184 replies to this topic

[Oberon]

I guess it depends what defines a planet killer?   

 

I would think a 20" Ostahowski f/3.4 would be a fine instrument for planetary observation once cooled and well collimated... 20" of well figured aperture would pretty much kill anything you throw at it.

 

I have to admit that even though I have a 16" f/4.5 scope with an Ostahowski mirror, I'm still working on an 8" f/~8.7 dob specifically for planetary use.   The 8" f/8.7 I have isn't necessarily because I think it would be better than my 16" for planetary...I'm building it just because I can, and out of sheer curiosity about how long focal ratio reflectors operate (all of my experience is with F/6 and shorter reflectors).   I'm using an old thick Pyrex mirror that I still need to get tested and maybe refigured, but I assume it'll do well in the planetary arena.   

 

I have a vintage C8 SCT which in theory should be great for planetary, but I have mixed feelings about how SCTs perform in relation to refractors and the big aperture of reflectors.

I have an f/4.5 16” Ostahowski mirror, and also an 8” f/7 by Steve Lee. Both appear excellent mirrors but the 16” slam dunks the 8” on everything. As it should. Every time I think of building a planet killer...like, say, a 10” f/7 or whatever, I remind myself I already have one.

[stubeeef]

I have an iOptron iEQ45Pro that can handle the Skymax 180 MAK no problem. The TTS alt az is nice but not going to spend those bucks on another mount. Wish the world made a $500 tracking alt az that handled a real 25 lbs

[Achernar]

I have an f/4.5 16” Ostahowski mirror, and also an 8” f/7 by Steve Lee. Both appear excellent mirrors but the 16” slam dunks the 8” on everything. As it should. Every time I think of building a planet killer...like, say, a 10” f/7 or whatever, I remind myself I already have one.

I have a 6-inch F/8 that is a planet killer for a small telescope, but the 8-inch SCT, 10-inch F/4.5 an 15-inch F/4.5 Dobs also do very well on the planets too when the seeing is good. I spent many hours looking at Mars, Jupiter and Saturn with my reflecting telescopes, and imaging them with the 8-inch SCT. The best views though came with my Dobs though, especially the 15-inch. That said, the 6-inch is no slouch and has excellent optics, the primary mirror was made by Meade back in the 70's. The 10-inch also has a Terry Ostahowski mirror, and the 15-inch has a primary mirror made by Optic Wave Labs.

 

Taras

Edited by Achernar, 17 January 2020 - 06:27 PM.

[Mike Spooner]

A well figured 10" as has been mentioned. Some of my best views of Mars were with a 10 f/5.4. Saw dark volcanoes on Mars years ago with 8" f/7. Mars was high up, seeing was spectacular and scope was working well at 600x. My eyes were a bit better, too.

 

Mike Spooner

[CHASLX200]

Any well built Newt is a brute for planets in my super steady seeing.  Fast or slow is fine.  Old school 8" F/8 Cave or other makes also make great planet scopes as well as used Starmaster Dobs at around F/4.3. 

 

Most mass made Newts the last 20 years just don't do it for me.

[SkyRanger]

On nights of fair to average seeing, I will be relying on my TSA 120; on good to excellent nights it will be my new-to-me Teeter- Zambuto 10” f/5.  It was damaged in shipping is still in limbo awaiting settlement from the shipper.

 

Gordon G

[cabfl]

Custom designed Newton:

Obstruction less than 20% = planet killer or APO killer

Cameras for planets: ASI / QHY - 290 / 224 = 6 - 7 mm diagonal.

 

Newton 12" F/5.3  (300x1600)
Primary Mirror: 303 mm
Secondary Mirror: 50 mm
Obstruction: 16.5% (Obstruction surface: 2.72%)
Illuminated diagonal: approx 9-10 mm

 

Newton 12" F/5  (300x1500)
Primary Mirror: 303 mm
Secondary Mirror: 50 mm
Obstruction: 16.5% (Obstruction surface: 2.72%)
Illuminated diagonal: approx 7-8 mm

 

Newton 10" F/6.4 (250x1600)

Primary Mirror: 254 mm
Secondary Mirror: 40 mm
Obstruction: 15.7% (Obstruction surface: 2.48%)
Illuminated diagonal: approx 10-11 mm

 

Newton 10" F/6 (250x1500)
Primary Mirror: 254 mm
Secondary Mirror: 40 mm
Obstruction: 15.7% (Obstruction surface: 2.48%)
Illuminated diagonal: approx 8-9 mm

 

Newton 10" F/5 (250x1250)
Primary Mirror: 254 mm
Secondary Mirror: 45 mm
Obstruction: 17.7% (Obstruction surface: 3.14%)
Illuminated diagonal: approx 7-8 mm

 

 

You can do the calculations on this website:

 

https://stellafane.o...b/newt-web.html

Edited by cabfl, 18 January 2020 - 11:56 AM.

[stubeeef]

Skywatcher Skymax 180 Mak $1200

OTA- 19lbs

 

Mirror 180mm

Secondary 41mm

Obstruction 5%

 

Focal length = 2700

Dawes Limit =.64

Limiting Mag = 13.8

Theoretical Max Mag = 354   (we all push way beyond these limit when we can)

 

 

Skywatcher Skymax 150 Mak $750

OTA-14lbs

Mirror 150mm

Secondary 47mm

Obstruction 10%

 

Focal Length=

Dawes Limit=.77

Limiting Mag=13.4

Theoretical Max Mag = 295

 

the 180 is just so much better numbers wise, but the weight gets to be the issue, gotta be 23+lbs when you get going with it.

 

But then again, is Saturn going to be that much better in the 180? and $450 is $450.

[KI5CAW]

My 8" f/8. Royce primary, Protostar quartz secondary. After midnight, when the seeing settles and the scope temperature has fully equalized.

[Galicapernistein]

My 8" f/8. Royce primary, Protostar quartz secondary. After midnight, when the seeing settles and the scope temperature has fully equalized.

I had a 6” F8 Discovery dob that had an excellent mirror. But my SW 100 ED was close enough that I sold the dob for the convenience of the refractor. But going from a 6” F8 to an 8” F8 or 9 dob puts you into an entirely different league. I would put my 8” F9 against any 5” APO refractor. That said, I still want a 5” APO refractor.

[ed_turco]

I wrote about my version of a planet-killer in 2015.

 

 

Ed 

[Jon Isaacs]

No place in the United Sates will a 20-inch telescope ever resolve to its full potential.

 

On few nights and in the best locations like south FL. a 12 to 16-inch will have a chance but in most locations just getting sub arcsecond seeing is rare. And not only do you need that seeing but the planets need to be high or at zenith or the seeing will be compromised even more.

 

For example…
"At the William Herschel Telescope site in the Canary Islands, even this superb viewing location (second best in the northern Hemisphere) has many nights of relatively poor seeing: the distribution is positively skewed, and at this excellent site, a 10-inch telescope will be seeing limited on 9 out of 10 nights.”

 

Bob

 

That's really not the question. A large scope maybe seeing limited but still outperform a smaller scope. 

 

The Dawes limit and the Rayleigh criterion are not the appropriate measures to determine the resolution/ seeing limits. 

 

In 1 arc-second seeing, a 10 inch will be seeing limited but dramatically out perform a 5 inch. 

 

Think about airy disk diameters and overlapping disks.. 

 

This my planet killer..

 

 

13.1 inch F/5.5 with a Royce mirror.  

 

The 16 inch and the 22 inch could be but are permanently located in the high desert where the seeing is not as good as it is near the coast.

 

Jon

[Asbytec]

...MAK lovers? 

 

What about f/12 or 1/15? Dimmer yes, but we're talking planets here.

If we're talking photographic speed, then yes f/15 is pretty slow, i.e., dim. But we're talking about afocal visual observing. Dimness is directly related to the exit pupil instead of the smaller relative aperture. Planets are not dimmer with, or because of, a slow focal ratio. They dim in all scopes because the light gathered by the aperture is magnified over a large afocal image scale on our eye.

 

The amount of dimming is proportional to the ratio of the entrance pupil to the exit pupil squared. In all scopes at the same exit pupil, planets will have (just about) the same surface brightness per unit of area. The only difference is a larger aperture means higher resolution and a higher ratio of aperture/exit pupil or, in other words, higher magnification.

[Asbytec]

The best planet killer is the atmosphere.

 

A scope also needs to be thermally stable and well collimated, as well. There are better scopes out there, of course, so get one. But prepping a scope for planet killing is probably the most important factor outside of seeing conditions and recognizing planetary detail when we see it.

[jakecru]

I'd say something in the 10-12" f/5.5 + range with good optics and proper cooling. 

[JoshUrban]

I have an 8" f/8 dob that is GREAT on the planets.  Plus, it's a lot of fun to use, too.  Really has the "OK, so it's come to this!" feeling when you aim it at anything.  Ha!  Something to keep in mind is binoviewers are excellent for planetary work, and will the scope/mount handle it.  The dob does great with this, too.  

[Bill Jensen]

Zambuto has an in stock 8 inch f/7 that is listed on his website. Those don't come up that often, and may be a nice solution to your planetary viewing desires. 

[starzonesteve]

Zambuto has an in stock 8 inch f/7 that is listed on his website. Those don't come up that often, and may be a nice solution to your planetary viewing desires. 

This 

 

I have a quartz Zambuto 8" F/7 on order that should be ready in about a year or so. I would like to find a lightweight scope to house the mirror and then possibly put it on an equatorial platform. 

Edited by starzonesteve, 19 January 2020 - 08:41 PM.

[Fatcat]

I have several Dobsonians and they can give stunning views of the planets but if I am observing from home my goto scope for planetary use is my C11 SCT on an equatorial mount with bino viewers. I know you said you dont like equatorials and I am a dobsonian user myself but for high power views of the planets tracking is a wonderful thing... and for me the bino viewers give an almost 3D appearance to the planets and there moons and SCTs work really well with binos.

 

If you have been around this forum for any length of time you probably know that SCTs have a stigma here and often take a beating. I have had the opportunity to use several modern samples from the past 5 years or so and they have all been great performers.

[areyoukiddingme]

This 

 

I have a quartz Zambuto 8" F/7 on order that should be ready in about a year or so. I would like to find a lightweight scope to house the mirror and then possibly put it on an equatorial platform. 

I got lucky on used parts  on AM that included a Zambuto 8" F7 quartz. It'll clobber any 6" refractor on the planets, and is basically up and running in a few minutes as the substrate doesn't have temperature equilibration issues.

 

The 8" is so good that I've decided to replace my 12.5" F5 Zambuto pyrex with one of his quartz mirrors. I'm hoping it'll be ready in the summer.

[25585]

Sounds like you largely have made the reflector decisions, so instead I'll comment on this portion, if somewhat off-topic.

 

To revive your 130mm refractor, the TTS Panther is a fabulous alt-az GOTO mount. I replaced my Losmandy G11 with one. Same load capacity, 27 pounds lighter, no leveling, no polar alignment, and much better ergonomics.

Would the Panther be better than an Az-Eq 6 in Az mode?

[starzonesteve]

I got lucky on used parts  on AM that included a Zambuto 8" F7 quartz. It'll clobber any 6" refractor on the planets, and is basically up and running in a few minutes as the substrate doesn't have temperature equilibration issues.

 

The 8" is so good that I've decided to replace my 12.5" F5 Zambuto pyrex with one of his quartz mirrors. I'm hoping it'll be ready in the summer.

Any plans for a structure to put the mirror in?

[areyoukiddingme]

Any plans for a structure to put the mirror in?

That's the big issue. It's a 20mm thick 12.5", so it's going to push the bounds of what can be done. My 8" F7 is siliconed to an Aurora Precision cell and I've had it beyond 400x and it shows no evidence of astigmatism.

 

My current structure is a 12.5" Portaball circa 2003. The original cell is wooden, and has 8 stainless L-brackets that are siliconed to the edges of the mirror (current mirror is 1.625"). And the cell has 6 points, also siliconed. I've found that can give me some issues with astigmatism, so I'm sure it will with the thinner quartz.

 

I've also got the cell from the recent generation, which appears to be made by Aurora Precision. That one is also 6 points, but has no edge supports.

 

My plan at the moment is to use the original cell with no silicone, and to cut vertical slots into the L-brackets. I will then be able to screw in stainless bolts with nylon tips that can be placed to just touch the edge of the mirror. The slot would allow those to be positioned right on the center of gravity, and the nylon (I hope!) will have sufficiently little friction that it will not induce any astigmatism.

 

If that doesn't work, I'm thinking of looking into putting two whiffle-tree supports on the Aurora cell. Mike Lockwood's site on edge support is helpful, but hasn't been applied to super thin mirrors in a Portaball structure. But he seems convinced by the whiffle trees.

 

If all of that fails, I will sell the Portaball with original mirror, and find a more standard Dob structure with a modern cell from someplace.

[stubeeef]

I need to hit a big star party, pure and simple. 

 

Big DOB is a cure for a crap load of desires, probably wait till it get here. Steve, not much to try on other than Venus, but love to work on that, and of course its easy and early.

[starzonesteve]

I need to hit a big star party, pure and simple. 

 

Big DOB is a cure for a crap load of desires, probably wait till it get here. Steve, not much to try on other than Venus, but love to work on that, and of course its easy and early.

 

This will probably sound like heresy, but Venus just doesn't get it for me. No contrast. If I'm doing something wrong I'd be happy to hear what works for others.