Better DIY planetary telescope: 8 or 6

[ehilge]

better DIY planetary telescope: 8" or 6"

Hey all, I'm starting to design my first home built telescope. I already own a 10" dob which works great but I've decided I would like something more transportable that is easier to take camping or wherever else. So right now I'm leaning towards a 6 or 8" truss dob for simplicity and I'd like to gear it more for planetary and lunar observation as opposed to deep sky. I don't want to go any smaller as I want to retain a some deep sky ability. From what I've read, even though aperture is almost always most important, something with a slower focal ratio (f/10 as opposed to f/5 if I understand it right) is better for planetary of viewing. Using this logic, a 6" scope would be better because a larger f-ratio could be used for the same length of tube. With a desired focal length of around 40 inches I could probably get something around f/7 for a 6" or f/5 for an 8". But this wouldn't be much different than my 10" with is f/4.7. So, my question is essentially even though an 8" mirror has greater aperture, is the 6" with a larger f ratio going to be a better option for planetary viewing?
Thanks!

 

[Philosophaie]

I have a Sky-Watcher 130mm EQ2 reflector telescope with a 900mm or 36" Tube. It is a little cumbersome to lug around. I can see why you want to downsize.

Its specs are:
•130mm (5.1 inch) diameter primary mirror made from grade-A glass and multi-coated for optimal reflection
•900mm focal length, f/6.9
•20.4 sq. in. light grasp
•307x maximum theoretical magnification and 1.08 arc-sec resolving power
•High-quality super Plossl fully-coated 25mm and 12mm 1.25” eyepieces provide clear, sharp images at magnification of 36x and 75x with a 52° apparent field-of-view and great eye relief
•1.25” 2x Barlow lens doubles magnification of the included eyepieces to 72x and 150x for up-close planetary and lunar viewing

 

[Chronos]

I think a 6" f/8 is the sweet spot. An 8" dob has a lot more [~80%] light gathering power, but will be twice as heavy, and f/7 is pushing the limit to even pretend to be portable. Futhermore, f/7 is on the edge of being too fast for a good planetary scope. The 'good' planets are bright enough you will rarely miss the extra aperature. You will never miss the extra bulk.

 

[ehilge]

I think a 6" f/8 is the sweet spot. An 8" dob has a lot more [~80%] light gathering power, but will be twice as heavy, and f/7 is pushing the limit to even pretend to be portable. Futhermore, f/7 is on the edge of being too fast for a good planetary scope. The 'good' planets are bright enough you will rarely miss the extra aperature. You will never miss the extra bulk.

That's what I was leaning towards for largely the same reasons, thanks for your input.

 

[chroot]

I wouldn't use a Dob of any sort for planetary work. I'd just buy a short-focus refractor, preferably apochomatic. They're extremely portable, tack sharp, and will outperform almost any big light bucket with a homemade mirror cell.

- Warren

 

[turbo]

It can be difficult to find or build a Newtonian that can match the performance of modest-sized APOs. Small refractors can provide excellent planetary views. They can be figured quite well, and have no central obstruction, unlike SCTs, Newts, and classical Cassegrains. You give up aperture, but there are some great advantages.

Still, ATMs skilled enough to accurately figure long focal-length primaries and sharp enough to optimize secondary obstruction, etc have been building nice planetary scopes for years. It bears mentioning that it is easier to accurately figure a long f:ratio mirror than a faster one, and a decent planetary scope of this type is less demanding when it comes to selecting EPs that will perform well WRT coma and field-flatness. Not so for very fast light-buckets. Planetary Newts can be forgiving.

I have a 6" f:8 Astro-Physics APO and a Vernonscope APO as a finder/guidescope, so I voted with my wallet. Luckily, I ordered my 'scope before Roland Christen became famous for top-quality refractors, and his prices and wait-time were very reasonable.

Edit: The Vernonscope is an 80mm f:6.25. It is very compact, and with the 2" focuser and mirror diagonal and 1-1/4" adapter, it works really well with all of my EPs and Barlows. Planetary views are wonderful for such a small scope - surpassing those through my old Questar.

 

[ehilge]

I think I'll go with the 6" f/8 dob for the aforementioned reasons. I realize that a DIY dob won't be as nice of a planetary telescope as an APO refractor, but my initial motiviation for this project was that I wanted to build a telescope and everything else followed after that.
Thanks for your input!

 

[chemisttree]

Right now I'm building an 80 mm f15 achromat using a Carton doublet lens in a custom made cell. It's going to be my planet-killer. I don't think you need an APO for great planetary views. Long fl achros were doing it long before the exotic glasses became popular.

I guess we'll see...