Basic Dobsonian questions

I'm considering getting a 6 or 8 inch Dobsonian. They seem to offer a lot of aperture for the money. But, I've got a few questions:
1) What kind of "cool down" time can I expect from a 6-inch and an 8-inch scope going from a warm apartment (72-degrees) to 30-degree winter weather? What about summer viewing going from a 74-degree apartment to 85+ degree summer heat?

2) How often do you have to collimate the scope?

3) How long does it take to set up your dob at an observing site?

4) How much bulkier is an 8-inch scope than a 6-inch scope?

Thanks. I'm going to go to work and I'm gonna try not to look for answers until I get home. I'm gonna be good.

I'll try and answer a couple of these...

#1 I have no idea...the temperatures here in Sydney, Australia don't vary that much (I don't have air conditioning)

#2...I check collimation after every set up and before viewing...with some practice it becomes fast and simple...

#3 No joke for my tube style dob it takes just a few seconds...with my truss dob it takes a few minutes...

#4 bulkier is a very subjective measure...???...I would not expect there to be a huge difference...I keep my 8 inch on a handcart (strapped on with bungee cords) see #3...I wheel it out of the house, slide it off the handcart and viola...

Cheers!
Scott

Lee:

Here's the way I look at it:

1. Cool down: It will take some time for any scope to cool down from room temperature to 30 degrees. With a fan, it will probably take 30 minutes to provide reasonable views, an hour to provide great views. Normally I set up my scope and start the fan an hour before I plan to start if possible. Obviously a 6 incher will cool faster.

If the scope has not cooled down, it can still be used, it just will not provide the same high magnification views it would if it were cooled and the seeing great... Refractors cool the most quickly, in general it's a non-issue. Newtonians cool faster than CATs (MAKs and SCTs) because they are open but I think they suffer more from tube currents while cooling.

2. Collimation: How often I collimate depends on the scope, the focal ratio and my previous experiences. For example I collimate my 12.5 inch F4.06 Discovery every time I use it because it is so sensitive. Normally it does not need anything. On the other end, I rarely collimate one of my 6 inch F/8s because they are tolerant and stay in collimation well. My 10 inch F5 GSO DOB stays in collimation for months at a time so while I should check the collimation, I often don't. The last time I used it, I took it to 600x to split a very tight double and I hadn't checked the collimation. It was fine...

Of course, with experience, one can tell by the images.

3. How long does it take to setup...

Well, no doubt about it, tube DOBs are the quickest setup there is. More imporantly, when you are tired and cold, they are the fastest tear down as well.

Last year, I decided to time myself setting up my 12.5 inch Discovery. I was driving my 1992 Ford Escort so it wasn't the ideal situation. Still..

I pulled up to the site, turned off the motor and started my stop watch. I pulled the trunk release, opened the door and went for it.

I first removed the base, set it where I wanted it. Pulled out the OTA, set it on the base. Pulled out the chair, the finders and installed them. I removed the covers.

At that point the scope was setup. I hit the button on the stop watch...

Total time: 41 seconds. It took me longer to setup my 70mm refractor.

4. Bulkier: As far as transportabiity, portability, the difference is small. Typically an 6 inch F/8 and an 8 inch F/6 are about the same length, the bases weigh about the same, the only difference is the diameter of the tube, this only makes a small difference because it is the length that is most important.

My older 6 inch F/8 Starhopper is actually longer than my 10 inch F/5 GSO so the 10 inch fits across the backseat more easily than the 6 inch.

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My thinking: Both these are good choices, the provide nice views of planets, they have enough aperture to show many DSOs, they are solid, inexpensive scopes that provide some amazing views. If money is a big issue, then the 6 incher is a good choice but otherwise, I think in the long run, the added aperture of the 8 incher gives it enough more capability that in the long run, it is the one to choose...

Jon

Lee,

I is interesting that you ask these questions. THey are certainly the types of things telescope owners like to discuss on a cloudy night. THerefore, I think, that they tend to interest beginners. However, I am not sure that I give much thought to them any more as a practical astronomer with some experience. In short, they just aren't anything you have to worry about. A six to eight inch Dob cools to usefulness quick enough. Probably as quick as anything except a refractor. Collimation takes a minute or two once you learn how to do it. (It takes a whole lot longer to read the directions on how to do it than it does to actually do it--once you learn it. It takes a bit to learn it, though.) Either the eight or the six are on the same scale of bulk. The six is a bit easier, but either fits in a car pretty easily, and neither takes up all that much closet space. And neither takes much more than plopping down and sticking an eyepiece in it. (Collimation two minutes, aligning the finder, 30 seconds..... Eyepiece, fifteen seconds. Ready to view.)

>> I'm going to go to work and I'm gonna try not to look for answers until I get home. I'm gonna be good.<<

And, by the way, the computer at work is for work only--not for checking out Astromart.

Alex

>>>All things considered my smallest telescope is a c8 that I picked up used for $300. It came with pre-installed scrathces and dings and I pack it any old way with no particular precautions. It is coffee can sized and I can put it on the floor in front of a car seat.
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Just a note to first time SCT owners:

As an ex-C8 owner, I took great care in how I packed the scope because of the vulnerability of the corrector plate. These are rather thin and can/do break. When the corrector plate is broken, one cannot simply replace it. The only option seems to be to send it to the factory where they will replace all the optics as they are "matched." This is not cheap and normally costs enough that people don't do it.

I imagine that Greg takes reasonable care when packing his SCT to protect the corrector. I keep my C-5 in a camera case, I don't count on the plastic cover alone to protect the corrector, whacking a DOB on something might damage the tube, whacking the corrector end of an SCT can be real trouble.

With larger SCT's I see many owners with cases designed for transport.

Just something to be aware of...

jon

Thanks for the quick feedback. There are a lot of factors to balance in considering another telescope purchase. This week, I've gone from GOTO to push to and from tabletop to super-sized and back about four times.