Baffled and blown away!

[Blackheart]

Hi all

Just wanted to share my experience with a mod that has improved the view through my 12" SW dob.

I've had the scope for 3 years and have always been disappointed with the views of planets and I was never able to see difraction rings on a star test due to 'poor seeing'. However, having become convinced that no one is that unlucky I began to explore other possible causes and soon learned about the Thermal Boundary Layer. This is a layer of warm air that sits above the surface of the primary mirror. Big mirrors struggle to reach thermal equalibrium so the TBL is nearly always in evidence. One solution is to fit the scope with a Mario de Lio baffle. This draws air down the tube and over the mirror surface.

I've attached a couple of pics of my version, which fits over the rear end of the scope (a further collar is fitted inside the tube a few inches above the primary mirror. A search for the baffle will show you the details). I wouldn't say it was a perfect solution but for the first time in 3 years I can see clear difraction rings on stars and I am starting to see some real detail on Jupiter!

If anyone wants to give it a go I have my 'prototype' which I used to tape over the rear of the scope and power with a small battery pack. It's not exactly Astro Engineering but if anyone wants it they can have it for free. It might just help to solve similar problems with your big dob.

Anyone interested just drop me a line and I will pop it in the post.

Here's a couple of pics of the fan section and the prototype in question...

Cheers

BH

ps. I found the details here (somewhere around page 6!)

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4910402/page/0/view/collapsed/sb/5/o/all/fpart/5/vc/1

[Beulah]

Nice one - I have a few computer fans kicking around the house - need to get the soldering kit out...

[sologuitarist61]

Thanks for the information - very useful

[Daniel-K]

that looks tidy

[JamesF]

It's certainly an interesting idea to seal the lower end of the OTA to force air into the tube above the mirror. With Skywatcher mirror cells that are pre-drilled for fan fittings that might be very easy to do, using the fan bolts to clamp the closure between the underside of the cell and the fan.

James

[Blackheart]

Hey James

That was my original thought but I wanted something that I could slip on and off so as not to interfere with collimation. Don't know if you have similar troubles with your 10" dob...

BH

[JamesF]

I have a flextube OTA, so it doesn't suffer so badly. I'd have thought the collimation screws would be able to go through the closure as well, but without trying it who knows what problems might occur

James

[DaveGarland]

As luck would have it I have an old PC I'm about to throw out. I think I'll be keeping the fans

Have you noticed if the air movement has any effect either way on dew?

[Blackheart]

Good point Dave

Yes, it has a marked effect on dew, namely you don't get any! I use a homemade shroud but even so I normally get problems with the secondary fogging up. However, now I can leave the scope out for hours and so long as the fan is running the mirrors stay clear. It also seems to be extremely effective at cooling. Normally takes at least an hour to cool but I checked after half an hour last night and the image was pretty clear. Just in time to spot Europa beginning its transit of Jupiter. (see my other post in the lounge) Wouldn't have been able to see this before I started using the baffle...

[John]

Very interesting

I have an Orion Optics 30cm F/5.3 newtonian which is solid tube of course. It has a small "pusher" fan behind the primary but it might be interesting to install a suitably sized baffle a few CM above the primary mirror and see if the air flow can be deflected across the primary to disperse that thermal boundary layer.

I've heard of fans being mounted on the sides of the tube, blowing across the primary, to try and achieve this but the baffle idea is a new one to me.

Orion Optics newtonians don't have much room between the tube walls and the edge of the primary though so the baffle will need to be quite a thin strip around the inside of the tube if it's not to vignette the primary.

Sounds like it would be worth experimenting with this though

[8kids]

This mod seems simple enough, and seems like it should work. Being a simple design a before and after comparison should be no problem.

[JAS]

I have a similar setup on my 200pds,

The colomation screws come through the 6 holes and it is fixed to the tube with velcro, I am able to colomate with it in position but I made sure it is removable just in case.

I also added a filter for a PC just to stop any dirt being pushed through.

I dont have any pics of it place tho.

Jason.

[JamesF]

That looks very neat Jason.

James

[John]

As I understand it, it's the baffle just above the primary mirror that is the key component here. Without that, any fan behind the primary is just either blowing air up the tube sides or drawing it down them. The target here, as I understand it, is to deflect the airflow across the primary mirror upper surface, thus quickly dispersing the boundary layer which appears to be considered to be a significant contributor to reducing resolution. Or have I got the wrong end of the stick ?

[Blackheart]

Hi John

That is my understanding too. I suspect that drawing the air down the tube would have some effect but not so much as with the baffle. If I get chance tonight I will try and take a vid of an out of focus star. It's amazing to see the difference in thermal currents with the fan switched off and on. Without the fan you can see 'worms' squirming across the image... switch the fan on and the worms disappear to be replaced by diffraction rings. Still depends on seeing conditions but certainly an improvement.

BH

[Blackheart]

Hey Jas

Yep, that's a nice job. Bit more professional than mine