Hello, and Scope Advice

[anomaly]

First, hello!
I'm new here and new to astronomy as a hobby. I see alot of good advice in this forum from alot of nice helpful people. Nice to meet you all.

The factors for me are:
I will be most interested in deep space objects.
Once a year I visit family in Idaho and would like to be able to take the scope on a plane as a "carry on". So 26" or less is ideal.
About $500 is my max budget.
Though I am not interested in astrophotog right now, I wouldn't be surprised if I were interested later.

Surfing around, I see that a 6" f/8 Newt is a highly recommended scope for novices, along with the 6 and 8" dobs.
Why is f/8 better than f/5 for a 6" Newt? For deep sky, would the f/5 have a noticable disadvantage for me?

Here are two 6" f/5 Newts that I am considering because they are not more than 27" long:
Orion Astroview 6 EQ (http://www.telescope.com/shopping/product/detailmain.jsp?itemID=374&itemType=PRODUCT&iMainCat=4&iSubCat=8&iProductID=374)
Celestron Omni XLT 150 (http://www.optcorp.com/product.aspx?pid=1-600-602-1122-9225)

And this refracter looks intriguing.
Orion AstroView 120ST EQ (http://www.telescope.com/shopping/product/detailmain.jsp?itemID=287&itemType=PRODUCT&iMainCat=4&iSubCat=13&iProductID=287)
It's built toward deep sky but the 4.7" diameter might not show deep objects good enough (?).

Your opinions of these scopes and advice is welcome.

About collimation- Since I am new, I have some concern. What would make collimation the easiest and fool proof?
Collimating Eyepiece (http://www.telescope.com/shopping/product/detailmain.jsp?itemID=12&itemType=PRODUCT&iMainCat=6&iSubCat=30&iProductID=12)
or
Lasermate Collimator (http://www.telescope.com/shopping/product/detailmain.jsp?itemID=106&itemType=PRODUCT&iMainCat=6&iSubCat=30&iProductID=106)

[redshifter]

A 'faster' focal ratio scope requires more exact collimation and have a more involved manufacturing process IIRC. A 6" f/8 dob is a great scope because you get an easy to manufacture f/8 mirror which will offer both low and higher magnifications pretty easily. The dob mount will also be inherently very stable, whereas a stable EQ mount will cost some $. You don't want a cheapy mount that won't settle down when observing. However, a dob likely wouldn't work as a carry on. What I'd suggest is a 8" f/6 dob and leave it at home, then get a short tube 80mm or something compact as a travelling scope.

If $500 is your max budget, I'd probably stick with the 8" dob and not worry about taking it on the plane (IMO there's just too many potentially bad things that can happen to a fragile instrument like a scope to even think about taking it on a plane). Also, getting into astrophotography is a substantial investment and can have a steep learning curve, so I'd stick with the visual astronomy for now and move on to astrophotography down the road should you still be interested.

[anomaly]

Will an f/5 have better quality optics and be noticable over an f/8? If the collimation needs to be more exact, is it more difficult to perform?

Thanks for your feedback.

[Dave Mitsky]

Fast mirrors are a bit more difficult to collimate. Nowadays there are a number of devices on the market that make collimation an easier process.

Generally speaking, the faster the mirror is, the more likely it is that its optics will be mediocre. (High quality fast mirrors are available from a few premium mirror makers such as Royce and Zambuto but they are rather expensive.) That is the price one pays for the convenience of transport and wider fields of fast Newtonians.

As redshifter said, a small refractor is much easier to transport on a plane than a solid tube Newtonian.

Dave Mitsky

[anomaly]

Thanks guys!

[ozark1]

Collimation apart, the fast mirrors (f/4, f/5) suffer from a phenomenon called coma. This causes stars to become elongated.

[redshifter]

I don't notice coma on my f4.7 scope execpt on very low power, and then it's a non issue for me, as the coma is only on the edge of the field.

[Dave Mitsky]

There's a good article on coma, which is dependent on both f/ratio and field position when it comes to parabolic mirrors, here (http://www.opticalmechanics.com/about_coma.htm).

Dave Mitsky

[ozark1]

BTW a 6" scope including mount weighs something like 15-20 kg. My 8" is 30 kg all up and stands nearly 5' high. You shouldn't even think of taking them on a plane. The only plane portable scopes would be the little ETX-90s (Mak-Cas).

I'd suggest buying 2 scopes - 1 for each location (A pair of XT6 dobs is $530) or an 8" XT8 dob and a pair of binoculars