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Sweet spot of cool-down time and aperture?

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#1 Michael Rapp

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Posted 18 July 2013 - 02:07 PM

Okay, before I inadvertently end up asking a "what's the best scope?" question (:lol:), let me set some parameters.

I've recently, to my great surprise, been bitten by the lunar observing bug. So, I am looking for a scope optimized for lunar observation. However, my observing time seems to be exceptionally limited therefore cool-down time of the optics is very important.

Of course, I know that resolution scales with aperture. However, the larger the scope, the larger the optics, and the longer it takes to acclimate (not to mention the rapid decrease in portability).

The two scopes I use for grab-and-go sessions lasting less than an hour are my Williams Optics 66SD refractor and my ETX-125 Mak. I use an LXD-75 for the mount.

With the 66, it is difficult to get high magnifications and I start hitting the resolution limit of the scope. The ETX, on the other hand has a lovely 1900 mm focal length, but to me, seems to take quite a bit of time to acclimate to the outside temperature.

Would a 100 mm refractor acclimate faster than my 5" Mak by an appreciable amount? The Mak has two optical surfaces to stabilize (mirror and corrector plate) whereas the refractor just has the elements of its objective lens.

So to recap.... I want to look at the moon, with observing sessions less than an hour, with a typical temperature delta from inside to outside of about 10-15 degrees....

...what is your suggestion for the sweet spot of resolution and cool-down time? A new scope, or is one of my existing scope already the best match I would reasonably expect?

#2 csrlice12

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Posted 18 July 2013 - 02:58 PM

a basic 4" refractor would probably cool pretty quickly and throw up acceptable views till it does cool. The 102mm f9.8 is a fantastic planetary, and are reakily available and very affordable and come either separately or on a whole plethora of packages. The 4" f9.8 is probably the closest scope there is to being an "all round" scope...

#3 Pinbout

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Posted 18 July 2013 - 03:43 PM

The 4" f9.8 is probably the closest scope there is to being an "all round" scope...



that's how I feel about my antique tv genesis sdf.

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and being used was not that expensive for how well its figured.

#4 auriga

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Posted 18 July 2013 - 05:24 PM

Okay, before I inadvertently end up asking a "what's the best scope?" question (:lol:), let me set some parameters.

I've recently, to my great surprise, been bitten by the lunar observing bug. So, I am looking for a scope optimized for lunar observation. However, my observing time seems to be exceptionally limited therefore cool-down time of the optics is very important.

Of course, I know that resolution scales with aperture. However, the larger the scope, the larger the optics, and the longer it takes to acclimate (not to mention the rapid decrease in portability).

The two scopes I use for grab-and-go sessions lasting less than an hour are my Williams Optics 66SD refractor and my ETX-125 Mak. I use an LXD-75 for the mount.

With the 66, it is difficult to get high magnifications and I start hitting the resolution limit of the scope. The ETX, on the other hand has a lovely 1900 mm focal length, but to me, seems to take quite a bit of time to acclimate to the outside temperature.

Would a 100 mm refractor acclimate faster than my 5" Mak by an appreciable amount? The Mak has two optical surfaces to stabilize (mirror and corrector plate) whereas the refractor just has the elements of its objective lens.

So to recap.... I want to look at the moon, with observing sessions less than an hour, with a typical temperature delta from inside to outside of about 10-15 degrees....

...what is your suggestion for the sweet spot of resolution and cool-down time? A new scope, or is one of my existing scope already the best match I would reasonably expect?



Hi,
In your place I would consider the Lymax cooler. Much cheaper than a new scope. Call them and ask if it works with a scope as small as a 125 Mak.
http://www.lymax.com/sct/order.php

Bill

#5 auriga

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Posted 18 July 2013 - 06:13 PM

Why not try a Lymax cooler, it's cheaper than a new scope.
http://www.lymax.com/sct/order.php

Bill

#6 Jon Isaacs

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Posted 18 July 2013 - 06:56 PM

Would a 100 mm refractor acclimate faster than my 5" Mak by an appreciable amount? The Mak has two optical surfaces to stabilize (mirror and corrector plate) whereas the refractor just has the elements of its objective lens.



There is no doubt that refractors stabilize more quickly and are less affected by thermal issues when they are cooling than other types of scopes. There are a number of reasons why this is so but what is important here is that if one takes a reasonable sized refractor outside and begins observing immediately, you will be getting good views immediately with no need to wait to begin using high magnifications.

A cooler can help speed up the cool down of a Mak or SCT but unlike a Newtonian, it's not possible to observe while the scope is cooling.

Jon

#7 tomharri

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Posted 18 July 2013 - 07:33 PM

8" is the perfect size. My f/5 dob with a quality mirror (let me see your papers) is able to perform like a chinese refractor, still large enough for clusters and nebula, compact enough to be easily handled, thin mirror cools in 15-20 minutes, and cheap enough to be under appreciated.

#8 coopman

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Posted 18 July 2013 - 08:26 PM

If you take an 8" Newt out and let it cool down for 20 min. or so before you start cranking up the power it should perform great. You can do some low power, wide field scanning while it cools down. A 100mm ED doublet refractor is pretty much going to perform great immediately after setting the tripod down on the ground, but you'd be giving up a lot of light gathering & resolution capability to get that instant high power capability.

#9 Jon Isaacs

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Posted 18 July 2013 - 08:45 PM

If you take an 8" Newt out and let it cool down for 20 min. or so before you start cranking up the power it should perform great.



That has not been my experience. An 8 inch Newtonian is capable of operating at 300x-400x in excellent seeing and beyond that when viewing close doubles. Even in the mild climate here in coastal San Diego, to really be rock solid thermally, it takes much longer than 20 minutes even with a fan cooling the mirror.

Jon

#10 stevew

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Posted 18 July 2013 - 09:41 PM

Would a 100 mm refractor acclimate faster than my 5" Mak by an appreciable amount? The Mak has two optical surfaces to stabilize (mirror and corrector plate) whereas the refractor just has the elements of its objective lens.

Yes, a 100 mm refractor should acclimate somewhat faster than a 125mm Maksutov.
You also have to remember that when using your Maksutov the light passes through the scope three times before reaching your diagonal, so any heat plumes or unstable cooling air will interfere with the image more than the refractor in which the light only has to pass through once.

Steve

#11 EFT

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Posted 19 July 2013 - 09:30 AM

Cool-down questions are always problematic. The problem is that the "sweet spot" that you are looking for will vary from place to place due to environmental conditions so I think that is fairly hard to answer. In relative terms, a refactor will probably cool faster than an SCT which will in turn cool faster than a MAK. However, the chuck of glass in a triplet refractor is not necessarily fast cooling either. Newts on the other hand are largely dependent on the thickness of the primary mirror. Factoring in active cooling on either the SCT, MAK or Newt can substantially change the speed of cool-down and stabilization, but the relative speeds between the three will remain the same. The active cooled SCT or Newt may stabilize faster than the refractor though. MAKs tend to be slow overall because of the two thick pieces of glass that none of the others have.

If you like the size and long focal length of the MAK you could move to a 6" SCT and probably get faster cooling. The C8 EdgeHD might also cool faster than the MAK and still be possible on the LXD75 mount. The advantage of the EdgeHD is that it is vented, unlike any other SCT at this time. There are also a variety of MAKs and MAK-Newts available with active cooling (mostly from Intes) that will certainly cool faster than a non-vented MAK.

For wider field views you could certainly move to a 90mm or maybe 100mm refractor, but the cool down times will naturally increase compared to your 66mm. Sticking with a doublet achromat will help things but will usually give you pretty obvious color fringing on the moon. A triplet APO will usually provide better color but will take longer to cool.

Unfortunately, the sweet spot will depend a lot on where you are setup, so comparisons from others in the same area will be the most useful. If you want to push any of the scopes to high magnification right away, you are likely to have trouble. If you don't mind starting off at lower magnifications and working your way up during your observing session, you might be better satisfied.

If you want to keep a long focal length instrument, then I would recommend switching to an SCT (preferably active cool or at least vented) or an actively cooled MAK. If you are looking for a more multipurpose scope, then a larger refactor might be better, but could result in greater cool-down times than you want.

#12 EFT

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Posted 19 July 2013 - 09:36 AM

A cooler can help speed up the cool down of a Mak or SCT but unlike a Newtonian, it's not possible to observe while the scope is cooling.

Jon


There are actively cooled MAKs and MAK-Newts as well as SCTs that can be actively cooled while using the scope. But if you use rear-cell coolers (or Fastar hole coolers for Celestron SCTs), then you are limited to cooling only while not using the scope, unlike the active cooled Newt that uses low-vibration fans.

#13 Michael Rapp

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Posted 19 July 2013 - 09:53 AM

Thanks everyone for the replies, especially your detailed one, Ed.

Much to think about.....and, indeed, more evidence that there is no free lunch when it comes to telescopes. :)

I was able to do some lunar observing last night, and actually was able to spend nearly two hours doing it. The Mak took about an hour to cool down to where 150x was usable to my eyes.

But wow, when setting up and taking down (more on taking down), that Mak is so nice and compact (compared to a long-focus refractor) and I don't have to worry about smacking the tube against the back door's frame when I take it in and out while looking for the cats trying to sneak outside.

Ah, no free lunch.

#14 SteveG

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Posted 19 July 2013 - 11:54 AM

My light-weight ED100 cools nicely in my Pac NW climate after only about 15 min. My 10" reflector requires about 1.5 hours on the same night.

#15 Jon Isaacs

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Posted 19 July 2013 - 01:57 PM

In relative terms, a refactor will probably cool faster than an SCT which will in turn cool faster than a MAK. However, the chuck of glass in a triplet refractor is not necessarily fast cooling either.


There are two factors to consider, the time it takes to cool and the effect on the image of a scope that has not cooled. Refractors seem to be superior to reflectors of all types in both.

A refractor that is taken outside and subject to a significant thermal change will still perform quite well without a wait to cool, it will resolve binary stars very near it's limit without having had time to cool down. Part of the reason is that the change in figure is less important because the effect of a surface error is multiplied the index of refraction -1, generally about 0.5 where as the surface errors of a mirror are multiplied by a factor of two, the wavefront error is more greatly affected in a reflector.

Another reason is that in a refractor there is a single converging light cone and it spends very little time near the tube where localized tube currents disturb the image. With scopes with mirrors, the majority of the light first passes down the tube close to the tube wall where the convection currents do cause serious disturbances.

But whatever the reasons, if you want a scope that is capable of providing top notch images without waiting for it to thermally equilibrate, a refractor is really the only choice.

Jon

#16 Widespread

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Posted 19 July 2013 - 02:47 PM

Dang, Jon. That is good stuff about the reflector light path passing nearby the thermally unstable tube walls. Makes sense!

Now I just need to figure out all that other stuff you were talking about, like surface and wavefront errors.

Best,
David

#17 Michael Rapp

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Posted 20 July 2013 - 12:03 AM

Jon, that does indeed make a great deal of sense. Thank you for posting that explanation.

#18 Ed Wiley

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Posted 20 July 2013 - 12:49 PM

I fitted my C11 Edge with Deep Space Products fans. When the sun gets towards the horizon I open the observatory, turn on the rear fans and by the time it gets dark I am ready to go. I also bought the front unit since it was a good deal to buy the entire package. Also, I have done high resolution work when I forgot to turn off the fans -- no apparent effect on images.

Interestingly: How do the pros do this with really large telescopes? On a visit to McDonald Observatory I was told that they cool the entire observatory to the prediction of the temperature expected that night. I would hate to see their utility bill in August. :shocked:

Ed

#19 KerryR

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Posted 20 July 2013 - 01:18 PM

My personal 'solultion' to sweet-spot aperture:cooldown is a DGM OA-4 off-axis newtonian, mine's on a dob mount.

At 98mm clear aperture, there's very minimal distortion to the glass as it cools (confirmed by star testing). The tube is comparatively large, and the cell very open, so the glass cools quickly. Because the light cone of the design hugs one side of the tube, it's an easy matter to turn the ota so that the ep sticks out the side 90deg: This puts the light cone to the side, where it is largely unaffected by tube currents arising from the warm mirror and/or rapidly colling top of the ota.

The figure is first rate, as nearly perfect a star test as I've ever seen, and completely chromaticism free. Coma is about 1/3 that of what would be viewed in the parent mirror, which really amounts to very little with a 32mm ep, and nothing at high powers. Collimation is slightly trickier because it has to be finalized by star testing, but it's pretty easy after a bit of practice. The moon and planets are typically high enough that the normally uncomfortable (on small ota's) 90deg ep angle is comfortable.

I've found it provides high power sharp images much, much sooner than my ED100. My ED100 is often sharp right out of the gate, but, as the top of the tube cools due to it's exposure to the sky, tube currents often begin to creep in, and it can be quite a while before everything settles, if it does at all. The OA, on the other hand, when used as described above, presents far less thermal issues at any point.

One more point worth mentioning: it's always extremely helpful to observe on grass instead of a driveway-- far less heat rising off the grass and up the rear of the tube. Just mentioning it because it actually took me quite a while to abandon the convenience of my driveway, but it made a huge difference with the OA (or any open cell newt).

The down side is a comparative lack of aperture, but this is a necessary evil when cooldown time vs. high power is the primary concern.

#20 GeneT

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Posted 20 July 2013 - 06:41 PM

I have a low tech solution that works great for me. The sun is in my backyard in the afternoon, and evening. My front porch is on the opposite side of the house, and shielded from the sun. Two hours before I plan to view, I put my telescope on the front porch. When I load it in the vehicle to leave for viewing, the optics have reached ambient temperature. Maybe you could do something similar. When you get home from work, put the telescope outside, out of the sun, have dinner and so on, then go out and view.






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