121 replies to this topic

[Deep13]

In my mind, the ideal planet telescope is a 10 or 12" EQ Newt (split ring?) in a permanent location with a clear view of the south and overhead. Add a good binoviewer, pairs of long ZAOs, and an easy way to reach the EP, and I'd be all set. In reality, it would be too expensive and I have no place to set it up permanently. So-o-o-o, I've arranged to buy a used 8" f/8 EQ-mounted Newt. I'll need to have some servicing done on the mirrors. I'm thinking that within the realm of likely possibility, this may very well be my ideal set-up. Right now it has no fan and a tall R&P focuser, so I may change those things. And I'll built a cart for the Meade RG mount. I already have a tall adjustable chair and a Denk II with pairs of TV Ploessls.

 

Any thoughts? What's your ideal planet scope?

[RalphMeisterTigerMan]

Actually, I had had the good fortune to view through 2 separate 8-inch f/8 reflectors. Each one had optics ground and polished by the owner. Unbelievable! These guys did an amazing on their respective mirrors. Jupiter at the Mount Kobau Star Party in Aug. 1985 I will never forget!

Neither will I forget the Oct. 1988 opposition of Mars through Lance Oklevic's 8-inch f/8 self fabricated newtonian reflector. It was a Sat. night, I believe, and Terence Dickinson gave a Mars lecture in the auditorium at the H.R. MacMillan planetarium. Afterwards many amateurs set up their scopes on the large concrete entrance to the Gordon Southam Observatory. A member's AP 6-inch f/8 Apo was also pointed towards Mars.

 

So yes, an 8-inch f/8 can make an ideal planet killer!

 

Clear Skies!

RalphMeisterTigerMan

[scngc7317]

I think I just put together the "ideal planet killer" but I haven't tried it yet. (too big to get out the front door)

 

It's a 6 inch F-15 refractor, the best part is because of the Looong tube you are closer to the planet !!!   

 

[Deep13]

Those long refractors are really nice, just big for their size for storage and mounting purposes.

[Smug]

My ideal planetary scope would be the biggest, longest newtonian I could afford - biggest aperture, smallest central obstruction.

 

In practice I have a 12" F/5 on an NEQ6 which I occasionally use for visual, it's great but also a giant pain in the **** - the EP is 2' above my head in most positions and I don't like standing on wobbly ladders in the dark, and it's a bit awkward to mount solo. Once it's set up I prefer it to my 14" dobsonian since I think it gives better views and the computerized mount is a million times better than tracking by hand, but the dob gets more use because it's much less hassle.

[stubeeef]

A 100”+  refractor north of f/50 mounted on a dry dessert plateau 

Edited by stubeeef, 14 December 2018 - 09:39 AM.

[mic1970]

I think I just put together the "ideal planet killer" but I haven't tried it yet. (too big to get out the front door)

 

It's a 6 inch F-15 refractor, the best part is because of the Looong tube you are closer to the planet !!! 

 

Man... would I like to look through that bad boy.  Did you post a build thread on it?  What is the finder scope?

[Deep13]

A 100”+ refractor north of f/50 mounted on a dry dessert plateau

You know the biggest refractor on earth is only 40".

[starcanoe]

There look to be 2 wonderful 8 inch f8 mirrors for sale at a modest price in the classifieds even as we speak....ultimate modest sized planetary scopes..and still just small enough to be grab and go dobs....for dso and all but WIDE field work to boot.

 

If I wasn't just now finishing a 10 inch f8 mirror of epic specs and a 6 inch f15 refractor (well that and hurting for cash) I'd be all over those babies....

[mic1970]

What would a 4" or 6" F15 Dob look like?  Would it be 8ft tall?  Would you get the same contrast as a F15 refactor?

[stubeeef]

You know the biggest refractor on earth is only 40".

Just not ideal

[Auburn80]

What would a 4" or 6" F15 Dob look like? Would it be 8ft tall? Would you get the same contrast as a F15 refactor?

It would look like a refractor with the focuser at the wrong end. 🤣

Not sure if you could find a small enough diagonal to be of benefit. A 6" f10 (1" diagonal?) or 8" f8" would probably be the ideal "small" dob.

Edited by Auburn80, 14 December 2018 - 09:50 AM.

[Jon Isaacs]

I owned a Meade 12.5 inch F/6 RG for a number of years.  Honestly, for large Newtonians, GEMs are a pain in the rear.  

 

My thinking: 

 

- In getting good planetary views, seeing is the number one priority.  It all starts with the seeing.  One wants a scope of sufficient aperture than on a good night, it is not limited by it's aperture.  

 

- In terms of planetary contrast and detail, aperture is more important than focal ratio.  This is particularly true for scope on a tracking mount.  Pick a focal length that is ergonomically acceptable, pick the largest aperture that is affordable.  

 

-  Important are high quality optics.  

 

- Thermal management is critical.  

 

- The mechanical structure must be stable and free from vibration.

 

- A small secondary is of some consideration but a little bigger than the minimum means alignment is easier, the illuminated circle is larger and any edge issues with the secondary are of less significance.

 

My solutions:

 

My best views of Jupiter and Saturn were with "Junior", my 25 inch F/5 Obsession.  The seeing in the high desert where Junior lived was rarely more than average so such views were few and far between.  Our home in San Diego is often blessed with very good seeing and at time excellent seeing, under an arc-second is relatively common.  A scope that big is not practical. For my backyard, this is what I consider my best planetary scope.

 

-  13.1 inch F/5.5 Starsplitter with a Robert Royce mirror.  It is a robust scope for a 13.1 inch, it's heavy but stable, the secondary is right at 20% and it has Feathertouch focuser.  It has enough aperture for the really good nights while still being ergonomically comfortable.

 

- Tracking:  I parted ways with 12.5 inch Meade RG when I acquired the Starsplitter.  The Starsplitter was a package deal which included a Tom O, dual axis aluminum Equatorial platform.  For visual observation, I think EQ platforms are superior to GEMs. A good one is rock solid and retains the superior ergonomics of the Dobsonian.  The mount is rated for a 16-18 inch and yet weighs less than 30 pounds and can be carried in one hand.  Truthfully though, I actually prefer manual tracking, I like the intimacy and the issues with nudging don't arise until well past 400x., 

 

 

My backup planetary scope is my 10 inch GSO Dob. I've had it for 15 years, it has good optics and it's a quicker setup.  I had an Orion 120mm Eon ED/apo for a couple of years but I found the 10 inch Dob was enough better on the planets and double stars that the Eon just sat in it's case so I sold it.  The 10 inch on the dual axis EQ platform.

 

 

I think that matching the scope to your local conditions is important.  I am about 4 miles from the Pacific Ocean and generally south of the jet streams.  The flow off the ocean can mean very good seeing and so in general, I have good seeing enough of the time that I do not need to fight it because there will soon be another night.  

 

Jon

 

[Kent10]

An article by Roland Christen of Astro Physics

 

https://www.cloudyni...-telescope-r402

 

I've got the 7" APO with an ~8" APO on the way.  I view the planets a lot with the 7" and as Jon says in post #13, seeing is so important.  I get good views but only rarely do I get views to see what the 7" is capable of.  After years of viewing Jupiter, I finally could easily see detail inside the Great Red Spot.  So exciting.  Too bad it is so rare from my backyard.  I'll keep trying though.

Edited by Kent10, 14 December 2018 - 11:24 AM.

[Deep13]

One of those big premium Dobs with Servocat would be good. Too pricey and not portable for me.

[rowdy388]

I like how Jon uses helium balloons instead of counterweights to balance his dob!   

[havasman]

The best planetary view I ever had was via Allan's 32" f3.3 SDM/Lockwood/Pratt ultra-premium Dob from the Australian boonies when Jupiter was nearly overhead. I've never seen anything else like it as far as planetary goes.

 

But I'd like to give that 100" refractor a go too. Especially from Oz or maybe the Namib.

[dscarpa]

 I had this in mind when ordering my  11" F5 tube dob with Zambuto and SIPS. Wanted to go F6 but told it would have been unstable. My thinking was the tube dob would hold collimation better. At this point it's a work in progress. David

[Galicapernistein]

It would look like a refractor with the focuser at the wrong end.

Not sure if you could find a small enough diagonal to be of benefit. A 6" f10 (1" diagonal?) or 8" f8" would probably be the ideal "small" dob.

I have an 8” F9 reflector, and I agree that one f ratio less would have made the scope much more manageable.

[Jeff Morgan]

In my mind, the ideal planet telescope is a 10 or 12" EQ Newt (split ring?) in a permanent location with a clear view of the south and overhead. Add a good binoviewer, pairs of long ZAOs, and an easy way to reach the EP, and I'd be all set. In reality, it would be too expensive and I have no place to set it up permanently. So-o-o-o, I've arranged to buy a used 8" f/8 EQ-mounted Newt. I'll need to have some servicing done on the mirrors. I'm thinking that within the realm of likely possibility, this may very well be my ideal set-up. Right now it has no fan and a tall R&P focuser, so I may change those things. And I'll built a cart for the Meade RG mount. I already have a tall adjustable chair and a Denk II with pairs of TV Ploessls.

 

Any thoughts? What's your ideal planet scope?

 

While refractors have come and gone in my collection I have always had Newtonian.

 

So it pains me to say this, but the ideal planet scope is a refractor.

 

Newts can be very very good ... but never ideal. Taking 7% of the signal (Airy Disk) and dumping into noise (diffraction rings) by adding an obstruction just can not be undone. And that is before the issues of scatter and thermal performance.

 

But, playing along with the topic at hand ...

 

Permanent set-up is good. From there you go straight to accessories (and that's ok), but let's stay on the scope itself for a moment. I would suggest these additional features:

 

• Solid Tube with minimum 1.5" air gap between tube wall and mirror, regardless of what that implies for secondary mirror size.
• Wire spider, minimum thermal effects, diffraction is least, and spikes minimal (though perhaps more noticeable than curved spiders).
• Push-pull locking collimation on primary mirror cell - f/8 has a wide collimation envelope, but why explore the edges?
• Tracking mount.
• Cooling fan(s) of course. Scrubber fan if mechanically feasible.
• Rear baffle against ground reflections back up the tube.
• Nose extension (stray light concerns again), especially for a low-profile focuser.
• Ground surrounding the mount (or dome) is grass covered. Else, light colored materials.
• If the scope is housed in an observatory, air conditioning that can be activated six hours before intended use.

Getting back to accessories, the one I would add for sure is an autocollimator. I really like the CatsEye with dual pupils.

[CDallas32]

A 100”+ refractor north of f/50 mounted on a dry dessert plateau

Imagine how long that would be lol

Edited by CDallas32, 14 December 2018 - 06:17 PM.

[Jon Isaacs]

So it pains me to say this, but the ideal planet scope is a refractor.

 

Newts can be very very good ... but never ideal. Taking 7% of the signal (Airy Disk) and dumping into noise (diffraction rings) by adding an obstruction just can not be undone. And that is before the issues of scatter and thermal performance.

 

What you say is true if you are comparing a refractor with a Newtonian of the same aperture.  The added energy in the diffraction rings does reduce the contrast, not a lot but some.  This is an experiment one can perform, I have done it.  Just take that refractor and add 20% CO.  The effect is surprisingly small. Even a 40% CO is surprisingly small.

 

However, one is rarely comparing a refractor to an equal aperture Newtonian and the diffraction effects of a smaller aperture of the refractor can easily have a much larger effect on contrast than a central obstruction, particular a central obstruction of 20% or so.  Conceptually, one can think of contrast loss as a function of encircled energy.  Perfect contrast means a point on the object is mapped to a point on image.  But this is not what happens.  The point on the object is mapped to an Airy disk pattern in the image.  If one compares say a 12 inch Newtonian to a 6 inch refractor, few own refractors larger than 6 inches, the Airy disk of the Newtonian is 1/2 the size of the refractor's.  This means the energy the Newtonian is much more concentrated and has greater contrast.

 

 Again it comes down to a question of seeing and matching the scope to the seeing.  The diameter of the Airy disk to the first minima of a 6 inch scope is 1.8"  Sometimes people point to the Dawes limit or the Rayleigh criterion as a measure of the resolving power of a scope.  These involve overlapping airy disks and are applicable to double stars.  I personally think the diameter of the Airy disk is a reasonable measure for the relationship between seeing and aperture.  It is similar to using the pixel size to measure the resolution of a screen. 

 

Something to think about anyway.

 

Jon

[dgoldb]

I'd say something like a 30" f/3.5 with a binoviewer and tracking with a permanent setup somewhere dry and high.   In theory yes a refractor is ideal but in reality, they don't scale up.  And the aperture difference between an 8" refractor and a 30" dob makes it no contest.  

 

If I was forced to observe from home, though, I might consider an APO because my skies rarely let me go over 250x.  

[stubeeef]

Imagine how long that would be lol

and focusing using a 1.25" ep!

 

Well we are taking IDEAL not affordable or practical, if you think the 100" is a bit nutz, think of the building and mound underneath just to mount it!!  But hey you can see the Wal-Mart on Titan I bet.

[barbie]

The IDEAL planet scope is one that can be easily managed and used often enough to justify having it in the first place!!

FOR ME, it is my trusty 6" F8 dob and 4" F9 apo refractor.