Latest Dob Mods - Fan and Balancing Weight

[Moonshane]

Hi all

I have made a couple of simple mods to my dobsonian recently.

The first is that I cut a hole in the end cap at the back and inserted a fan. This blows air into the tube and I wired it so I can have both fans on at 12v or at reduced voltages / speeds (I have a variable voltage transformer) or just the rear fan flowing air across the back of the mirror. The other fan is on the side of the tube blowing across the mirror. I think that the rear one at low speed is better left on while viewing. Anyone got any views on this?

The next mod was to try and counter the tendency for the dob to sag a little when the 35mm Panoptic is in the chair. I robbed an idea off Russ and though about a dovetail bar with a puck and some weights. As luck would have it, Tom sold me a dovetail puck for £10 and for another £10 I got three of the 80mm x 6mm washers and a 1/4 UNC Meade thumbscrew off Astroboot and a short dovetail bar off the main SNS site. These guys really seem to have their act together now..

This seems to work well and although I may have to fiddle to get good balance, it's easy to add and remove. I may get another OO tube ring so I can move the weight down a bit and therefore the tube up a bit but I'll see how it goes. It looks good and I thought may assist others.

Credit to Russ for the initial seed of an idea.

It's actually clear here currently so everything is crossed.

Cheers

Shane

[russ]

Looks excellent Shane. I have something similiar for mine. I filled the EQ6 puck with fishing lead weights and then capped it off.

[Moonshane]

top quality mate!

I think I'll be moving mine down and getting another tube ring. the weight is either on or off and easy to remove so it will work OK. I was dubious about putting yet another hole in my tube (so to speak).

[Doc]

These look really cool guys and pretty easy to do.

[Moonshane]

cheers Mick - cool and easy - that's the way I like it

[Rusty Strings]

Very elegant. I love the way dobs are so mod-able.

[kirkster501]

This is a great idea. I think I could rig something up with two three-inch pieces of broom handle - a piece either side of the support post of the flextube. This broom handle just fits certain dumbell 1kg weights a treat.

I will post some pics later.

[Moonshane]

good luck. it's an easy mod and does work very well indeed.

[estwing]

this may be silly,i can understand havin the fan at the back,coolin/dew,but having one blowin across the mirror face?wouldn't this affect the viewing..we wait for seeing conditions to be good and still and your creating "movement" in the tube? pls go easy on beating me up!!!sorry

[Moonshane]

the theory is that a mirror which is not in thermal equilibrium with the environment (either as it's not yet cooled or the local temps have dropped or risen quickly) creates a boundary layer (a bit like heat haze shimmering on the distant road on a hot day). this creates a weak lens effect and therefore makes the image poor. air blowing across the mirror destroys this boundary layer and clears up the images.

I never really found it made a big difference and removed it.

a lot of scopes have them https://www.google.co.uk/search?q=boundar+layer+fan+telescope&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a&um=1&ie=UTF-8&hl=en&tbm=isch&source=og&sa=N&tab=wi&ei=BaFZUb6pJo3ZPLLpgJgC&biw=1024&bih=605&sei=IqFZUfWvHOKH0AXxlYCwBA

[estwing]

so to beat this,keep scopes outside to combat it..which i know isn't possible for most.

[Moonshane]

to a large extent yes. however, some nights (although not necessarily in the UK) the temperature drops (or rises) so rapidly that a big mirror cannot keep up so that's when these fans come into their own. that said, if you are at a dark site with a dob scope, I'd imaging high power bright things are far from your mind so it should not really matter.

I think they may also help with dew to some extent.

[kirkster501]

My scope kept in the garage. So always within a degree or two of outside. However, being kept in garage has its own challenges - dust and creepy crawlies for instance !

[Moonshane]

true and that's a good start but these fans are really for when the scope is already outside and at ambient temperature but there's a relatively rapid shift in temperature (or of course if you cannot store outside and want to observe soon after putting it out). then the mass of glass in a large mirror always lags behind without assistance.

[estwing]

My scope kept in the garage. So always within a degree or two of outside. However, being kept in garage has its own challenges - dust and creepy crawlies for instance !

..i find bin bags and lacky bands useful

[Ohmless]

clean work. kudos