First observations with 180 Mak

[morls]

Well, there were enough gaps in the clouds over Melbourne for me to get my first observations with the new scope. Tantalising...

Keeping the Mak in the garage, I was hoping it would be pretty close to thermal equilibrium. I was able to see several bands of colour on Jupiter after setting up, and over the next 90min, before conceding defeat due to clouds, the images became steadily clearer. From this experience I reckon cooling won't be too much of an issue.

After about 60 min I was able to make out the Cassini Division between Saturn's rings, which I was very pleased with as I'd not seen this before. This was with my 13mm eyepiece at around x207, I say around because I think the diagonal adds focal length to the 2700mm.

The moon was blinding, of course.

Overall I'm extremely happy with the setup. The mount (Vixen Super Polaris) seems up to the job, and it was tracking pretty well with just a basic true-south alignment. One modification I'm thinking about is the addition of a quality focuser, but I'll wait and whether I get used to the slight image shift that's happening. I'm not sure if the mount could take much more either.

Cheers

Stephen

[Wavytone]

I know this sounds counter-intuitive after all the hype about thermal equilibrium, but please bear with me on this as I have had two similar scopes and my current one is a considerably bigger mak.

Insulating the OTA works wonders for killing the internal tube current, and it ensures the corrector won't dew over, either. Find a sheet of foam plastic or rubber a few mm thick - a yoga mat works well - and cut a piece big enough to go round the OTA and cover the OTA from end to end and then some - a short dewcap. Make some cutouts for the dovetail and finderscope. Wrap this reasonably tightly around the OTA - anything will do - adhesive tape, even your belt... I use a pair of velcro straps.

The point is that the internal tube current is caused by the thermal gradient within the OTA - ie the difference in temperature between the internal central baffle, and the air near the wall of the OTA (which is metal).

Ordinarily the night air is much cooler than the innards of the scope, so the wall of a bare OTA is chilled and a significant difference is formed, which drives the tube current. Note also that in the Skywatcher maks the primary mirror is supported on the central baffle and this amounts to a large thermal mass which continues to pour heat into the baffle for ages, as it has no other way to cool apart from convection.

Insulating the OTA stop it cooling, and hence the OTA stays at much the same temperature as the air inside and the baffle, hence no thermal gradient, and no tube current.

With the insulation, the main way heat escapes is then through the corrector plate, and its enough to keep the corrector plate warm and stop it dewing over, for hours.

It also means you won't need a heater strap.

There's another reason to extend the insulation beyond the OTA, as well - this makes a significant improvement in stray light baffling for maks, and the contrast on extended DSO's should be noticeably improved.

[morls]

Hi Wavytone,

That all makes a lot of sense, thanks so much for sharing your experience with this. What a great way to take care of heat currents, reduce dew issues AND increase contrast!

I'm going out today to buy a yoga mat...

Cheers

Stephen

[ZeroID]

Hi Wavytone, and thanks from me for that as well. Easy to try and makes sense. I'm enjoying discovering my ( new to me ) Mak. We may have to setup the 'Mak Users' club on here. :thumbsup: :D