What is the use of the fan on the bottom of my dob?

[mrsnipey]

Hi there, Sorry if this seems like a daft question.
I have just bought myself a 12" Dobsonian and I've noticed on the forums
that everyone keeps mentioning that they need to cool their dobs down
before they use them but I take mine outside, put the eye pieces in and
start looking and I can't see any difference between the beginning of the
night and the end. Is there something I'm missing?

Regards.

[xelasnave]

Try turning on the fan.
alex

[astro_nutt]

Welcome MrSnipey!
Yep..do turn on the fan..this will help cool down the primary mirror and the air in the tube..this helps maintain an even temperature and therefore a clearer image..try it and see!
Cheers!

[iceman (Mike)]

The fan is to help cool the mirror down.. it's passive cooling, by blowing ambient air onto the mirror.

If the mirror is much warmer than the air outside, you get tube currents in the tube and a boundary layer of air above the mirror which distort and refract the light as it comes down your tube.

It is usually interpreted as bad seeing.

You may not notice it in an hour or two of observing because it can take many hours to cool the mirror down properly. During winter months it may never catch up.

The daytime/nighttime temps in Brisbane are pretty close too, compared to places like Canberra so the effect may not be as dramatic in your part of the world.

[MarkN]

Well I took one look at the battery holder that came with my 10" Newt and swore that it would forever remain empty.

Talk about stupid design! No less than eight AA batteries. Don't know how long they would last and don't really care but I'll bet it's not long.

I'll just wait for convection to do it's job thanks very much.

Mark.

[mrsnipey]

Thanks for the replies everyone.
I'll try out the fan on Friday night (weather permitting) and let you know how it goes. There was one night where it looked like there were "heat shimmers" for lack of a better term (like heat coming off the road in summer). Is that what you are referring to?
I've only seen it the once though.


Anyway, thanks for the advice.

[GrahamL]

I found the pack supplied lasts a night and a bit which isn't very
usefull,most buy a small single rechargeable cell .
I bought 3 flat (8 AA) packs off amember here for alittle over $25
with a charger which work great .
Where your scope is stored figures in as well a hot room late in the day
for me means it takes most of the night to cool down and probably never does.. so it goes out on the back as early as possible .

[iceman (Mike)]

I didn't know some of the fans came with battery packs!?

Definitely replace it with a simple 12V powered CPU fan. You can pick them up from Dick Smith cheaply.

[ballaratdragons (Ken)]

Originally Posted by MarkN

Well I took one look at the battery holder that came with my 10" Newt and swore that it would forever remain empty.

Talk about stupid design! No less than eight AA batteries. Don't know how long they would last and don't really care but I'll bet it's not long.

I'll just wait for convection to do it's job thanks very much.

Mark.

Originally Posted by nightstalker

I found the pack supplied lasts a night and a bit which isn't very
usefull,most buy a small single rechargeable cell .

I bought my 12" dob about 2 years ago and it came with one of those little 8 AA battery packs. I have used it very regularly, run it all night most nights, and even had it running in the house to de-Dew the mirror the next day quite often.

Now this is where it is strange, It came with El-Cheapo no-name AA batteries and yet they have never gone flat! They still run perfect after 2 years of heavy usage!!!

[GrahamL]

thats weird ken .. it was the cheapies I tried first and they were slowing down by the end of night 2..heard someone on the radio the other day with a similar experiance.

must be ghost batteries.. or maybe the familys been takeing the ****
for a couple of years and changeing them every week on ya

[Gargoyle_Steve (Steve)]

The "heat shimmer" that you're referring to (often really obvious when looking at the moon) is possibly due to atmospheric conditions - especially if you are looking at an object lower in the sky (ie looking through more atmosphere to see it) or looking across a neighbours roof, etc, where heat from the sun that was soaked up all day is still bleeding up into the air.

A warm mirror does give distorted views, but seeing as you're in Brisbane and we haven't had any nights that you'd call cool at all in recent times I suspect it's more the atmospheric shimmering that you noted.

Mirror temp is all about how warm the mirror is relative to outside air temp - the guys down south have more trouble because sometimes in winter the ambient temp drops faster than the mirror can cool - ie the difference between mirror temp and air temp actually increases, even though the mirror is cooling.

The reason the battery holder has 8 batteries is that the fan is a 12v unit - 8 batteries at 1.5 volts each gives the requird 12 volts. As was mentioned above many of us have bought either 12v rechargable batteries, or one of those 12v jump start units from a car accessory place. (The single battery is cheaper and more compact by the way.)

One reason that you may not be seeing much difference between viewing at the beginning and end of the night is possibly the collimation (read: optical fine tuning) of the scope. A reflector (be it dob mounted or other) needs to be well collimated to get really good results, it could be that the difference visually between a warm mirror and one at or near ambient temp is being lost due to the collimation not being as good as such a great scope deserves!

For what it's worth I took the standard fan off my 12" dob and replaced it with a up-specced computer fan - larger diameter, more aggressive blade design, blows more air to cool quicker - and I wouldn't be without it.

Cheers!

[Tannehill]

Check our www.fpi-protostar.com/bgreer/ and the topics discussing fans and thermal stability in newts.

See also the yahoo group skyquest_telescopes Folders and Photos sections for some discussions on how some folks modify their scope to address mirror cooling and the issue of boundary layer.

A nice trick is to select a bright star or planet, and defocus until you have as large a bright disc as possible. Look at the bright disk - study it for a few minutes - and you will see one or two types of phenomenon. One is a slow churning type of distortion without any directional pattern. These are thermals due to your mirror not being at ambient. If you turn on a fan at this point, and the system is designed to somehow move air across the front of the scope to "scrub" this boundary layer, you'll see this slow churning distortion resolve or improve. If your mirror is perfectly equilibrated with ambient temperature you will not see these mirror thermals. Or, if it isn't but you have a very effective boundary layer scrubbing fan system, you also won't see it or it will be much less noticeable.

If the mirror is at ambient, or even if it is not and you study the image, you'll see a second type of distortion: An extrememely fast almost eyeblink quick distortion that seems to sweep straight across the image. This is atmospheric seeing effects. Cooling your mirror will have no effect on this, obviously.

Lotsa opinions on impact of fans on image quality in newts, very little hard data, but greer's page above has some of it, and Alan Adler had an article in S&T also about this. http://www.pha.jhu.edu/~atolea/WAS/t...newtonians.pdf

You'll not notice undetectable (edited typo from original post) impact on low contrast low resolution objects like galaxies and nebula. That's why DSO viewers will often dismiss the value of fans. You will notice the most impact from mirror thermals on bright high contrast objects like Jup, Sat, and the Moon at high power. I think this is one reason why newts traditionally have a bad rep for such objects, compared to refractors..that and the propensity for poor collimation to show bad images of planets and moon, which isn't an issue for refractors.

s

[White Rabbit]

Originally Posted by ballaratdragons

I bought my 12" dob about 2 years ago and it came with one of those little 8 AA battery packs. I have used it very regularly, run it all night most nights, and even had it running in the house to de-Dew the mirror the next day quite often.

Now this is where it is strange, It came with El-Cheapo no-name AA batteries and yet they have never gone flat! They still run perfect after 2 years of heavy usage!!!

I think you may just have stumbled onto the much talked about Zero Point Energy. Keep a hold of them batteries mate your a millionaire lol .

[sejanus (Gavin)]

I've never noticed it myself but i guess it's more relevant for people who drag their scopes out from a warm house.

I leave mine in my garage so it's pretty much near outside temps all the time.

[mrsnipey]

Whoa, haven't been on in a couple of days and there's a few more posts than there was before.

I finally turned the fan on the other day. Sorry to say that I didn't notice any difference (other than Jupiter which seemed to be perfectly clear - no change in ngc's).
I think you might be right Gargoyle_Steve. My collimation might be off. I'm in the process of saving up for a laser collimator at the moment.

[Tannehill]

On the stock installed fan, you wouldn't notice much acutely because the fan as installed just blows air against the back of the mirror, which then bounces back and away. Very little acutally goes up the sides of the tube, and almost NONE goes across the face of the mirror, which is where this boundary layer is. The stock fan will just help drive the mirror's temp down which helps, but it takes time...it's just speeding mirror equilibration with ambient. While there are many ways to address this, if you store your mirror where it'll be closer to ambient normally, it's probably not worth the effort. Or, put a box fan up against the back of the scope for the 30-45 min while you are setting up and waiting for dark. That's what many with big dobs (stored inside) do to drive the mirror temp down and minimize these effects.

This phenomenon is much more noticed in the very large mirrors (18" and up). My 12" cools down fairly quickly, but on my 18" the fan set up really helps esp in the 1st 1-2 hours of the night...unless the mirror was at ambient already.

I'd agree with Gargoyle_Ste tho, that collimation issues are MUCH more likely - across a broad range of users - to be an issue than tweaking temperature issues in these scopes.

s

Originally Posted by mrsnipey

Whoa, haven't been on in a couple of days and there's a few more posts than there was before.

I finally turned the fan on the other day. Sorry to say that I didn't notice any difference (other than Jupiter which seemed to be perfectly clear - no change in ngc's).
I think you might be right Gargoyle_Steve. My collimation might be off. I'm in the process of saving up for a laser collimator at the moment.

[mrsnipey]

Bought myself one of those 12v jump starter kits for the car on the weekend so I could power my dob fan and laptop. Holy Cow batman!

I left the fan running for an hour before having a look (family visiting) and when I went out I couldn't believe how clear everything was. Tuc 47 was absolutely amazing. I've never seen anything like it. It was if I could see each star perfectly clear, almost 3d.

Best $55 I've ever spent.

[Gargoyle_Steve (Steve)]

Very glad to hear you've got that sorted out with a very positive outcome, now go grab those photons!!