The question of Newtonian primary mirror fans

[Mr Spock]

A simple question: which do you believe is best method of mounting a Newtonian primary mirror fan - suck or blow?

[JeremyS]

I’m not a fan of mirrors 

[Macavity]

ISTR it is held that... "blowing on stuff" induces more turbulence?
Computer CPU fans blow rather than suck? Misspent-youth, "Case
Modder", reluctantly concedes (brute force) blowing is better? 😅

Edited January 21, 2023 by Macavity

[AstroKeith]

Blow.

If you measure the air movement around a fan, the exhaust is quite a well formed 'beam' diverging slowly.

The input air is drawn from 'all over' and hence the flow rate is much lower, around 1/10.

So if you are cooling or heating a specific surface, always blow.
If you are regulating an enclosure, then suck!

[great_bear]

The general consensus on CloudyNights was that fans should blow on to the mirror. It’s also how my 8” GSO Dob was made.  

[Mr Spock]

The idea of the fan is not to cool the mirror but to remove unstable air from the tube and from the mirror surface. So, wouldn't suck be better, providing there's a good seal around the fan?

[CraigT82]

Suck. Draws an undisturbed flow of air down the tube which washes over the face of the primary removing the boundary layer, also sucks the micro boundary layer off the secondary.

Fan isn’t for cooling the mirror in my opinion, more for managing the air in the tube - trying to avoid temperature deltas.

Edited January 21, 2023 by CraigT82

[Robindonne]

Remember reading somewhere that blowing over the surface from the side gave the best result

Edited January 21, 2023 by Robindonne

[rl]

I've reversed the fans on my 8" so that they blow on the back of the mirror. The fans are not 100% efficient so the "wasted" power helps put a small amount of heat into the mirror which helps with dew. 

[scarp15]

Blow, have them set up on both my Newtonians but never use them.

 

[Stu]

I always understood that a fan blowing across the face of the mirror is excellent for removing the boundary layer, which sounds like what you are trying to achieve?

[Macavity]

I imagine a large "Palomar-sized" (Amateur Super-Dob?) might be a
concern re. such things. A typical shop-bought reflector, rather less so?
But I may be quite wrong! If you can demonstrate such effects! 😎

Edited January 21, 2023 by Macavity

[Mr Spock]

I'm not sure I want to make holes in the side of the scope for fans though. 
I was thinking of swapping the small fan for something larger - maybe even 200mm. Not sure if that's a good idea or not.

What I see from a defocussed star image is small fluctuations. I'm assuming these are tube currents. 

The scope last night was performing really well. I just wondered if it could be even better with a stable air mass in the tube and over the mirror.

[Stu]

3 minutes ago, Macavity said:

I imagine a large "Palomar-sized" (Amateur Super-Dob?) might be a
concern re. such things. A typical shop-bought reflector, rather less so?
But I may be quite wrong! If you can demonstrate such effects! 😎

In this case I think you are wrong 😉. Even on 12” and upwards scopes, removing the boundary layer makes quite a difference.

My 8” Portaball had a tiny fan blowing across the surface specifically for this.

[Mr Spock]

I should add my 12" has one small fan in the centre. I note some other designs have three or more. I can't imagine a small fan on a 300mm mirror is doing much.

[Macavity]

4 minutes ago, Stu said:

a tiny fan blowing across the surface specifically for this.

I suspect that's the VERY thing! (Two/more issues?) 🙂

Edited January 21, 2023 by Macavity

[niallk]

I want to install boundary layer fans, and they'll blow 😉

Edited January 21, 2023 by niallk

[bosun21]

I only used the fan on my dobsonian during the cooling period. I then found that the stars were sharper with it off during observing time. 

[Guest]

On the 10 inch the fan sucks air out, on the 16 inch the 3 fans suck air out.  I will use them initially to do some air exchange and then shut them down.   Only in the rarest of circumstances have i ever seen a benefit of running fans while observing. 

Edited January 23, 2023 by Mike Q

[dweller25]

9 hours ago, JeremyS said:

I’m not a fan of mirrors 

Except your C9.25 and Mewlon 🤣🤣

[dweller25]

8 hours ago, CraigT82 said:

Suck. Draws an undisturbed flow of air down the tube which washes over the face of the primary removing the boundary layer, also sucks the micro boundary layer off the secondary.

Fan isn’t for cooling the mirror in my opinion, more for managing the air in the tube - trying to avoid temperature deltas.

What Craig said 👍

[Peter Drew]

I always blow on my chips, never suck.     😀

[Jeff Marsden]

These links might help. They are a little long especially the second link so they are more than a quick simple answer.

 

https://www.cloudynights.com/topic/61238-cooling-fans-and-newtonian-thermal-issues-research/

 

This one has thoughts on curved spiders as well as fans and their placement. Some of Daniel Mounsey’s (long) musings about observations with a ‘dialled in’ reflector are interesting. Post 49 perhaps addresses your immediate question.

 

https://www.cloudynights.com/topic/24123-refractor-vs-reflector/

 

There are some really good ideas going on here but a little hard going. In typical forum style, veering off does happen a few times.

 

Enjoy the bedtime reading.

 

Jeff