Dew heater for the Tal - discussion topic.

[Kaptain Klevtsov]

Firstly, I need to describe the beastie as its got an unusual design. The front of the 'scope is open as there is no corrector plate like you get with the SCT, just the secondary assembly on the curved support bars. The primary mirror is open to the elements as it were.

Dew gets on this thing in two ways, firstly on the primary mirror, which I can shift quickly with the hairdryer, but also on the secondary mirror. This is where the problem lies, as the secondary mirror is behind a lens (this is instead of the corrector plate in the SCT, smaller and cheaper to make). That means that I can't get to blow it with the hairdryer, so once this bit dews up it takes ages heating the whole secondary assembly, then the whole tube is warm and needs to cool down again.

I'm wondering if I can use a heated dew shield in the same way that would work with an SCT, as there is no corrector to conduct heat towards the secondary, or should I fit some resistors in the secondary to heat it directly? Then do I need a dew tape around the back of the 'scope to warm the primary as well, or might the secondary bit warm the entire tube up enough?

I've been scratching my head over this for a while now and its time to canvas opinion before I make/buy something that may not work.

Kaptain Klevtsov

[moondog]

KK - Using heater strips both on my SCT and achro I would think that you would be better with 2 strips. One by the main mirror and one by the secondary.The spider should conduct some heat to the secondary.

The heater strips provide very gentle heat and are best switched on before you start a session. If you wait for the dew to collect first the strips take an age to clear it.

MD

[FLO]

Have you tried a regular dew-shield first? That is sometimes enough.

[moondog]

Steve

I have a dew shield for the C8 - it holds back the formation of dew for a reasonable length of time.

Main problem is if it's a heavy dew and you're wanting to spend an hour or so imaging. Then the dew shield alone just doesn't do the business I'm afraid.

MD

[FLO]

Steve

I have a dew shield for the C8 - it holds back the formation of dew for a reasonable length of time.

Main problem is if it's a heavy dew and you're wanting to spend an hour or so imaging. Then the dew shield alone just doesn't do the business I'm afraid.

Yes, I am hoping KK's location is less prone to dew and that a regular shield will suffice. It might, particularly when you consider that the Tal 200K doesn't have a front corrector lens so is less of a dew magnet. Just wrapping some card around the tube will tell us whether it will be enough.

[Kaptain Klevtsov]

OK guys, first go seems to be the old "wrap some cardboard round it" technique. As its nearly free, I'll try that one first. Once the clouds clear off (and the heavy rain!) I'll report back and let you all know what happened.

Kaptain klevtsov