Sky-Watcher Mirror Cooling

[Charic]

I generally find that about 60 mins is a good time for me to leave my scope outside before use, this gives me ample time to ready myself, collate what I want to view, wrap up well, and to get the eyes dark adapted for my site.

Given that a 12" Pyrex Mirror is less susceptible to thermal expansion / movement, surely having more glass would still need as long, or a longer cooling period,  to get the larger mirror to the same ambient temperature within the mirror itself compared to the 8" mirror if sat side-by-side.

I don't imagine my 8"  mirror actually expands/contracts too much, I'm more concerned about the warm air currents over the mirrors surface, affecting the final image, until these currents disseminate. I don't particularly want to convert my telescope into a fan assisted Astro-colander?

Has  anyone compared the visual results of an 8" standard to a 12" Pyrex Sky-watcher mirror, what was your impression?

I wouldn't  buy a 12" standard until I have tried one first. I need to iron out these little issues. Bigger is better for DSO, but I don't want to introduce some unwanted issues. If I can use a 12" happily with the present collection of EPs, then an upgrade could be feasible. There needs to be a substantial difference to warrant the upgrade, if not, then I'll stay with the 8" (200P) f/6 (f/5.91) Skyliner for a lot longer.

[John]

Personally I feel that cool down is more about equalising the air temperature in the tube and allowing for air currents to dissipate than allowing the glass of the primary to cool and achieve a slightly better figure although the latter does happen of course.

I find I can use my 12" Orion Optics dob for viewing DSO's at low / medium magnification within 15mins or so after being put outside. I wait for 40-50 minutes to pass before presssing on higher powers for the Moon and planets. The scope does have a fan behind the primary but I don't use it. I've no idea what glass is used for the primary.

I seem to recall the 8" and 10" scopes that I used to have behaved in a similar manner.

With Skywatcher scopes, I thought it was only the 10" that uses Pyrex ?

[todd8137]

Not sure but my sixteen goes out for at least two hours in the obsy before I even think about it I bet it's ready in a hour plus but I always leave it that long ,not sure why you think a fan is a bad idea I had one on my 12 solid tube skywatcher a few year ago and it did the job and it kept tne dew at bay

A astro colander ?does it have holes drilled in the side's Am sure a extra 4" would make a difference the bst eps might show a few elongated stars at the edges but am sure you could live with it

With that said my next plan is for a bigger mirror not sure when big is going to be big enough ,good luck with you choice

Pat

[Tinker1947]

I think its a waste of viewing time waiting for a scope to cool, set it up, collimate, get to viewing, as it cools all that's needed is a slight focus adjustment, of course if you have a obby of sorts then there cool already.....

[Charic]

John....the timings are just personal. If I want to view the Moon, In with the panaview, no cooling required, and as the system cools, I go lower with the focal lengths. problem is I use ALL my EPs, so to get the best, the scope needs to be just right. Skywatcher site says the Dob 12" Traditional is a Pyrex mirror.

todd8137.....I've considered a rear fan, but not sure there is enough space to allow for a decent amount of flow to pass the mirror? An LED light does't allow much light pollution around the mirror cell when viewed from the secondary, so I suppose airflow would have the same restriction. In fact, I'd go as far as saying a small 9V powered  rear mounted PC fan would just bounce the air back off the surface of the rear mirror, only trying to cool the rear face.  Its the surface layers that need to be dissipated, and cross-flow fans with entry-exit holes around the OTA would help. Ive also considered a flexible pipe installed, maybe silicon, drilled at an angle to give some sort of circular motion to the airflow around the perimeter of the mirror cell, in an attempt to dissipate the surface currents. I also cool my scope vertically, with the 2" aperture cap removed from the OTA dust cap, incase a rain cloud appears, I do live in Scotland!  warm air rises quicker if straight up rather than crawling up the sides, but then only leaving a 2" aperture exit can cause a restriction?

Tinker1947....I put the scope  out just as its getting dark, for possible use later, so no wasted time for me!  Sometimes the scope just comes back in, depending if anything else has distracted me for the evening. The s cope has sat outside all night on one occasion? But as I mentioned above. I can go out using the 32mm almost straight away.

[John]

John....the timings are just personal. If I want to view the Moon, In with the panaview, no cooling required, and as the system cools, I go lower with the focal lengths. problem is I use ALL my EPs, so to get the best, the scope needs to be just right. Skywatcher site says the Dob 12" Traditional is a Pyrex mirror.

I find the same - higher power needs a better cooled scope.

The 10" Skywatchers do use Pyrex primary mirrors, hence the "PX" in their title. I'm really not sure that the 12" ones do though, from looking at the specs on the OVL website and the worldwide Skywatcher website. Not that I think its a big deal.

Dealing with the surface layer of warm air just above the surface of the primary is an interesting issue and a number of solutions have been used. Fans blowing across the mirror surface are one and a rear mounted fan with a baffle just above the surface of the primary is another.

[Charic]

I noted the Pyrex here http://www.skywatcher.com/product.php?cat=6&id=250

I've read about the fans, hence my comment about  my Colander scope? but hesitant to drill some holes. Funny though, I'll wash my mirror with 4 system windolene as a project tester, but won't drill a hole?

I could imagine a collapsible Dob would be less of a problem with surface currents, as the mirror is more exposed! so a collapsible could be another solution to the issue?

[John]

I noted the Pyrex here http://www.skywatcher.com/product.php?cat=6&id=250

The solid tube is out of production now, at least here in the UK. Shame becuase I've had some great views with them. The 12" Flextube specs don't say Pyrex although that could be an oversight I guess.

Truss designs can let heat in as well as out - I'm thinking of body heat getting into the optical train through the struts here. You stand right beside the tube with a dobsonian.

The Flextubes seem to work pretty well though, from the ones I've looked though.

[bingevader]

Storing the 'scope outside somewhere to begin with would alleviate a greater degree of the problem.

Mine lives in the shed.