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10" DK vs SCT

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#276 Peter Natscher

Peter Natscher

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Posted 24 September 2014 - 07:46 PM

a comment on cool down ... i do not have any modifications to my LX200, but i don't cool it as daniel describes. i leave the focuser in place and turn the tube to point straight up. heat rises, the corrector has maximum view of the sky, and cool down proceeds fairly rapidly in my experience. my issues with daniel's method (also recommended to me by the manufacturer) are: (1) the open back allows dust, debris to blow into the OTA, where it remains until the thing is taken apart; (2) heat rises, so heat migrates across the mirror from bottom to top, producing a thermal gradient that can distort the optical figure. (wolfgang rohr makes some mention of this in one of his reports on the mewlon.) i know there is the story that infrared is reflected from the mirror coating and therefore has to go out the back of the mirror, but the reflective coating is also heated by conduction and i believe that largely negates the supposed reflection effect.

 

in any case, i'd be interested to hear how far the cool down issues with the SCT can be remedied with different cool down methods, and any demonstration that venting through the visual back is really optimal.

Do you like your Royce optics on the 250?  How do you compare the planetary views between your two DK's -- Tak and Royce?



#277 drollere

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Posted 25 September 2014 - 01:24 AM

 

a comment on cool down ... i do not have any modifications to my LX200, but i don't cool it as daniel describes. i leave the focuser in place and turn the tube to point straight up. heat rises, the corrector has maximum view of the sky, and cool down proceeds fairly rapidly in my experience. my issues with daniel's method (also recommended to me by the manufacturer) are: (1) the open back allows dust, debris to blow into the OTA, where it remains until the thing is taken apart; (2) heat rises, so heat migrates across the mirror from bottom to top, producing a thermal gradient that can distort the optical figure. (wolfgang rohr makes some mention of this in one of his reports on the mewlon.) i know there is the story that infrared is reflected from the mirror coating and therefore has to go out the back of the mirror, but the reflective coating is also heated by conduction and i believe that largely negates the supposed reflection effect.

 

in any case, i'd be interested to hear how far the cool down issues with the SCT can be remedied with different cool down methods, and any demonstration that venting through the visual back is really optimal.

HI,

How fast does your Royce 250 take to cool down?  It is open at the front and allows warmer air to escape vs. an SCT.  Since the DK's are open-tubed, how often do you need to clean the primary and secondary mirrors on them?  Once a year as on my Dob?

 

 

the royce takes longer to cool down than anything else i own. the conical mirror is supposed to facilitate cool down at the thin edges, but in fact the fat core acts as a heat sink. even with the cooling fan running, two hours is often insufficient time in the moderate temperature variations of coastal northern california. when it is cooled down and properly collimated, the views are impressive.

 

the point with orienting the SCT tube upward is that the heated air does *not* have to escape -- it's the heat in the mirror and back plate that has to escape. this it can most easily do through a corrector exposed to the sky, and heat rises evenly from the mirror to this cooler zone, equally from all sides because the mirror is flat. i cool all my reflector telescopes in the same way, and use the tube as a chimney.

 

i clean the corrector on the SCT once a year, or less often. dobs are relatively exposed and low to the ground, but i feel you pay a sleek tax every time you clean if you touch the mirror with anything except a liquid jet of detergent or water.




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