121 replies to this topic

[Deep13]

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.

Refractor?! Heathen! Obviously the short comings of a refractor are costs and physical size. While you're right about the obstruction, the Newt lets me go to 8" for $700 including the mount. I have a custom 5" f/12, and while it's really sharp and contrasty, it looses brightness with magnification quickly.

Yup, I'll set it up on grass.

Yes, the tube is ventilated which is simply a must-have. Nothing will kill an image faster than thermals. Same with the fan.

I already have all those collimation tools for my 12.5" f/5 DOB and my 8" f/6 Dob. The 8" f/6 is a sonotube scope with 9" tube. I have some black foam weatherstripping on the mirror end as a baffle. It's visual images are astonishingly sharp.

I don't know how the collimation screws are, but I'm sure they work. Also the secondary vane are the typical flat variety. Yeah, wire would be better, but probably beyond my technical ability. I'm pretty sure the tube is long enough not to need an extension. It comes with a Meade RG mount, so it will have tracking.

I'm having Terry Ostahowski check the mirrors and recoat them,

[daquad]

Imagine how long that would be lol

Don't have to imagine it. 5000 inches or 417 feet, 1/1/3 football fields?  Still wouldn't meet the Sidgwick criterion, though.  

[Eddgie]

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?

First, I would not go with a split ring Newt for exactly the reason Jon mentioned.   If you wanted that option for tracking, just get a Go2 Dob or a tracking platform.   Split ring Newt can put eyepiece in just horrible location.  

 

A 10" f/6 sounds great, but to get the small secondary (5mm fully illuminated field) it will be no better than a 12" f/5 with the same size fully illuminated field.  In other words, you get a scope in the 10" f/6 that is as good for planets, but not as good for just about any other use (mirror quality being equal).   The 10" though would be lighter and easier to manage.  I have a 12" with a fine mirror that delivers outstanding planetary views, but it takes a hand truck to move it (though it is easier to move than my 6" Apo on a GEM mount was by many orders of magnitude!)

 

The major issue with either of these is that depending on your location, seeing may limit both of these to working at less than their full capability on both nights, and the 12" will suffer a bit more than the 10".

Adding boundary layer fans will up the weight of both, but since the weight of the 12" OTA is already pushing 50 lbs, adding fans is just that much more to handle.   The 10" could be kept under 40 lbs with fans and will be easier to boundary layer scrub (and with with boundary layer fans, cool down is going to be far less of an issue because boundary layer scrubbing means you don't need to cool the mirror.

 

I think there would be fewer occasions where the 12" would outperform the 10" if you live somewhere with poor seeing, but if you live somewhere with lots of excellent seeing, 12" will just be a better all around scope and unless the only use for the instrument is planetary, then the 12" to me seems to be the way to go.  While the OTA will be 50 lbs, this is still manageable by many people, and if it is not manageable by you, then I would think that we would not be having this conversation.   

 

A highly optimized 10" though would probably keep up, but only at the cost of loosing some of the all around capability of the 12".   Again, if only use is planetary, a highly optimized 10" would be hard to beat on a night of typical seeing for many.   But only hard.  Not impossible.  

[Jeff Morgan]

Refractor?! Heathen! Obviously the short comings of a refractor are costs and physical size. While you're right about the obstruction, the Newt lets me go to 8" for $700 including the mount. I have a custom 5" f/12, and while it's really sharp and contrasty, it looses brightness with magnification quickly.

Yup, I'll set it up on grass.

Yes, the tube is ventilated which is simply a must-have. Nothing will kill an image faster than thermals. Same with the fan.

I already have all those collimation tools for my 12.5" f/5 DOB and my 8" f/6 Dob. The 8" f/6 is a sonotube scope with 9" tube. I have some black foam weatherstripping on the mirror end as a baffle. It's visual images are astonishingly sharp.

I don't know how the collimation screws are, but I'm sure they work. Also the secondary vane are the typical flat variety. Yeah, wire would be better, but probably beyond my technical ability. I'm pretty sure the tube is long enough not to need an extension. It comes with a Meade RG mount, so it will have tracking.

I'm having Terry Ostahowski check the mirrors and recoat them,

 

I know, I know. Heathen. But you have to call them like you see them. My primary scope has always been a Newtonian (well, for the last quarter century anyways). Great optics too, including Royce and Zambuto. But the Newtonian just has too many inherent disadvantages to be called the ideal planetary telescope.

 

Not to say I discourage you from trying. Just the opposite really! A finely tuned scope is a joy to use.

[Deep13]

Eddgie,
I'm sure all that's true. I've had lousy luck with EQ Dob platforms. I had a Roundtable for a short time for my 12.5", but it stopped working and the maker went out of business.

Scope I've arranged to buy is 8" f/8 on a Meade RG mount. The seeing here is intermittent. When good, it supports quite a bit of magnification. When bad, well.... I find with my 5" f/12 refractor I run into aperture limitations--image is too dark for more magnification. The 8" f/6 Dob I already have and the 12.5" give really nice views, but no tracking.

[Jon Isaacs]

I know, I know. Heathen. But you have to call them like you see them. My primary scope has always been a Newtonian (well, for the last quarter century anyways). Great optics too, including Royce and Zambuto. But the Newtonian just has too many inherent disadvantages to be called the ideal planetary telescope.

 

Not to say I discourage you from trying. Just the opposite really! A finely tuned scope is a joy to use.

 

The Newtonian has some inherent advantages that make it the prime candidate for a planetary scope.  It is very simple, there are only two optical surfaces.  Those two components can be made essentially perfect . Large apertures are very doable so they do not suffer the limited resolution and fine scale contrast of smaller aperture scopes . Very small central obstructions are possible. 

 

The potential is there. As with any instrument , the challenge is in the execution and in the operation .

 

Jon

[Stephen Kennedy]

This is my planetary telescope.

 

 

It is a Mikage 210 mm F/7.7 Newtonian on a Pentax MS-5 GEM.  It gives great images of the planets through my 7mm Pentax XW EP

[Jon Isaacs]

This is my planetary telescope.

 

IMG_1168 (3).JPG

 

It is a Mikage 210 mm F/7.7 Newtonian on a Pentax MS-5 GEM.  It gives great images of the planets through my 7mm Pentax XW EP

 

Stephen:

 

I'm glad you posted the photo of your scope.  When I wrote:

 

"The potential is there. As with any instrument , the challenge is in the execution and in the operation ."

 

I had your scope in mind. 

 

Jon

[MitchAlsup]

The Ideal planetary scope is the 36" at Lick--now if one could just get rid of the light pollution from San Jose....

[Adun]

Ideal planet scope.

 

Maybe not ideal, but the best so far, is a"14-element all-refractive lens with a nominal focal length of 11 mm, F/3.2 with five front elements made of radiation-hardened glasses, and optics shielded by a thick titanium housing", fabricated by Rockwell-Collins Optronics. It has to be mounted on a spacecraft to really deliver, so that takes it outside the realm of likely possibility.

 

For mounting on Earth's surface I settle for a Mak

[Deep13]

This is my planetary telescope.

IMG_1168 (3).JPG

It is a Mikage 210 mm F/7.7 Newtonian on a Pentax MS-5 GEM. It gives great images of the planets through my 7mm Pentax XW EP

Really nice. Have you been to Cherry Springs, PA? I feel like I've seen it before.

[Jeff Morgan]

One other thought came to mind for your Performance Tuning Checklist: Turned Down Edge of the primary mirror. This would be a disaster for planetary performance.

 

If you don't have access to a Focault tester, you can make a simple aperture mask for an observational test. If the image improves with the mask, you have an edge problem (which is likely TDE).

 

Usually, only the very outer edge is affected. The mask is a simple annulus that fits over the mirror. Medium or heavy grade black construction paper from the hobby store is sufficient. Make the mask to cover a small portion of the outer edge - 1/4" should be plenty. So your 8" mirror becomes a 7.75" mirror. Do your best to cut a clean surface. If it is not smooth and even, start over with another piece of construction paper.

 

Don't worry about the "loss of aperture". An edge that is spraying light somewhere besides but the image only hurts, not helps.

[areyoukiddingme]

I agree with the view that aperture is key, but the cooling is a biggie too.

 

About a year ago I acquired the parts for a 8" F7 Newtonian. The mirror is a 20mm thick quartz made by Zambuto, and the secondary is tiny. I forget the dimension, but this scope is optimized for high powers/planets.

 

The first time I got a good view of Saturn with this scope I was seriously impressed. Compared to a nearby 18" Obsession, the little 8" was showing a much sharper and more stable view (this was after several hours in the field).

 

The quartz primary is the key with this thing (well, apart from the fact that the quality is superb). This scope actually produces stable images more rapidly than my 80mm triplet refractor.

 

The only thing holding this scope back is that it is only 8".

[Deep13]

One other thought came to mind for your Performance Tuning Checklist: Turned Down Edge of the primary mirror. This would be a disaster for planetary performance.

If you don't have access to a Focault tester, you can make a simple aperture mask for an observational test. If the image improves with the mask, you have an edge problem (which is likely TDE).

Usually, only the very outer edge is affected. The mask is a simple annulus that fits over the mirror. Medium or heavy grade black construction paper from the hobby store is sufficient. Make the mask to cover a small portion of the outer edge - 1/4" should be plenty. So your 8" mirror becomes a 7.75" mirror. Do your best to cut a clean surface. If it is not smooth and even, start over with another piece of construction paper.

Don't worry about the "loss of aperture". An edge that is spraying light somewhere besides but the image only hurts, not helps.

I'm sending it to Terry Ostahowski. He's going to check it for aberrations.

[Jon Isaacs]

Worth a read:

 

What is the Best Planetary Telescope  Roland Christen

 

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

 

"Whatever system you choose, you might want to consider your local viewing conditions. For planetary, light pollution has zero effect, so you can observe right from your backyard in a downtown area. The most important thing is the stability of the air above. The better your seeing i.e. steadiness of the image, the larger the instrument I would install. The farther south you live, the larger the scope that will be most effective."

 

Jon

[JeffreyAK]

 What's your ideal planet scope?

 

I think, the one I'd use most often and enjoyably for viewing planets.   What that means depends on the person, location, ability/desire to set up the scope up permanently, ability/desire to mess with collimation regularly, wallet thickness, and other factors.

 

But planets mean high magnification and high resolution and contrast, so that means larger aperture (higher resolution), long focal length (so you can achieve high magnification with a reasonably long eyepiece with reasonable eye relief), and small obstruction (improves contrast).  To me that's, well, what I have, when coupled to other factors like portability (I haul my gear to dark sites) and holding alignment well (so I don't have to collimate every time I take it out).  A large long-focus refractor might or might not provide a better view (no obstruction, no collimation, but relatively small aperture), but it would lose a lot on the portability factor and wouldn't do as well with faint objects that I go to dark sites to view.

 

So it's all tradeoffs, I reckon.

[Jeff B]

Worth a read:

 

What is the Best Planetary Telescope  Roland Christen

 

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

 

"Whatever system you choose, you might want to consider your local viewing conditions. For planetary, light pollution has zero effect, so you can observe right from your backyard in a downtown area. The most important thing is the stability of the air above. The better your seeing i.e. steadiness of the image, the larger the instrument I would install. The farther south you live, the larger the scope that will be most effective."

 

Jon

Sage advice by Roland that's always worth reading. 

 

Thanks for posting it Jon.

[Deep13]

I think, the one I'd use most often and enjoyably for viewing planets. What that means depends on the person, location, ability/desire to set up the scope up permanently, ability/desire to mess with collimation regularly, wallet thickness, and other factors.

But planets mean high magnification and high resolution and contrast, so that means larger aperture (higher resolution), long focal length (so you can achieve high magnification with a reasonably long eyepiece with reasonable eye relief), and small obstruction (improves contrast). To me that's, well, what I have, when coupled to other factors like portability (I haul my gear to dark sites) and holding alignment well (so I don't have to collimate every time I take it out). A large long-focus refractor might or might not provide a better view (no obstruction, no collimation, but relatively small aperture), but it would lose a lot on the portability factor and wouldn't do as well with faint objects that I go to dark sites to view.

So it's all tradeoffs, I reckon.

Well, my refractor is 5" f/12 and this "new" Newt is 8" f/8. So, I won't see any portability savings. But then, I rarely travel with the refractor because it's mostly for planets and I can see them in my backyard.

[Kunama]

The best planet scope I have owned (and somewhat foolishly sold....... kicking my own RRRs) is the TEC MC 200 F15.5.  

It gave me the best views of Saturn in 40 years of viewing it..... 

Edited by Kunama, 18 December 2018 - 03:05 AM.

[barbie]

Fortunately, I haven't sold my best planetary scopes which are my 6"f8 newtonian and 4" F9 apochromat refractor.  These have given me my best views ever of the planets in over 30 years  and I plan on keeping them for a very long time!!

Edited by barbie, 19 December 2018 - 12:07 AM.

[MikeRatcliff]

My best planetary view was many years ago looking at Jupiter through someone's 18" Zambuto mirror dob with a Televue binoviewer. Tak LE eyepieces of unknown focal length. At the time I didn't have a lot of experience to ask more questions or to know if the seeing was unusually good. I don't recall the magnification, but I would now estimate 350x or higher going by memory. A lot higher than I normally use now. It was driven. Cooling fans I don't know. It was in Joshua Tree National Park, an area not known for great seeing, but it must have been pretty good that night. That evening I went back to my own un-optimized 16" scope and realized I had a lot to do to catch up! 

 

A club member has an 8" f/8 ATM dob, and it works well. Trapezium and the E and F stars were very sharp one night with better than average seeing night here in the inland area of S. California. I think the 8" f/8 dob would be a great scope for you.

 

There is a good thread by Daniel Mounsey a few years ago on some of their friendly competitions on planet viewing in the Los Angeles area. They have exceptional seeing. 

 

Mike

[Redbetter]

My best planetary views have been through my 20" f/5 Obsession with a Galaxy mirror.  Mars was best with the scope further south in frequent stable seeing (500 to 750x).  I have had some of my best views of Jupiter and Saturn through it here, despite seeing that has not been as good for planets and has topped out around 357 to 417x on the best nights...but it is just idling because of the seeing.  I would like to get the scope back south again to do some of the things I planned, like an albedo map of Ganymede. 

 

I made an off axis mask that gives me 8" of unobstructed aperture in the least thermally disrupted part of the mirror, but I find the full 20" aperture provides more detail on nights that are worth observing planets at 250x or above.        

 

A very large Dob in excellent seeing would be very difficult to top...particularly in the southern hemisphere with the planets high overhead.

[Jon Isaacs]

I made an off axis mask that gives me 8" of unobstructed aperture in the least thermally disrupted part of the mirror, but I find the full 20" aperture provides more detail on nights that are worth observing planets at 250x or above.

 

I think if you place the mask on top of the upper cage, the effect is essentially the same but it does not thermally disrupt anything.

 

Jon

[Jeff Morgan]

My best planetary view was many years ago looking at Jupiter through someone's 18" Zambuto mirror dob with a Televue binoviewer. Tak LE eyepieces of unknown focal length. At the time I didn't have a lot of experience to ask more questions or to know if the seeing was unusually good. 

 

I have a 6" Apochromat coming, it seems like an ideal platform for binoviewing and I am seriously considering it. I really need to get up to speed on today's bv offerings ... 

 

Back to the topic at hand. With regards to the "best ever" experiences ... they all have something in common.

 

Mine was seeing spokes in Saturn's rings, and observation confirmed by Chicago optician Dan Joyce. The scope was a 10" f/9 Newtonian (Dan helped me grind the mirror), 1.25" E&W quartz diagonal (I am dating myself now!), and a 9mm Nagler with 2x barlow.

 

Some 20 years later, still have never duplicated that observation using a refractor, Mak-Cass, Mak-Newt, or larger Newtonian. All of them featured "famous name" premium optics.

 

So does that mean a 10" f/9 Newtonian is the ideal planetary scope?

 

No. It means that was the scope I happened to be using when that moment of "orbital" seeing happened.

 

So, get the best optics you can and make lots of observations.

Edited by Jeff Morgan, 19 December 2018 - 02:45 PM.

[spencerj]

...

 

So, get the best optics you can and make lots of observations.

 

The ideal planetary scope isn't the next one or the one in your closet or the one in your garage.  It is the one you have set up and focused on your planetary target.  Especially on (what is very rare for my home location) a night of exceptional seeing.