telescopes for visual planetary observation

[sixela]

I think we all need to take a step back, as there are many good "planetary" scopes. Just asking for instances will get you many, many answers.

I hate to quote myself, but I just wrote this in another thread:

-you need aperture. Preferably at least 200mm, but this might not be feasible (see below).

-You need tracking, whatever you get. It even trumps the above rule (I'd rather have a 150mm reflector with tracking than a 200mm reflector without tracking, when looking at planets, even though I've become good at following Jupiter around with Dobs. That's why I have a platform.)

-You need good thermal control. On a Newt, that means a fan. On a large Mak or SCT, that means a Lymax cooler. On an ED doublet refractor, that means a little bit of patience (remember not to gloat while others still have mushy images while all you have is just a wee little bit of spherical aberration).

-On any scope that has lenses at the front, you need a dew prevention solution unless you happen to live in a very dry place (reflector people, remember not to gloat or the God of Dew will curse your secondary too --eventually).

So as you can see, there is a set of basic requirements and more ways than one to skin the cat --each of the possible scopes requires a slightly different set of accessories. But forget one of the requirements above and the scope won't perform on planets (or, in the case of too few dew prevention measures, will perform for a very short time).

Oh, and on any scope you will need to be at least proficient with one aspect of collimation --to make the optical axis cross the focuser axis at the focal plane. Without that, nothing will work (not even a refractor, though they are often well collimated when you buy them and remain well collimated until you dent the tube).

[brianb]

Well said Sixela!

[michael.h.f.wilkinson]

I have a GP-C8 and do a lot of planetary work. The C9.25 is no doubt better, but my old C8 does a very good job indeed. Especially since I got good planetary EPs (first Vixen LVs, now Radian, the images seem to be limited by seeing more than anything else. Cooling time can be a hassle (45min-1 hour in my experience). A longer focal ratio Mak should have smaller central obstruction (like the C9.25, which is a real planet killer), and be a bit better on planets. The C8 will beat them on deep sky, because of a larger FOV. The EQ mount sets up very quickly, and tracks very nicely (indeed a must for planets).

If you buy a C8, I would not expect you to be disappointed on planets, unless you get a dud one (which I have never encountered myself). I wonder if the difference in secondary obstruction between Mak and SCT is so great that a 7" Mak beats an 8" SCT (for a given optical quality of course, it is quite easy to produce a dud APO:D, though you will never get away with it).

Bottom line, given the choice between two similar-size, portable scopes, and if you only want to go for planets: go for a Mak; if you also want to go deep sky: get an SCT.

[Moonshane]

Quality of SCT's seems to vary quite a bit. Of the 9 i've owned, 2 were excellent, 3 good, 2 not so good and 2 were dire. The bests one were not the Celestron's surprisingly but the Meade's. My LX6 10" F10 and LX50 8" F10 ota's were superb, as good a SCT as you could hope for. The two dire ones were a Meade LX200 10" F6.3 and a Celestron C5 F10.

Based on that LX50 8", i would highly recommend an 8" SCT but there's a good chance you'll get a duffer. Rob's Celestron C8i was a good example of a poor performing 8" SCT. Just couldn't yield a sharp image.

No experience of the Tal K series scopes. I know they have issues of their own from reviews i've read but all scopes have flaws. Best to find some good reviews and judge from that I suppose.

My own personal recommendation would be a good planetary optimised newtonian or a Mak. Shane's Orion Optics 150 f11 Deluxe was simply gobsmacking for planetary detail.

same here Russ

it will be interesting to see what my latest 'bargain' (not sure it is one yet) will be like against the 6" f11. http://stargazerslounge.com/diy-astronomer/119740-8-5-f8-dobsonian-restoration-rebuild.html

based on the spec it should be a good planetary performer (8.5" f8 primary and 35mm (16%) secondary) but not sure what the figure of quality is like yet.

need to build a dob/truss base for it first!

[E621Keith]

I think SCT are good general purpose scope. Mak and apo do better on planets, RC and Newtonian do better on deep skies. Throwing more money at it will get you better instruments, but you have to think whether the extra money worth it. My best view of the planets so far was through a C6 with XLT coating follow by an old 8" Lx10 back in university.

[brianb]

Throwing more money at it will get you better instruments, but you have to think whether the extra money worth it.

What you're actually buying is quality control. You will occasionally get a great mass produced instrument, but that will be balanced by the lemon ... at the premium end you are guaranteed to get at least very very good.

Optical design actually matters very little - as stated, short focal length instruments are good for low power, wide field work whereas long focus scopes have an advantage for high power, high resolution work. Quality matters far more than optical design. A 1/10 wave PV instrument with a 33% central obstruction will out-resolve and out-contrast an unobstructed instrument with 1/4 wave RMS optics (which is about what mass produced scopes are).

[Mr Spock]

Any opinions on a 8" f8 with 1/10 wave optics? Sounds like the ideal planetary scope.

[brianb]

8" f8 with 1/10 wave optics?

The Orion Optics (UK) 8" f/8 and 10" f/6.3 Newtontians - with top grade 1/10 wave PV optics and the mirror cell & focuser upgrades - make superb planetary / lunar instruments but demand a good mount. With the sort of rotten seeing typical in the UK, I rather doubt there's a lot of point in going much larger.

[sixela]

I think SCT are good general purpose scope. Mak and apo do better on planets, RC and Newtonian do better on deep skies.

That's a meme, but rarely a fact in general (even at equal aperture, there is little reason to think that a Mak should best a Newtonian. In fact, it's plagued by thermal gremlins more and the central obstruction ratio is usually higher. The meme calls it "planetary" because it's usuallu a scope with a high f/ratio, but so is a slow Newtonian or a fast Newtonian with a barlow).

[sixela]

with 1/4 wave RMS optics (which is about what mass produced scopes are).

Uhm - I don't know in what century you live, but in mine mass produced scopes usually have 1/12-1/16 wave RMS on the wavefront optics and sometimes better. "1/4th wave" is usually the peak-to-valley error on the wavefront cited as a criterion (though admittedly, some fall short on that criterion).

[brianb]

mass produced scopes usually have 1/12-1/16 wave RMS on the wavefront optics

That may be what the machines are capable of, when new and if properly set up.

My impression is that, if you buy 10 mass produced scopes at the budget end of the market (where optics are not graded by wavefront accuracy or Strehl ratio), you get one great one, two or three good ones, three or four average ones, two or three poor ones and one which is rubbish. That hasn't changed in the last few decades.

You get the occasional duffer creeping through with premium prices scopes too, but at least you have an argument for getting the supplier or manufacturer to bring it up to spec, replace with a good one or refund your money.

[sixela]

My impression is that, if you buy 10 mass produced scopes at the budget end of the market (where optics are not graded by wavefront accuracy or Strehl ratio), you get one great one, two or three good ones, three or four average ones, two or three poor ones and one which is rubbish. That hasn't changed in the last few decades.

Over the years, the expected value of the quality has improved a lot, though. Not even ten years ago, if you got a mass market mirror, you were almost guaranteed a lot of roughness (there still is some roughness, but nothing as bad) and probably a turned edge plus at least one zone, and usually either a slightly undercorrected or overcorrected mirror. If you were lucky; if you weren't, the mirror was barely usable because it was so astigmatic.

Nowadays, the average ones don't have 1/4th wave RMS on the wavefront. That corresponds to a Strehl ratio of less than 0.08. That's rubbish. And they're not all rubbish - even the rubbish ones aren't that bad.

In fact, I'd be hard pressed to find a really bad primary with a Strehl ratio of less than 0.5 these days (which corresponds roughly to 1/8th wave RMS on the wavefront). If you look at independent tests in Germany (Germans love to have their mirrors tested independently), you see Strehl ratios in the range 0.86-0.93 for the good ones and 0.76-0.84 for the "bad" ones, with the occasional lemon yielding an estimated Strehl ratio closer to 0.7 (usually due to a dominant gross error somewhere) like this astigmatic one.

"The machines" are capable of mirrors that look like these. That's not an RMS error of 1/16th wave, that's an RMS error of 1/25th wave or less¹.

When GSO and Synta mess up a scope big time, it's usually with a really astigmatic secondary (which fortunately is a lot cheaper to replace).

Or by simply putting usable mirrors in a scope and then pinching them or making sure the mirror cell can't work properly (recent GSO added bonus: a liberal amount of GSOgoo applied so that you have a hard time fixing it).

Sometimes the most obvious things (and easiest to fix) are almost comically bad: I've recently seen a 16" GSO whose "edge support" was black GSOgoo, and it wasn't even touching the back support points at all on a third of the support points, hanging by the edge support GSOgoo instead!

¹For one of the mirrors, you do have to disregard a small central hole, but on the scopes that's well hidden by the secondary even for objects well off-axis.

[Ags]

That's a meme, but rarely a fact in general (even at equal aperture, there is little reason to think that a Mak should best a Newtonian. In fact, it's plagued by thermal gremlins more and the central obstruction ratio is usually higher. The meme calls it "planetary" because it's usuallu a scope with a high f/ratio, but so is a slow Newtonian or a fast Newtonian with a barlow).

The CO may be larger, but what about the vanes supporting the Newt's CO? The vanes seem to make a substantial contribution to contrast loss. Many people on this forum own both Maks and Newts and the consensus seems to be that the Maks provide sharper, higher contrast views.

I admit I don't quite understand this as Maks are supposed to beat SCTs on planets, but SCTs have smaller COs...

[sixela]

The CO may be larger, but what about the vanes supporting the Newt's CO? The vanes seem to make a substantial contribution to contrast loss.

Until you do the math.

Many people on this forum own both Maks and Newts and the consensus seems to be that the Maks provide sharper, higher contrast views.

I've never been a believer in "consensus" when neither the laws of physics nor my personal experience agree .

Many Newts are cheap and have rather careless users who don't fix obvious issues. Many Maks have quite dedicated users who will do anything to eke out the last bit of performance out of them.

But correlation is not causation, and to know the difference you have to try to understand the issues rather than wave your hands at them.

Maks are more practical to use for planets (you don't have to use very small and uncomfortable eyepieces, if they're GEM mounted the eyepiece doesn't end up in an uncomfortable position, they tax the mount less if it's a GEM,...).

That they're better than a planetary optimised Newtonian with good mirrors and above all good thermal control (or even a very good fast Newtonian with an owner with attention to detail and a really good barlow) is just a fable, though.

A fable that is repeated by many people in "consensus" is a meme, not universal truth through repetition.

I'm not saying Newts are "better" either. You can make any design perform very well, and I'd personally like one of Roland Christen's 10" Maks as a planetary scope.

[michael.h.f.wilkinson]

I think my 6" F/8 Newtonian was actually very good on planets (very thin vanes, and it kept collimation very well). One reason people might think SCTs are less suited (not my experience) than Maks is the fact that SCTs are faster. This makes Maks even easier on the EPs than SCTs. Even though at F/10 my scope is not that picky about EP quality, I certainly noticed the improvement from very decent Celestron (Vixen) Plossls, to LVs, and even going from LVs to Radians. The difference between the latter two may be less in Maks.

The remarks from Sixela on EPs also hold true: Plossls suitable for planetary work on F/15 scopes have much better eye relief than the circle T ortho 5mm I used in my F/8 newt. This means you can afford to use a monocentric or orthoscopic EP in a Mak, even when wearing specs, whereas in an SCT you would need a Radian or TMB planetary EP, which is (slightly) less crisp.

Having said that, at a star party a few years ago, my 8" SCT with Radian 8mm or Meade UWA 14 beat a 10" Meade SCT equipped with Plossls and even an Ortho (not a top brand one). If you put my Radian in the Meade it was quite clear the EP made the difference.

[beamish]

Uhm - I don't know in what century you live, but in mine mass produced scopes usually have 1/12-1/16 wave RMS on the wavefront optics and sometimes better.

I'm intrigued, please give examples. Mass produced mirrors with 0.083 RMS minimum?

[sixela]

Read the thread, follow the links, read the reports from Wolfgang Rohr.

1/16th wave RMS on the wavefront isn't that spectacularly good; you must be confusing it with 1/16th wave PTV on the wavefront. RMS errors (root-mean-square) are usually a lot smaller than PTV (peak-to-valley) errors, expressed in waves.

It's even so unspectacular that GSO actually mentions this as a minimum performance criterion for their primary mirrors:

Guan Sheng Optical

Of course, they're still cheating a bit because they mean "in 632nm light", but that's only a difference of a factor 632/550.

[KaStern_Former_Member]

Hello sixela,

I do nnot give anything on the reports of Wolgang Rohr.

Some years ago I let him test my 8"f/6 GSO mirror because in good seeing conditions

I thought I was seeing slight astigmatism.

Rohr tested my mirror with his bath interferometer and told me that strehl ratio was 0.98

and there was no astigmatism in the mirror.

But he disabeled astigmatism ("asti off") in the evaluation software.

At that time I did not know enough about optical testing.

Later I did evaluate the interferogram with a freeware testing software.

There was astigmatism in the interferogram and strehl ratio was lower than Rohr said:

0.90

So I asked a friend of mine to test my mirror with his interferometer he built in the meantime.

Kurt made several test, including tests when the mirror was turned 90° and 180°

The astigmatism turned around with my mirror.

Astigmatism lowered the strehl ratio of the mirro to between 0.90 and 0.91

But to be true:

This result is a good result. I recommend the GSO mirrors. My new mirror (tested by Kurt

(3 interferograms in position 0° and 3 interferograms in Position 90° ) has a very good

strehl ratio, no astigmatism, and since the mirror is mde from a low expansion material

I can use it very quickly after I set my scope up (fan needs to run then to remove

tube currents).

Cheers, Karsten

[sixela]

...

[sixela]

But he disabeled astigmatism ("asti off") in the evaluation software.

Well, he certainly doesn't on the reports I saw (a few of which are referenced above).

Perhaps he was doing a quick'n'dirty test and assumed he was causing test stand astigmatism and that's why he decided to remove astig in that one test; I don't know the details, but he certainly does detect astigmatism on all the tests I've seen from him (and even tests for it with a separate test these days, just to make sure it isn't test stand astigmatism).

He even got in a bit of a wrestling match with a well-known vendor who is known for always removing astigmatism completely from test reports claiming it "must" be test stand astigmatism. In that discussion, Wolfgang Rohr took the position that you shouldn't remove astigmatism from the test data (at least not unless you can prove it's test stand astigmatism that does not rotate with the mirror).

Whatever happened, at least you got the raw data form him (it's worth a lot, because many times there's no way for you to validate the data).

[beamish]

Read the thread, follow the links, read the reports from Wolfgang Rohr.

1/16th wave RMS on the wavefront isn't that spectacularly good; you must be confusing it with 1/16th wave PTV on the wavefront. RMS errors (root-mean-square) are usually a lot smaller than PTV (peak-to-valley) errors, expressed in waves.

It's even so unspectacular that GSO actually mentions this as a minimum performance criterion for their primary mirrors:

Guan Sheng Optical

Of course, they're still cheating a bit because they mean "in 632nm light", but that's only a difference of a factor 632/550.

Fair enough !

[KaStern_Former_Member]

Hello sixela,

Perhaps he was doing a quick'n'dirty test and assumed he was causing test stand astigmatism and that's why he decided to remove astig in that one test; I don't know the details, but he certainly does detect astigmatism on all the tests I've seen from him (and even tests for it with a separate test these days, just to make sure it isn't test stand astigmatism).

I was in the cellar when Rohr did the test. He created roundish pattern and from these

he told me that there is no astigmatism, cause if so the pattern would be oval.

I loocked through and saw that the patern moved around, fron oval to round then

oval to the other side and again round. He made several photos and then picked

the most-round one for the test report.

This is not an acurtae way to evaluate for astigmatism.

The problem is:

The Bath-Interferometer is blind for astigmatism.

If there is astigmatism in an interferogram you cannot tell if this is fron the tested optics,

or it is from the test equipment. So you have to make at least 2 interferograms:

1 and then another 1 where the optics is turned for 90° but nothing else is changed.

Then you see if the astigmatism turns with the optics, or not.

Or you have to use another type of interferometer. Maybe a Twyman-Greene or Michelsen.

but he certainly does detect astigmatism on all the tests I've seen from him

A friend of mine had 2 mirrors tested by Rohr and it was the same story.

One mirror suffered from quite severe astigmatism but that was unnoticed by Rohr.

Wolfgang Rohr took the position that you shouldn't remove astigmatism from the test data

Funny. Removing astigmatism is exactly what Rohr did with my mirror, the mirror of a friend.

There were some other test from him that created a lot of fury amoung german optics enthusiasts.

For example the mirror round robin test (you can get to the results of different testers there):

RR

There was a 200mm Vixen VMC test wich according to Rohr suffered from colour aberration

but was only tested off-axis by him.

There was a 6" f/8 achromat that was tested and then evaluated only on a thin line,

not the whole surface, so that the inherent severe astigmatism went unnoticed.

So after all I stay with my first comment: I do not give anything upon his tests.

I accept your differing opinion.

Cheers, Karsten

[Questions]

If its purely visual what about the cpc range?

Very easy to put up (5 minutes)

Very sturdy (no wobble like the 8se)

Quite managable weight wise (due to convienient handle/finger grip)

Great visuals.

No Eq mount

Celestron also appear to do regular discounts (down to £1295 for the 800), wich really isnt that far from the 8se

[sixela]

I was in the cellar when Rohr did the test.

Digging some more, I certainly see some of his older tests where astigmatism is indeed removed (incorrectly, I might say). Later data that I've seen, though, does not remove the astigmatism at all (AFAIK, he does two different tests which I assume are at different orientations --I certainly would change the test diameter if I tested twice-- and test reports include at least a couple with residual astigmatism after correction for inherent test stand astigmatism by the data reduction software.)

Incidentally, that's after his discussions with ***.

I'm wondering if Wolfgang Rohr changed his test methods over the years after getting flak about them...if so, that would be a Good Thing.

I disagree that you can't measure astigmatism with a Bath interferometer. There is inherent astigmatism present (it is inherently an off-axis test), but if you have a repeatable test stand you know how much there is (you can calculate it from the beam separation, and you can plug in the data in openFringe and it will compensate for it).

And indeed, if you just rotate the mirror 90° you know what part of the measured astigmatism is due to the test stand and what part is due to the mirror.

We don't just have to agree to disagree - I'm very eager to see someone else's view on things like that. I'm not "right", but the recent (>2008) test data that I've seen produced just didn't strike me as unreliable. I may, however, not have seen the complete picture.

[John]

If its purely visual what about the cpc range?

Very easy to put up (5 minutes)

Very sturdy (no wobble like the 8se)

Quite managable weight wise (due to convienient handle/finger grip)

Great visuals.

No Eq mount

Celestron also appear to do regular discounts (down to £1295 for the 800), wich really isnt that far from the 8se

Good for you for getting back to the original posters question