here are a couple of photos of the base of the scope. Still coming to terms with the size of it, after being used to a 6" and 80mm scope for the past couple of years

Thanks

Adam

[Deneb]

Im wondering will your Primary thumbscrews fit my Celestron C8 N Primary ?

[tooth_dr]

I'm wondering why you'd be wondering that ?

[Deneb]

mmm, Because they are all manufactured from the same company & coolimating the primary would be easier. Wondering about your fan cooling, how big are the holes for the fan fitting - will they take a fan screw, which is around 5mm in thickness. Maybe you don't have a cover due to your thumb screw size.

[tooth_dr]

Ah right, get you now.

There is another thread on SGL, just found it with some searching, it has the exact fan I've been thinking about:

http://stargazerslounge.com/equipment-discussion/83583-skywatcher-skyliner-250px-black-diamond-auto.html

[dweller25]

The back of my SW 8" dob was completely open when I bought it.

I added a computer fan with a baffle.

The baffle is made from an aluminium clock face !!!

The baffle sends all the cooling air up the tube for maximum efficiency.

[tooth_dr]

Very nice mod! I wonder if it will make much difference in the obs, but it will look cool, and I suppose that counts for something

[PhillyDee]

I have a cooling fan on my 200mm skywatcher. I used an Akasa Amber 80mm fan on the plate. Measure the pitch of the holes and find out the thread. Im guessing that they are meant for a fan. To mod one, I would make a cable with a 3pin header onto 2 wires. Then you can put a USB connector on the end. USB is only 5V, so the fan will run quite a lot slower. Better to put a cigarette lighter plug on to suit the rest.

[Keithp]

Looks and sounds like a good idea, got a 250px myself, one question though, would the air from the fan rising up the tube cause currents that might interfere with the reflected image back at the eyepiece?

Regards

Keithp

[tooth_dr]

I've no idea about that Ketihp, hopefully someone with the knowledge will help with an answer, or maybe a search might bring up some info.

I'll let you know how I get on with it.

[dweller25]

Looks and sounds like a good idea, got a 250px myself, one question though, would the air from the fan rising up the tube cause currents that might interfere with the reflected image back at the eyepiece?

Regards

Keithp

Hello Keith,

I have switched the fan on and off many times during observing as I get a buzz from seeing just how rock steady planetary images become when the fan is on.

I always thought the constantly changing softness of the image was due to the atmosphere - not so - it was due to the boundary layer on the mirror which this fan scrubs off.

The baffle must be used to push the cooling air forwards to do this.

The best £5 I ever spent on astro kit.

[Keithp]

Neat idea then, looks like I'll be getting one of my big case fans and trying it out.

Thanks for that I've only just started imaging and it will be interesting to see the difference.

Regards

Keithp

[tooth_dr]

Is there any specific fans better for the job? I've a fan here that will certainly fit, but was looking at some of the 'quiet' fans on the market, for about £5-10, and then get a nice grill to boot as well.

[dweller25]

I would not spend big bucks on a special fan, the one I used was a standard Coolermaster fan out of the back of an old PC.

[tooth_dr]

Firstly thanks for all the helpful advice in this and another thread on SGL.

I decided to remove the fan from an old unused desktop that was ready for binning, and I picked up a few bits and pieces to allow me to make a lead, and connectors for the fan to run off 12v, and 2.5mm jacks male and female.

I decided not to drill the OTA to mount the female connector, and the plan was to leave it as in the second and third pictures below. But it looked and felt a little flimsy (and was too unfinished looking for my liking), so I shrink-wrapped the red/black wires, and attached a rubber 'boot' over the rear of the connector. This was filled with glue from a glue gun before being sqeezed on. I think this will be sufficiently strong to withstand the weight of the cable going to the 12v power supply.

Should I keep the fan running all the time when I am using the scope, or just switch it off half an hour before using?

Thanks a lot for looking,

Adam

[dweller25]

Nice mod but most of the air will bounce backwards and will not remove the boundary layer on the front mirror surface. Try making a baffle - it will improve the effect of the fan.

[tooth_dr]

I wasn't sure what a baffle was but re-reading the thread I now see what it is. I take it there is 'space' for air to get up through the tube from where my fan is, and in fact the mirror doesn't seal the end of the tube? So does the baffle prevent air, and force it up the tube? Any suggestions of what I can make the baffle from?

[dweller25]

Yes there is a gap at the side of the mirror that allows air up the main tube. I used an old aluminium clock face to make the baffle.

Good luck.

[tooth_dr]

Ok, just made up a baffle using some foam board (it's about 5mm thick so pretty stiff) to see how much difference it makes. There was air moving up the tube without the baffle, but it is significantly greater with the baffle in place. Thanks for the advice, it has made a difference.

Here is an image of what I just made up, I may mount the female end into the baffle, but it's easy to remove and reattach as it is for collimation etc, so I may just leave the design as it is. I'm pleased now!

Regards

Adam

[dweller25]

Nice :-)

[tooth_dr]

Cheers, once I get something black sorted I think it'll look very OEM

[Astro_Baby]

Theres a pretyy comprehensive guide to fans here for fans of fans

Telescope Optics Topics

[johninderby]

One thing to watch out for if you do use a baffle is that you may need to leave it running all the time. A baffle will block cooling air from reaching the back of the mirror as temperatures drop during the evening if it isn't running. One solution is to make the fan/baffle removeable.

I found that it didn't make any real difference with or without baffle so I don't bother with a baffle any more. Of course some scopes may be different and need some extra help, particularly if they don't have a pyrex or other low expansion mirror.

John

[tooth_dr]

Astrobaby - thanks for the link, will have a read through that later.

John - I can easily remove the baffle, but found that the air flow felt much fast through a small opening in the scope cover with the baffle than without it.

The mirror in my scope is pyrex so perhaps I won't have much need for the fan anyway.

Regards

Adam

[johninderby]

I find it dosn't take too long for the 250PX miror to cool down anyway. I've found when observing the images were very sharp and didn't need the help of a fan while observing. However with the fan it cools down in half the time, great in mid February for example.

In comparison scopes such as The Meade Lightbridge have a plate glass mirror and take twice as long to cool down, and may really benefit from having a fan running while observing as the plate glass mirror (particularly the larger apertures) sometimes can't keep up as the temp drops.

A high airflow fan may not actually be as good at cooling a mirror as a slower one. If the airstream flows too quickly over the back of the mirror it doesn't have time to absorb heat.

John