To fan or not to fan, that is the question...

[Timebandit]

9 hours ago, nicoscy said:

Just remembered to add another reason: The low weight of the components!

Scope weights 14 kilos. Mount weights 12 kilos. In my case, open rear passenger door of my CRV and slide telescope on the back seats. Rings help a lot to move the scope about. 

Place observing table, observing chair, mount, eyepiece case and accessories case in the boot and I am off.

Setup time is slightly under 10 minutes if I am lazy. If in a hurry, I have been known to have everything set up in under 4 minutes with fan running 

Hi. Nico's you are right they are quick to set up , but 10min or 4min that is a bit slow ,and all that observing time you are missing. I open shed doors , slide out scope, put eyepiece in ,about one minute. But in England you have to be quick or the rain or clouds come in ?.  

But in all seriousness they are great scopes and I am sure DRT will love it .

To fan or not to fan ,I think OOuk put the fans in to keep their Cyprus customer's  happy, as we never have the temperature's here to need our fans ? 

[CraigT82]

On 12 July 2016 at 14:03, Timebandit said:

Hi, to fan or not to fan?  I was of the understanding that if the scope tube air/mirror assembly was at the ambient temperature already then by using the fan then there is really no noticeable benefit. 

And a fan to draw air in to the tube rather than push it up the tube nowerdays seems to be a more positive way of adjustment to your tube/mirror to ambient temperatures.   I would of thought unless there is a dramatic sudden change in temperature (when I can see that a fan would assist in bringing tube air temperature/mirror temperature more in line with ambient temperature then under normal temperature changes when observing then the fan going would not bring much benefit. Actually I think I read an article somewhere about fans that basically said a fan can be more of a hindrance when observing due to it disturbing air currents in the telescope tube that can effect the view through the eyepiece. I suppose if you think about it disturbing air currents in the tube is a bit like atmosphere disturbance which can be detrimental at times. But i am sure there are various options of thought on this.

By the way Derek ,now I have itchy feet also?. Had a parcel this morning containing some Pentax XW(  thanks Shaun) but the clouds are like a white blanket here presently so we are both in the same boat wanting to test our new purchases out?. 

 

 

It is not the movement of air which causes bad seeing, it is pockets of air at different temperature and density, which have differing refracting indices that cause the light passing through it to bend this way and that, creating the wobbling/boiling effect. Think of heat rising from buildings and bodies of water.  The function of the fan (one of at least) is to 'homogenise' the air in the tube. It does this by pulling in in ambient air through the tube and out the back.

Even if your scope if kept outside all day, as the ambient temperature drops from dusk through into night the temperature of the mirror will not drop as quickly, leading to the big lump of glass radiating warm air into the tube, creating the so called 'boundary layer'.  A fan therefore is required to: A- help the mirror cool quicker in line with ambient temp. B- help to disturb this boundary layer. Obviously the rate at which nighttime temperatures decrease is a big factor in all this.  You won't need a fan at all if the diurnal temp range is small in your area.

The only downside to fans on newtonian are the requirement for batteries and possibly high frequency vibration which could be visible at very high magnifications.  But then if the fan is properly installed with a variable speed this wouldn't be an issue.

As with most things in this hobby, you just gotta suck it and see... Either you will see some benefit or you won't!

 

 

[niallk]

I always throw on the rear fan to cool my 2" thick, 15" mirror.  Typically I put the scope out, do chores & put the kids to bed, and then go out to observe, turning off the fan.

I always read about people swearing by the boundary layer fans on say a Teeter mirror box on CN ... I have a notion to try something like the pic below clipped into the corner of the mirror box - angled to mix up the air for use while observing; or bent in for cool down blowing onto the front face, and then bent out of the way in use.

Maybe it's a silly idea Fairly cheap though to experiment!

 

[DRT]

I do hear what people are saying about fans and I will try mine if ever I get a chance, but, given that all this air turbulence and temperature variations inside the tube are being cited as the reason for doing so I struggle to imagine for one second that the minute quantity of air inside my tube and the negligible difference between that air and the air around it can even come close to competing with our temperate (moist) climate and the jet stream a couple of miles above my head and directly in line of sight of anything I could point the scope at. If the scope is (house) warm and outside is cold I get why this would be a problem, but not when a scope is always stored at ambient temperature.

If this was really the problem why don't we need to cool solar scopes? A whole day pointing at the Sun must cause the air inside a scope to "boil" - why don't we do anything about that? Is it because it is the air outside that determines the quality of the seeing and the clarity of the image?

Surely the enormous column of turbulent and variable temperature air above me is 99%+ of the problem, particularly in the UK?

[CraigT82]

Yes obviously you're looking through a column of heterogenous air many kilometres thick, but that column is free to move around and you will from time to time get moments of clarity.  The warm air in your tube (specifically right in front of the mirror) is trapped and not free to move, if undisturbed it can degrade the image permanently and it may never clear to give you those moments of clarity.

I understand your point about keeping your scope outside and hence at ambient temperature. My point is that if ambient temperature begins to drop rapidly when the sun sets, a 12" mirror may struggle to cool at the same rate, depending on it's thickness. This is where you may see the benefit need a fan, then again you might not. YMMV.

Have a look at this site:http://www.fpi-protostar.com/bgreer/sep2000st.htm.  There's some good vids of the effects of warm air in a tube. The site in general has some good info about thermal behaviour of newtonian and how cooling fans work.

As for your point about solar scopes, I have no idea.  Never had one.

 

EDIT: You'll see on that website that some users of big newts suspend a small fan in front of their mirror using fishing wire, in the shadow of the secondary. They take the boundary layer issue very seriously!

[jetstream]

2 hours ago, Pig said:

Derek maybe this will suffice for the hottest of days

 

I wonder if they could put a set of handle bars on this engine

Cooling is hugely important to help equalize the mirror, best views won't happen without it an any scope. Check out part 4 of Peach's great article for the effect of seeing.

http://www.damianpeach.com/simulation.htm

[CraigT82]

45 minutes ago, jetstream said:

I wonder if they could put a set of handle bars on this engine

Cooling is hugely important to help equalize the mirror, best views won't happen without it an any scope. Check out part 4 of Peach's great article for the effect of seeing.

http://www.damianpeach.com/simulation.htm

Agreed. Especially when a premium has been paid for the highest quality optics.  It's money down the drain if you don't employ some kind of thermal management (for high powered planetary observing at least) 

[jetstream]

This whole article is VG, I usually shut the fan off to obs, then give it another shot etc. Greer describes micro vibration in an article and this is why most experienced observers shut the fan off to observe IMHO.

http://www.fpi-protostar.com/bgreer/sep2000st.htm

[DRT]

Whilst I really do appreciate the advice given here in this thread I do wonder whether or not the discussion of "To fan or not to fan, that is the question" should be copied out by a clever Mob into a separate thread for the benefit of others?

[faulksy]

4 hours ago, Stu said:

I tend to agree that fans behind a mirror are probably marginally useful. I'm surprised that you don't find boundary layer fans blowing over the surface of the mirror more often as I think this has a more significant impact on the views.

this is what im fitting to my 20" just like the teeters, and using a speed controller to keep vibrations down

[scarp15]

This topic might get me to start using my fan again perhaps, as I seldom take it on dark sky trips when using my 14" dob. As has been already touched upon, perhaps it depends a bit on temperature variation. My scopes are kept in a fairly cool shaded (avoid direct sun light) and dry environment in doors. When transported out by car, I usually have the car fan blowing cool air. For late summer observing, there is a long cool down time once set up whilst waiting for darkness and the gradual temperature drop is not particularly severe. It also depends on where you go observing, one of my favourite locations is fairly lofty and situated on open moorland, so there are usually at the very least night time breezes, pointing into a breeze whilst cooling and in-between periods of observing helps gain and to retain ambient temperature. Low to medium power is of course usually stable anyhow. However probably good to have the fan to hand and give it a blast from time to time. 

Just out of interest my OOUK VX14 fan has a transformer lead, so a three pin plug rather than crocodile clips, which is very convenient to plug (and unplug) into an inverter which is subsequently connected to a standard 12V battery - the set up placed on the dob base.  

[Merlin]

Consider this article: www.skyandtelescope.com/wp-content/uploads/Alder-NewtThermal.pdf .

[CraigT82]

1 hour ago, Merlin said:

Consider this article: www.skyandtelescope.com/wp-content/uploads/Alder-NewtThermal.pdf .

That link seems to point to a 404 page not found error.. do you have that pdf saved somehwere?  I'd love to read it if you could share it!

 

[ollypenrice]

For a guess Merlin's article would have been the one in which research suggested that fans should blow across the reflective surface of the mirror, at right angles to the optical axis, and not up or down it. Despite the evidence nobody seems to use this method but I strongly suspect that they should. It makes perfect sense to me. Maybe an inlet fan on one side and on the opposite side an outlet fan. The key area in the story is the boundary layer just above the mirror. The lateral venting system destroys and removes it.

Olly

[Merlin]

Sorry you couldn't get the article Craig, it's very good. I'm not a computer expert, but type in "thermal management in newtonian reflectors", then click on the relevant item.

Let me know if you get it.

[Mark at Beaufort]

On 18/07/2016 at 11:35, Merlin said:

Consider this article: www.skyandtelescope.com/wp-content/uploads/Alder-NewtThermal.pdf .

The link works for me

[ian_bird]

On 19/07/2016 at 08:16, ollypenrice said:

For a guess Merlin's article would have been the one in which research suggested that fans should blow across the reflective surface of the mirror, at right angles to the optical axis, and not up or down it. Despite the evidence nobody seems to use this method but I strongly suspect that they should. It makes perfect sense to me. Maybe an inlet fan on one side and on the opposite side an outlet fan. The key area in the story is the boundary layer just above the mirror. The lateral venting system destroys and removes it.

Olly

Olly

I think Ian King may have done this with his Carbon Tube 10" RCT. He did mention chopping a hole in the side of it. Does he lurk here?

Cheers

Ian

[John]

An interesting piece on using fans with newtonians:

http://www.fpi-protostar.com/bgreer/fanselect.htm

 

[Mark at Beaufort]

On 17/07/2016 at 21:42, scarp15 said:

Just out of interest my OOUK VX14 fan has a transformer lead, so a three pin plug rather than crocodile clips, which is very convenient to plug (and unplug) into an inverter which is subsequently connected to a standard 12V battery - the set up placed on the dob base.  

Iain out of interest did you obtain your lead from Orion or did you purchase the product or did you make it yourself?

[scarp15]

Hi Mark, the lead with the transformer plug was provided with the scope and although I had purchased the scope second hand, as far as I can tell this was supplied by Orion with the original purchase in 2011. The lead supplied with my fairly recently purchased from new VX8L has the clips rather than a plug, which I haven't used at all. It might be obtainable from Orion UK or Maplins, it certainly is more convenient than just the clips, I can post a picture / provide specification if required. 

[Mark at Beaufort]

Iain that would be useful if you could provide a photo and detailed specification. I have bought both a VX8 and VX6L both second hand and neither had any power supply. Although my 12" Revelation has a battery containing 8 1.5v slots the device has a different connection. The Orion, I understand requires a 3.5mm Jack Plug adapter.

[scarp15]

Here are a couple of pictures, the lead is approximately just under 2M in length and yes the jack plug is  3.5mm (mine has some PVA tape on it as it must have been pulled at some point).

 

[scarp15]

This is what I plug it into.

 

[Mark at Beaufort]

Thanks Iain I might go into Maplins and see what I can find. I did a search on eBay - lots of cables but no one seems to make a 12volt - 3.5mm jack plug cable .

[scarp15]

Good luck with that Mark, yes Maplins might be able to source something or advise.