Another primer on Mak cooling

LewisM · #1 20-11-2018, 09:37 PM

In restoring a Mak scope, I have had to totally remove the paint. This has left a rather attractive bare aluminium OTA, and I was thinking ... would it help in cooling a Mak faster if it was left bare?

I seem to recall some doing similar things to OPEN tubes like Newts, but insulating Maks, so would I actually be better served painting the tube?

I like it shiny

(plus no paint 'n polish blues

)

Wavytone · #2 20-11-2018, 10:49 PM

Hi Lewis...

There really is a reason why telescopes and observatory domes should be white.

A bare aluminium OTA is about the worst choice possible as it’s an excellent thermal conductor. I guess you also aren’t aware that cooling a mak is also not the best thing to do to stop the internal tube current. It has been pretty well established that insulating the OTA of maks (and SCTs) will both stop the internal tube current and prevent the OTA from cooling rapidly - this helps significantly with keeping the corrector dew free.

If I were you I’d have it powder coated in pearl white, there are places that will do it for a reasonable cost. Or have some sort of close-fitting insulating sleeve that fits over it.

Secondly pearl white will stop the scope getting hot if it’s in the sun, and minimises the radiative cooling to the sky at night.

I know some will dispute this however some years ago I had the chance to test about 100 cars and some samples of sheet metals with an optical pyrometer to find out how they perform in the real world in order to assess the range of temperatures that would occur in trains stabled in maintenance centres, and power required for air-conditioning night and day winter or summer.

Pearl white was consistently the best followed by the lacquer whites.

LewisM · #3 20-11-2018, 11:01 PM

Dang it, more painting...

How about satin pearl enamel? I have that already from a prior restoration.

Thanks Nick.

I already have made a thick EVA foam insulation tube that tightly fits the tube (I made a dew/light shield for a Tak refractor recently so know how to do it rather neatly).

I'll start masking tonight. HO HUM!

Wavytone · #4 20-11-2018, 11:09 PM

Satin or gloss won’t make any difference. Personally I’d get it done professionally as it should be grit blasted - paint adheres much better to a rough surface than smooth - and apply a primer before the final coats.

A DIY effort is going to look pretty awful IMHO.

I did some research recently as I’ve been thinking of repainting the MK91 OTA.

Or you could have one of the weird bi-colour paint jobs with a white base then a coloured clear coat over that. These aren’t DIY though. Maybe add some yellow/red flames, a long antenna with foxtail (extra wifi reception) and mount it on some wide chrome mags under for portability...

Kunama · #5 21-11-2018, 10:42 AM

A very nice score on the Santel Lewis, bargain of the century !!!

I tend to disagree with Nick on the bare aluminium thing and I think there are quite a few who would also go with bare aluminium though perhaps not on fully enclosed Maksutovs. Horses for Courses comes to mind.....

There is a good reason why Antony Wesley, Trevor Barry and Phil Miles all use bare aluminium tubed Newtonians for their superb planetary imaging....

Look forward to you treating this Mak to some "MichelangeLewising".

(Lewis's DIY efforts are way above the usual DIYs we see...)

LewisM · #6 21-11-2018, 10:46 AM

Quote:

Originally Posted by Kunama

A very nice score on the Santel Lewis, bargain of the century !!!

I tend to disagree with Nick on the bare aluminium thing and I think there are quite a few who would also go with bare aluminium though perhaps not on fully enclosed Maksutovs. Horses for Courses comes to mind.....

There is a good reason why Antony Wesley, Trevor Barry and Phil Miles all use bare aluminium tubed Newtonians for their superb planetary imaging....

Look forward to you treating this Mak to some "MichelangeLewising".

(Lewis's DIY efforts are way above the usual DIYs we see...)

Thanks Matt - indeed, a super score!

It won't be bare metal - it's going to be Antique White USA from Dulux. All mixed up in epoxy 2pack ready to go.

Wavytone · #7 21-11-2018, 02:34 PM

Well I’m stunned to find you’ve joined this exclusive club, Lewis. Which Santel ?

I’m well aware this seems completely contrary to the long established mantra (must cool the OTA and force-ventilate it) but it has only been recently accepted that there really is an alternative - insulation - that is entirely effective at stopping the internal tube current and helps with dew as well.

If you do use insulation, the colour of the OTA is irrelevant if you’re going to wrap it in a cover of some sort. You could leave it bare or paint it black with pink polka dots if you like.

LewisM · #8 21-11-2018, 03:57 PM

Etch primed and ready for the top coats when the weather permits.

I should do a blog like Marc and the CN212 resto we did together

Wavytone · #9 21-11-2018, 05:12 PM

So which Santel is it - the 6”, 7” or 9” ? Pics ?

LewisM · #10 21-11-2018, 05:38 PM

6"

Wavytone · #11 21-11-2018, 07:09 PM

Have you tried it optically ? I think the CO is 33% so will be interesting to see if it’s any better than typical 6” SCTs

What’s the focuser on the back ?

LewisM · #12 21-11-2018, 07:16 PM

Quote:

Originally Posted by Wavytone

Have you tried it optically ? I think the CO is 33% so will be interesting to see if it’s any better than typical 6” SCTs

What’s the focuser on the back ?

I think a cardboard toilet roll tube will be better than typical 6" SCTs

Helical focuser.

Wavytone · #13 21-11-2018, 07:30 PM

Quote:

Originally Posted by LewisM

I think a cardboard toilet roll tube will be better than typical 6" SCTs

Helical focuser.

Hmm so it was not original. Any paperwork or a test certificate with it ?

And does it have the black intes finder with their unique attachment - there are at least 3 different variants depending on vintage...

LewisM · #14 21-11-2018, 07:44 PM

No, the focuser is original - paperwork proves it.

Intes finder is unique, yes... I wouldn't say the attachment is that "unique". Good, but hardly unique.

Wavytone · #15 21-11-2018, 08:40 PM

So its not this configuration re focuser and finder ?
http://www.stellaroptical.com/Images/Santel-MK6-b.jpg

Who issued the test certificate ? StellarOptical, Mike Palermiti, or Santel itself ?

FWIW In case anyone else is thinking of acquiring a large OTA secondhand from the US there is a way without paying a fortune - sign up with a freight forwarder (like MyUS) that charges by weight only - not volume - and have the vendor ship to them at their domestic rates. Then the freight forwarder will consolidate it in a larger shipment to Australia - which is where the big savings come in. And they can still use Fedex for that.

Shipping the MN86 this way cost a fifth of what the quotes were to ship it direct from the vendor. It was so reasonable that it was worth having the scope shipped inside its transport case rather than ditch that to cut the cost.

LewisM · #16 22-11-2018, 12:39 PM

The new paint was done yesterday afternoon.

Today, I redid the finder base and the primary backplate.

The front of the corrector plate is now spotless (came away as a BLACK kimwipe!). The primary was spotless already (and looks like it won't need a resilver for even another 20 years).

All mounting hardware either resprayed satin black, or cleaned with turpentine.

Interior blackening is FLAWLESS and VERY non-reflective, so nothing required there. Front tube cap resprayed satin black, with a touch of personalisation that will be revealed when it is finished. New data plate arrived from my printer today.

Starting on cleaning the visual back and focuser.

Decided at this point to use an new old-stock Vixen finder, as I find the Intes finder NOT user friendly eye-relief wise (for me). I may yet modify and fit a Baader Vixen shoe onto the spare screw spots.

This one was too far gone exterior finish wise to keep it totally original, hence I had the data plate made that indicates "RESTORED 11-18". No bluff and BS here. It is what it is.