SCT Cooler part II

[davidpretorius]

A question,

How often and how easy to one of the c9.25 come apart for cleaning the mirror????

Is it a no-no, don't go there situation???

If i was to three cut holes in the back of a OTA so that we could put in three peltier systems. The system would be sealed ie not introduce external air into the cavity, just cooling the internal air and circulating over the mirror???

Before I cut holes, I would like to remove mirror for dust etc

OK, you are over the shock of the suggestion, is it possible??

[Lester]

What size holes do you need to make?

Would the extreme cooling of the peltier cause temperature changes from the primary=very cold, to the secondary slightlywarmer?

Would you need a exhaust fan at the eyepiece mount to draw out some of the warmer air?

If all this works might consider doing my 14" SCT Meade.

[matt]

I'd say that's possible Davo.

I think you'd need to consider putting the primary back in exactly the same way it came out to maintain the optical alignment...???

but other than that, it sounds OK.

You got your hands on a 9.25 OTA????

[bird (Anthony Wesley)]

I have heard from various people that dismantling a SCT is not for the faint hearted - it comes apart easily enough but when you reassemble it you have to get the optical alignment of the corrector and primary back to exactly where it was, not something that you can do without specialist equipment. I don't know how true this is, but that's what I have picked up off the net. It might be worthwhile checking out where you could send the scope after the mod is done to have it optically checked and aligned.

You might want to consider adding fans on the inside of the OTA as well to move the cold air around, so you don't end up with a layer of cold air sitting in one place.

cheers, Bird

[davidpretorius]

going purely on the size of the cold plate off a $30 12volt d1ck smith fridge and its surrounding support for screw holes etc, the area would be say 2 1/2" x 3 1/2".

This little fridge is cooled by a single 4amp peltier This insulated volume of say 8" wide x 5" deep x 6" high chills your drinks.

The idea would not be for the peltier to cool the mirror directly, but cool the air to cool the mirror so it is even all over the mirror.

Exhausting means replacing the air you exhaust and that in my mind brings in dust?

Also, it would be ideal to get the temp of the internal air colder than ambient by a few degrees so that the mirror can keep losing heat to the air until it is close to ambient. Then let it settle for 1/2 hr.

Based on birds system for his newt, it seems to be effectively isolated, ie the air is continually recycled around the bottom of the tube and continually cooled. You don't want it interacting externally.

I have just had an excellent suggestion from a learned colleague, I will try and get my hands on an old c8 OTA tube only to try my hand so to speak.

Continually circulating the air via fans should hopefully mean all internals should be cooled as well.

It would not be as extreme as a fridge as we are not wrapping the ota in insulating foam, just cooling the inside.

One 4amp peltier at the moment can cool my primary mirror at up to 5 degrees an hour, birds sealed system is even quicker I believe.

[davidpretorius]

thanks bird, anyone ever pulled one apart??

Also bird, do you agree that an isolated system is better to cool than exhausting and intrcoducing air from the outside??? from a cooling point of view. obviously less dust

[matt]

Dave?? You got a 9.25 you're working on?

[davidpretorius]

Matt, no sct for me just yet.

maybe in a few years. it is more out of interest that i am playing with the idea

[bird (Anthony Wesley)]

Originally Posted by davidpretorius

thanks bird, anyone ever pulled one apart??

Also bird, do you agree that an isolated system is better to cool than exhausting and intrcoducing air from the outside??? from a cooling point of view. obviously less dust

I'm convinced that one of the best ways to cool a scope is to have the mirror end sealed to prevent any air exchange with the outside and cool around the mirror with peltiers and internal fans.

Doing this on my scope has totally stopped any tube currents and slow seeing effects. With temp sensors inside and outside it's possible to get the air around the mirror to a stable temperature, and because the tube is not open at both ends there is no airflow inside the tube.

This also prevents excessive dewing etc, as once the air in the tube has dumped its moisture then it stays dry for the rest of the night.

cheers, Bird

[Lester]

Great topic, I have a mid 1970's Celestron 8" SCT that I dismantled many years ago to drill 3 x 1/4" holes and thread them to put in bolts to stop the primary from shifting during long exposures.

The primary can only go back in one way because the focuser attaches to a spiggot on the back of it.

I put a texta mark on the edge of the corrector and tube so that it went back in the same spot.

I have never had to recollimate the C8. Unlike the 14" SCT Meade that I have just purchased; but I had to just tweek a couple screws on the secondary and now its fine.

So I don't see any problems dismantling a SCT.

[davidpretorius]

thanks lester for dismantling update, sounds promising.

the mirror could always stay there whislt drilling/cutting, but I would almost prefer it to come out ie dust, metal shards etc