Cooling Off Period

[michael.h.f.wilkinson]

I store my scope in an unheated garage, which cuts down cooling time. I generally get good views with my C8 in 30-45 minutes at high power. As others have said, the main issue is convection currents due to temperature difference in the tube, rather than the temperature of the optics. It is the comparatively wide, short tube of the SCT and Mak design which makes it particularly sensitive to this. The longer, more slender design of both refractors and to a lesser degree Schmidt or Mak Newtonians has two effects: the relative surface area for heat exchange with the outside is larger, and stable convection currents in the tube do not form easily (in particular in narrow tubes with baffles).

Regarding dew strips: I only use a dew shield, and only once or twice did this prove insufficient (both times at the end of a 5-6 hour session). I would first make a dew shield, and only get a dew strip if necessary. Note that heating the corrector plate in a Mak or SCT does not induce convection in the tube (or only very little), as the heating is at the top of the tube (assuming your are pointing the scope upwards). If you overdo the heating, you can set up convection currents outside.

[Moonshane]

you need to recheck collimation after the scope is cooled of course.

What?

Nobody told me that!!!!

How is it with this hobby that, each week, I discover that I still need to do something else to get it all right??

So my routine should now be,....

1. switch on heaters on 10% power, switch on fan (blowing over primary from behind)

2. allow to cool down so air inside is equal temp. ( say 40 mins for 8 inch dob)

3. Collimate with laser ( check laser with jig first)

4. Star alignment

5. Start looking?

Cheers

Wise

really it depends on how large and fast your scope is and how tightly held in the cell. mine is a heavy 16" f4 which basically floats on the cell with virtually no pressure points or places holding it to potentially pinch the optics. I always check before cooling and after but also sometimes during the session if the stars look odd as the mirror can occasionally shift during the night.

my 6" f11 hardly ever moves and therefore I hardly ever check this after cooling, but do so once, just after putting out.

as I said though, you can start observing at lower powers almost instantly with a newt although the view does improve as the temperatures stabilise.

[John]

So my routine should now be,....

1. switch on heaters on 10% power, switch on fan (blowing over primary from behind)

2. allow to cool down so air inside is equal temp. ( say 40 mins for 8 inch dob)

3. Collimate with laser ( check laser with jig first)

4. Star alignment

5. Start looking?

Cheers

Wise

You missed out step 4.1 - notice clouds moving rapidly across the previously clear sky.

Step 4.2 is to order a grab and go refractor

[Yendor]

Thanks again for all the input, and the extra questions too.