34 replies to this topic

[GTom]

Refractors are mostly out of any serious planetary photographer's toolkit as I see it, but what about visuals?

 

A 6" f8 refractor needs approx. the same mount as a c9.25, cools faster (doublets at least), usually better Strehl, stock focusers expect some very low end models (if that category exist for 6" APO's) are way better, image shift is completely unheard of. Obviously, 6" is less than 9, still, that 9" is quite a compromise.

[quilty]

We seem to have found that the C9 does better in general

[DVexile]

The very rough rule of thumb when comparing apertures for resolving low contrast planetary detail is diameter minus central obstruction.  For the 9.25 the central obstruction is 3.35 so the figure of merit is just about equivalent to a 6 inch refractor.  So we should expect them to be “similar” with the devil more in the details as to how one uses them and what one prefers:

• Depending on the refractor it could suffer from CA that reduces its low contrast detail.
• If you primarily do filtered observing then CA might not matter.
• The C9.25 will operate at a larger exit pupil for the same magnification which can help folks with floaters.
• Insulated C9.25 might not be very different cooling wise as a 6” triplet.

And so on and so on!

 

But I’m probably not the person to explore those nuances as there are so many more experienced folks here!  

[12BH7]

Have you seen a 6" refractor in use? My friend Gene had one and it was bigger than he was.

 

A 6" refractor in the end, is a 6" scope. And there's no getting around the physics that aperture is still king. Even in planetary.

 

But getting back to the logistics of actually using a 6" refractor. Even though a mounts spec can handle the weight, that is not the problem. The huge lever arm of the OTA needs far more stability as compared to the compactness of a SCT OTA. Otherwise it's going to be a battle try to view at any decent magnification. Also the large lever arm will at most times put you in a very awkward viewing position. Sometimes even laying on the ground.

 

Here's a little story from the Lowell Observatory in Flagstaff. They special ordered an 8" refractor specifically for planetary use. After they got it they realized that it just wasn't that special. Now, it's basically a show piece for visitors sitting mostly unused. 

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[GTom]

At f8 (1200mm FL) it certainly won't be longer than me, but getting the point. The c925 isn't the lightest scope either, approx the same weight - more compact though.

 

 

Have you seen a 6" refractor in use? My friend Gene had one and it was bigger than he was.

 

A 6" refractor in the end, is a 6" scope. And there's no getting around the physics that aperture is still king. Even in planetary.

 

But getting back to the logistics of actually using a 6" refractor. Even though a mounts spec can handle the weight, that is not the problem. The huge lever arm of the OTA needs far more stability as compared to the compactness of a SCT OTA. Otherwise it's going to be a battle try to view at any decent magnification. Also the large lever arm will at most times put you in a very awkward viewing position. Sometimes even laying on the ground.

 

Here's a little story from the Lowell Observatory in Flagstaff. They special ordered an 8" refractor specifically for planetary use. After they got it they realized that it just wasn't that special. Now, it's basically a show piece for visitors sitting mostly unused. 

[GTom]

A third, possibly a fourth scope entering the race:

 

- Skymax 180 MC: still a Cat, but smaller CO - no need to collimate

 

- Classic Cassegrain 8" - good stock focuser

Edited by GTom, 11 November 2024 - 03:37 PM.

[Cpk133]

I’ll steal Chas’ thunder and throw an 8” f6 Newt with a custom mirror and a co <20% or a 10”.  Conventional wisdom is a Mewlon next with a little bigger co but much better portability.  I would not expect any SW180 who’s co is about the same as an sct, to beat my C9 on planets.  I’d like to compare it to a Mewlon if i could find one in the wild or maybe I’ll break down and buy a 210 while they’re on sale, but it’s a big pain for something that I doubt would out perform by much if anything at all.  The Newt seems like the best bet to me and i’ve compared an 8” Zambuto to my C9.  On Mars it beat the C9 with the Maria appearing darker (more contrast) but we were setup in a sun soaked asphalt parking lot, less than ideal especially for 3 light passes.  No radiant insulation in those days, just a black tube.  On another occasion, head to head on Jupiter under good seeing, I wasn’t able to see much difference between the two and it’s going to take a big improvement to get me to give up the portability and versatility of the sct in favor of something that requires more fiddling and diffraction spikes.  By the way, 7 years without touching collimation, it holds, no nobs, no probs. 

[GTom]

I’ll steal Chas’ thunder and throw an 8” f6 Newt with a custom mirror and a co <20% or a 10”.  Conventional wisdom is a Mewlon next with a little bigger co but much better portability.  I would not expect any SW180 who’s co is about the same as an sct, to beat my C9 on planets.  I’d like to compare it to a Mewlon if i could find one in the wild or maybe I’ll break down and buy a 210 while they’re on sale, but it’s a big pain for something that I doubt would out perform by much if anything at all.  The Newt seems like the best bet to me and i’ve compared an 8” Zambuto to my C9.  On Mars it beat the C9 with the Maria appearing darker (more contrast) but we were setup in a sun soaked asphalt parking lot, less than ideal especially for 3 light passes.  No radiant insulation in those days, just a black tube.  On another occasion, head to head on Jupiter under good seeing, I wasn’t able to see much difference between the two and it’s going to take a big improvement to get me to give up the portability and versatility of the sct in favor of something that requires more fiddling and diffraction spikes.  By the way, 7 years without touching collimation, it holds, no nobs, no probs. 

Newt's are great, but I wouldn't underestimate their needyness of collimation. I know very little of the MAK, will give it a little research to be on the safe side.

[quilty]

The weight of a C9 and a 6" f/10 frac. might be the same but the use of the C9 is much more comfortable.

On the fork the ep is never farther off the axis than 12". Which means I can easily aim all targets without changing my seat, neither for the finderscope.
No Newt or frac can do that.
And the small lever makes the C9 (if you want to) possible on an EQ5.

There's a great side by side report with a 6" frac by Damian Peach with the outcome (when I remember correctly) that the C9 levels the frac at some targets and outperforms at other ones.
The mtf? was same or better than the unobstructed 6" depending on frequency. That was a simulation but it matched Damian's visual findings

[Sebastian_Sajaroff]

I'm sure the 9.25 will perform better, but in both cases you'll need some serious mount.

Either the 6" refractor or the 9.25 SCT + the mount + accessories will easily exceed 100 lb, not the kind of weight and bulk I'd be happy to carry around.

 

IMHO, any of these beautiful instruments deserve an observatory (to be mounted on a peer, with a concrete slab and a dome).

[12BH7]

The OP is located in the UK. I'm not sure but most of that area is limited by weather. So splitting hairs over CC, SCT, Mew's is pretty much pointless when your local conditions will be the main limiting factor. 

 

Under those conditions I would also be choosing a scope that I could move about quickly. 

[Astrojensen]

My personal experiences with a 6" f/8 APM ED as a planetary instrument are positive, but there are some caveats: 

 

- it's a fairly large scope and needs a beefy mount, if you want to take advantage of the good optics and use high magnifications.

 

- the pier/tripod needs to be fairly tall, even if you observe seated. It's a long scope. 

 

- it needs substantial cooldown! This must NOT BE IGNORED. This means that it is not an instrument to use at a moments notice, unless you can house it in an observatory. If taken from indoors out in a cold night, cooldown takes well over an hour. The glass is very thick and steeply curved. If at all possible, I plan ahead and let the scope sit outdoors for a while, before starting observations. For lower power observing (below 100x), this is not necessary. 

 

- if used with this in mind, I've found it to be a very reliable performer. Much more so than the C8 I once had. 

 

- it requires unusually good seeing (for my location) to show everything it's capable of.   

 

- it likes to dew up at the most inconvenient times, so have the hairdryer ready. Or make an additional dewcap. 

 

- In addition to it being a fine lunar-planetary scope, it's a SUPERB deep-sky scope, despite its modest aperture. 

 

- It's also a superb solar scope, when outfitted with a Quark Chromosphere.   

 

6"-class refractors are not everyone's cup of tea, but they can be very pleasing telescopes to use, if you understand their needs and behavior. I find them to be a very good balance between portability, power, reliability, versatility and relative affordability. They can do a lot of things really well. 

 

 

 

Taken with APM 152/1200 ED and 4mm KK ortho, handheld Sony Xperia phone. No image editing whatsoever. 

 

 

Clear skies!

Thomas, Denmark

[12BH7]

Over the years I have consider a 6" refractor for planetary. However, having had a friend who owned one, your observations were spot on.

 

Also, I prefer to keep my viewing habits on the more flexible side. And a 6" refractor is not a more flexible type setup. 

[GTom]

The OP is located in the UK. I'm not sure but most of that area is limited by weather. So splitting hairs over CC, SCT, Mew's is pretty much pointless when your local conditions will be the main limiting factor. 

 

Under those conditions I would also be choosing a scope that I could move about quickly. 

<<-This. Actually seriously debating if a c8 could be the most productive scope for me:

All-in-one portable, compatible with the lightest category harmonic mounts (HAE16, AM3, Umi 17r, Juwei 14), half weight suggests 2x faster cooling (cooling is not too critical here though as near the sea, temp gradients are moderate). Under seeing-limited conditions, I am unsure if large apertures bring much...

Edited by GTom, 17 November 2024 - 06:31 PM.

[Astrojensen]

And a 6" refractor is not a more flexible type setup. 

I do consider a 6" refractor to be very flexible in what it can do. Widefield, high power, planetary, solar, deep-sky, visual, photo, use binoviewers and large eyepieces, etc. Many people consider the 8" SCT to be the "Jack of all trades" telescope, but in my opinion the 6" f/8 ED (or apo) refractor is actually far more versatile, and will do many more things really well than the C8.  

 

But it's not super portable, if that was what you meant with being flexible. 

 

 

Clear skies!

Thomas, Denmark

[12BH7]

I do consider a 6" refractor to be very flexible in what it can do. Widefield, high power, planetary, solar, deep-sky, visual, photo, use binoviewers and large eyepieces, etc. Many people consider the 8" SCT to be the "Jack of all trades" telescope, but in my opinion the 6" f/8 ED (or apo) refractor is actually far more versatile, and will do many more things really well than the C8.  

 

But it's not super portable, if that was what you meant with being flexible. 

 

 

Clear skies!

Thomas, Denmark

Great, now you got me rethinking a 6" refractor. Just when I thought I was out - you pulled me back in.

[quilty]

My personal experiences with a 6" f/8 APM ED as a planetary instrument are positive, but there are some caveats: 
 
- it's a fairly large scope and needs a beefy mount, if you want to take advantage of the good optics and use high magnifications.
 
- the pier/tripod needs to be fairly tall, even if you observe seated. It's a long scope. 
 
- it needs substantial cooldown! This must NOT BE IGNORED. This means that it is not an instrument to use at a moments notice, unless you can house it in an observatory. If taken from indoors out in a cold night, cooldown takes well over an hour. The glass is very thick and steeply curved. If at all possible, I plan ahead and let the scope sit outdoors for a while, before starting observations. For lower power observing (below 100x), this is not necessary. 
 
- if used with this in mind, I've found it to be a very reliable performer. Much more so than the C8 I once had. 
 
- it requires unusually good seeing (for my location) to show everything it's capable of.   
 
- it likes to dew up at the most inconvenient times, so have the hairdryer ready. Or make an additional dewcap. 
 
- In addition to it being a fine lunar-planetary scope, it's a SUPERB deep-sky scope, despite its modest aperture. 
 
- It's also a superb solar scope, when outfitted with a Quark Chromosphere.   
 
6"-class refractors are not everyone's cup of tea, but they can be very pleasing telescopes to use, if you understand their needs and behavior. I find them to be a very good balance between portability, power, reliability, versatility and relative affordability. They can do a lot of things really well. 
 

 

 
Taken with APM 152/1200 ED and 4mm KK ortho, handheld Sony Xperia phone. No image editing whatsoever. 
 
 
Clear skies!
Thomas, Denmark

Thomas,

for planetary which does better the APM or the Meniskas? Which one do you prefer?

Both scopes can be insulated to reduce the need for any acclimatisation.

Stephan

[quilty]

At moment I'm fine with my very portable C9 Deskdob:





Insulated and dewtubed it takes no time for "acclimatisation" but does 350x at once as soon as seeing allows


Easy to stay seated when changing target, one of my must have features, even so for all targets +45° in the finderscope

Edited by quilty, 18 November 2024 - 03:46 AM.

[Astrojensen]

Thomas,

for planetary which does better the APM or the Meniskas? Which one do you prefer?

Both scopes can be insulated to reduce the need for any acclimatisation.

Stephan

The APM can't be insulated. That would mean disastrously bad undercorrection that would linger on for hours and hours. The front element is always exposed to the cold, nighttime air, and it's the differential cooling between the outer and inner lens that causes the undercorrection. Insulating the OTA would make the problem much worse. What I really need (and need to buy or make) is a cooling fan for the OTA that can quickly bring the inner lens down to the same temperature as the outer one. 

 

In terms of planetary performance, the APM ED is much superior to the Meniscas. Especially on Jupiter. They're more equal on the Moon in terms of resolution, but the APM is much brighter and can more easily use higher magnifications. 

 

But the insulated Meniscas has one, very powerful advantage: It's ready for use almost immediately. For those evenings, where I have been surprised by a sudden clear sky, the Meniscas can be set up in less than five minutes. There are a few minutes with unstable images, but it settles down very quickly. The Meniscas is also smaller overall, and uses a smaller mount, so it's faster to set up and there are no extremely heavy parts to carry. The heaviest part is actually the scope itself. 

 

It's hard to say, which scope I prefer. If I have the time, the APM clearly shows more and is the better scope, but on the many evenings, where time is too short, and I must make do with a short period of clear weather, the Meniscas is the winner. 

 

 

Clear skies!

Thomas, Denmark 

Edited by Astrojensen, 18 November 2024 - 04:04 AM.

[quilty]

Thomas,

thanks for your detailed answer. Which puts the Meniscas a bit into relation. I guess colours and contrast in the frac are much better, aren't they? But the frac is much brighter, too which I didn't expect.
Why can't the frac be insulated? Do you state a certain, permanent delta T between the lenses is calculated and necessary? Can't quite believe. Insulation (and insulated long dewtube) prevent from huge T drops both in time and extension and I think must help at huge fracs like it does for Maks.

I think it's easy to say, which one you'd prefer if there wasn't the hassle factor :-)

I never have had to choose between them because I never had a seriours frac yet. For the above reason.

Stephan

Edited by quilty, 18 November 2024 - 04:29 AM.

[Astrojensen]

 

Why can't the frac be insulated? Do you state a certain, permanent delta T between the lenses is calculated and necessary?

Quite the opposite. A delta T between the lenses must be avoided as much as possible.

 

 

Insulation (and insulated long dewtube) prevent from huge T drops

The problem with the refractor is that the front lens is exposed to the cold nighttime air, while the rear lens is not. This means that the front lens cools down a lot faster than the rear one, and this situation is made even worse by insulating the tube. When the front lens is colder than the rear one, its curves become steeper, relative to the rear lens, which is thus no longer able to fully correct the aberrations of it, and this causes spherical undercorrection. How much the objective will be undercorrected depends on several factors, such as lens thickness and steepness of the curves, spacing, as well as delta T between the two lenses. ED refractors, which often have very thick and deeply curved lenses, are hit pretty hard by this.  

 

In a Maksutov or an SCT, the correcting lens/plate is also a lot colder than the rest of the optics, but since the optical power of the corrector is much weaker than the front lens element in a refractor, this does not give any noticeable problems. 

 

Once the refractor has cooled down completely, insulating it will prevent tube currents, and can give a noticeable improvement in stability, but the objective MUST be fully cooled first. 

 

 

Clear skies!

Thomas, Denmark

[quilty]

My idea is that a long, insulated dewtube might slow down cooling the front lens.
It cools down by ratiation loss into the sky. The smaller that hole to the sky the better. I think it helps to the very same extent as it helps against dewing up.
Dewing up to my experience doesn't stem from fog falling down onto the scope but rather any surface dews up which has cooled below dew temperature and the more a glass surface is exposed to the clear sky the sooner it dews up.
But I see the difference to the Mak. Until now I thought insulation would do for fracs as well. Always learning something. Another plus for cats.

Stephan

Edited by quilty, 18 November 2024 - 05:36 AM.

[Astrojensen]

 

My idea is that a long, insulated dewtube might slow down cooling the front lens.

It definitely will, but not much, since the cold air can still reach it. What a dewcap will do is prolong the time before the lens is cooled BELOW the dewpoint from radiation. 

 

 

Dewing up to my experience doesn't stem from fog falling down onto the scope but rather any surface dews up which has cooled below dew temperature and the more a glass surface is exposed to the clear sky the sooner it dews up.

Absolutely correct. 

 

 

Until now I thought insulation would do for fracs as well. Always learning something. Another plus for cats.

Absolutely!! The ability to immediately use the Meniscas at high magnification despite a delta T of 15°C is quite amazing and very useful indeed. 

 

 

Clear skies!

Thomas, Denmark

[quilty]

I'd still like to do a side by side of my CC6 and your Meniscas. Recently I learned that the CC is most probably inferior to any Skymax 150 but yet, both scopes share the same huge CO.
Which to me (from outside view) is the HUGE degrading factor with respect to your 6" Apo.

[Astrojensen]

I'd still like to do a side by side of my CC6 and your Meniscas. 

That should be interesting and fun! 

 

 

Clear skies!

Thomas, Denmark