SCT vs DOB on planetary viewing??

[ausastronomer (John Bambury)]

Originally Posted by icytailmark

SCT's are the closest to a perfect scope.

That depends on how discerning a visual observer you are. If you want one scope for both visual and imaging they are a great choice and probably the best choice.

If you are a specialist visual observer their optical performance level is below that of a premium grade Newtonian IMO. As I indicated in another recent thread physics supports my experience having looked through in excess of 100 different SCT's over the past 30 years from both Meade and Celestron ranging in aperture from 5" to 16". I am yet to look through one that can equal a top grade Newtonian of equal aperture as a visual planetary telescope. They do a very good job of planetary imaging because unlike for visual use the larger central obstruction and increased number of reflective and refractive surfaces does not affect their performance and their long focal length becomes an advantage due to the larger image scale.

I would happily put my money and my 14"/F4.5 Zambuto powered SDM up against any SCT, currently in Australia, in a visual shootout on the Moon or planets. In fact I would happily put my money on Rick Petrie's 14" Skywatcher against any SCT that anyone cares to throw in to the mix. Rick's 14" Skywatcher has a better mirror in it than any mass produced budget telescope has a right to have, but it is what it is. The downside of mass produced scopes, of course, is that you take the good with the bad and not everyone gets a mirror as good as Rick's.


Cheers,
John B

[Camelopardalis (Dunk)]

Originally Posted by ausastronomer

I would happily put my money and my 14"/F4.5 Zambuto powered SDM up against any SCT, currently in Australia, in a visual shootout on the Moon or planets. In fact I would happily put my money on Rick Petrie's 14" Skywatcher against any SCT that anyone cares to throw in to the mix. Rick's 14" Skywatcher has a better mirror in it than any mass produced budget telescope has a right to have, but it is what it is. The downside of mass produced scopes, of course, is that you take the good with the bad and not everyone gets a mirror as good as Rick's.

I'd love a look through your scope, John, should you wish to let me if our paths ever crossed, seriously curious to see what I'm missing, but then there's the inevitable wanting for a cherry-picked, unique, or at least very expensive scope

The crux of the issue is summed up by your last sentence...for the rest of us, unless we dig deeply we're all at the mercy of whatever Skywatchers, Meade, GSO or Celestron churns out. So unless we win the lottery, it's a lottery

[AG Hybrid (Adrian)]

To be fair. SKywatcher/synta mirrors consistently perform well. You rarely hear a story about anyone who has bought one recently that has optics that needs to be refigured. Meade/GSO mirrors on the other hand. You hear stores of getting truly exceptional optics and sometimes complete dogs.

If I had to choose between either Synta or GSO. I'd choose Synta every time.

That being said. I thoroughly enjoyed the views I saw in Dunk's C11HD of Jupiter. The color contrast on the bands of Jupiter were excellent. The upside of the Celestron HD series scopes is they are made in the US and quality control is better. The standard models are built in Taiwan/China

[MortonH]

It also depends where you do most of your observing from. I'm stuck on a small balcony most of the time, so an 8" SCT would fit, but even my 8" f/5 Newtonian is too big. A 14" Dob is definitely out of the question!

[Shano592 (Shane)]

Robert, if you get the chance, try to borrow a Televue, or Pentax, or other high-end eyepiece off of someone nearby.

As has been mentioned previously in this thread, the telescope is only half of the optics. Generally, the glass in the higher-end eyepieces is magnificent, and can really make your night.

I have the 10" LX200, and view Jupiter with a 10mm Ethos. Aside from being visually very bright, and quite hazy due to the low altitude, Jupiter generally resolves quite nicely... although a moon filter or some other dimming filter of the like wouldn't go astray.

Would a Barlow help to dim the view? (A genuine question from me, as I haven't used one.) I know they extend the focal ratio, and in my head, that makes the image "slower," therefore dimmer.

[Gem (Grant)]

Originally Posted by Shano592

Would a Barlow help to dim the view? (A genuine question from me, as I haven't used one.) I know they extend the focal ratio, and in my head, that makes the image "slower," therefore dimmer.

Yes it would. Its like halving your EP focal length (if a X2 barlow). But you only want to turn up the magnification if the seeing conditions are good enough. My Televue 4.8mm Nagler rarely gets used since the conditions never suit that magnification.

[Amaranthus (Barry)]

The dimness is nothing to do with slow/fast f/ratio, visually. It is simply the the Barlow will yield a longer FL/magnification.

[bigjoe (JOSEPH)]

Hi Rob.

I find that No.1 Altitude and seeing

Then. No.2.Central obstruction size

Then. No.3 optical quality of eps and scope and scope type etc.

All DO MATTER greatly esp on Jupiter if it is low in the sky, as its features are low in contrast. A good dob should beat an Sct.
PS: use a light/ mid blue filter. It DOES make a difference.
Maybe this is why my 7" bd MAKSUTOV. slays my 10"SCT ,6"SCT, 80MM REFR
ETC on Jupiter, in fact all the planets and some Dsos.
Cheers bigjoe.

[bratislav (Bratislav)]

Originally Posted by ausastronomer

If you want one scope for both visual and imaging they are a great choice and probably the best choice.

Disagree on all accounts. Visually SCTs suffer from huge central obstruction. There is no way around that - you can have a perfect SCT optically (and they are nowhere near that) but damage to contrast transfer has already been done. Not really that objectionable on Moon or Saturn (rings), but any ultra low contrast feature (all of Jupiter, detail in Saturn belts and fine features of Mars) will be hard hit.
Imaging wise, SCTs are hard work because they are dreadful in keeping collimation (I have to check and usually recollimate every time I change elevation more than about 20 degrees with my C11); but main issue is enclosed primary which lags in temperature and causes internal heat plumes. Short of actively tackling that via pelitiers and internal fans, you are at the mercy of local temperature gradient. Quite often SCT users will report "bad seeing" while Newtonian and Cassegrain counterparts will work close to their maximum resolution - same place, same time.

The only real advantage of SCT is comfortable observing. Using binoviewer (compulsory for people with floaters like yours truly) and having comfy chair is a real game changer. I know you can use bino in a Newtonian, but they often end up in all sorts of neckbreaking angles and it is impossible to seat back and relax. Planetary observing is a waiting game, being comfortable is a definite advantage.

Bratislav

[Gem (Grant)]

Originally Posted by bratislav

Disagree on all accounts.

While some of this is subjective - I find it hard to see how you think a dob is better than a SCT for imaging. Unless you are comparing a GEM Newt (rather than a dob) to a GEM SCT, the SCT will always be a better imager. If you are comparing GEM scopes, then the newt's size and weight means you will need a massive mount to have anything over 10". A similar aperture of SCT will be lighter and easier to get good tracking than a newt on the same mount.

I take you're point on everything you say about optics. I always choose my dob over my SCT for visual observing (partly since it is choosing 16" over 9.25") - but my SCT will always be the best compromise scope for size/portability/ability to image/visual observe. Jack of all trades, but master of none.

[Don Pensack]

An SCT is hampered in planetary observing by 3 major factors:
1) a large secondary obstruction. This will prevent seeing the smallest details because of insufficient contrast. I would want to note, however, I'd rather have an SCT in superb seeing than a dobsonian of the same size in mediocre seeing, if planetary observing is the aim.
2) Cooling. Simply put, the night is not long enough for a 12" SCT to achieve thermal equilibrium. Some form of active cooling is essential (Lymax SCT Cooler, which can be used in an ACF, but not in an Edge HD; or a home-made equivalent) or the mirror will never cool down and allow resolution the aperture is capable of.
3) contrast robbing scattered light. Now, a dob can have this too, but it is easier for extraneous light to get into the scope and bounce around in the typical SCT design. What can be done? Flock the inside of the dewshield (and always use one), the inside of the tube, the outside of the primary baffle, the inside of the secondary baffle, and the inside of the primary baffle. Make sure the inside of the star diagonal is really non-reflective. I did all this to an 8" SCT and the difference was profound. It yielded superior contrast at all powers and allowed a couple tenths fainter stars to be seen on a regular basis at high powers.

Dobs have smaller secondaries and typically have fans to cool the optics. Other than that, there is no particular reason an SCT shouldn't give good planetary images so long as the optics are good. It may be easier to get 2 excellent surfaces in a dob than 5 excellent optical surfaces in an SCT. I note that optical testers find excellent optics more frequently (though not a high percentage of the time) in dobs than in SCTs. It's hard to tell about SCTs, though. 95% of the ones I've looked through aren't anywhere near cooled down.

[bratislav (Bratislav)]

Originally Posted by Gem

While some of this is subjective - I find it hard to see how you think a dob is better than a SCT for imaging.

One word - equatorial platform.
Sorry that is actually two words

[Amaranthus (Barry)]

SCTs are hard work because they are dreadful in keeping collimation (I have to check and usually recollimate every time I change elevation more than about 20 degrees with my C11)

Wow, I find that staggering. What criteria are you using to judge collimation accuracy? I personally have never needed to collimate my SCT -- it holds it beautifully -- let alone feeling the need to do it every time one raises the OTA's altitude axis!

[bratislav (Bratislav)]

Originally Posted by Amaranthus

Wow, I find that staggering. What criteria are you using to judge collimation accuracy?

I normally use high power extrafocal images (500x or more).
In this case I actually use CCD chip (QHY5L II) at focal length of over 7m.
If we think of that chip as a shortish eyepiece (chip is only 5mm across; a typical 5mm eyepiece would have itself ~5mm field stop) we are talking about 1500 times effectively to check collimation when imaging planets. And believe me it is necessary if you want a last ounce of resolution SCT can offer.

Try it yourself. I haven't really seen through too many SCTs, but of couple of dozens or so that I did, they ALL had the same problem.

If you don't believe me, check what Thierry Legault has to say about SCTs and collimation.

"People who think that a SCT does not need to be collimated very often probably do not realize the level of precision required for this type of instrument. The constraints are so high that a simple car trip always changes the alignment in small amounts, and sometimes in large amounts. Collimation can even vary according to the orientation of the optical tube (with a German mount, an interesting experiment is to point at the same star at the meridian successively from the left and the right of the mount, to observe the modification of collimation due to the reversal of the tube). This is the reason why it is advised to choose a star in the same area as the object of interest (planet or Moon). If a small misalignment can be tolerated in deep-sky observation, it is taking a big risk not checking the collimation before a planetary session. The idea is that the alignment becomes as familiar as checking the oil level or the pressure of the tires in a car before a trip !"

http://legault.perso.sfr.fr/collim.html

[ausastronomer (John Bambury)]

Originally Posted by bratislav

Disagree on all accounts. Visually SCTs suffer from huge central obstruction. There is no way around that - you can have a perfect SCT optically (and they are nowhere near that) but damage to contrast transfer has already been done. Not really that objectionable on Moon or Saturn (rings), but any ultra low contrast feature (all of Jupiter, detail in Saturn belts and fine features of Mars) will be hard hit.
Imaging wise, SCTs are hard work because they are dreadful in keeping collimation (I have to check and usually recollimate every time I change elevation more than about 20 degrees with my C11); but main issue is enclosed primary which lags in temperature and causes internal heat plumes. Short of actively tackling that via pelitiers and internal fans, you are at the mercy of local temperature gradient. Quite often SCT users will report "bad seeing" while Newtonian and Cassegrain counterparts will work close to their maximum resolution - same place, same time.

The only real advantage of SCT is comfortable observing. Using binoviewer (compulsory for people with floaters like yours truly) and having comfy chair is a real game changer. I know you can use bino in a Newtonian, but they often end up in all sorts of neckbreaking angles and it is impossible to seat back and relax. Planetary observing is a waiting game, being comfortable is a definite advantage.

Bratislav

Bratislav,

I don't think you actually took the time to correctly read and comprehend what I was implying. At no time did I imply that a SCT was the best at anything, either visual or imaging. Truth is I wouldn't own a SCT if someone gave it to me.

Originally Posted by bratislav

Disagree on all accounts. Visually SCTs suffer from huge central obstruction. There is no way around that - you can have a perfect SCT optically (and they are nowhere near that) but damage to contrast transfer has already been done.

Isn't that exactly what I said in my post above, excepting that I expanded on it somewhat ?

Originally Posted by ausastronomer

As I indicated in another recent thread physics supports my experience having looked through in excess of 100 different SCT's over the past 30 years from both Meade and Celestron ranging in aperture from 5" to 16". I am yet to look through one that can equal a top grade Newtonian of equal aperture as a visual planetary telescope. They do a very good job of planetary imaging because unlike for visual use the larger central obstruction and increased number of reflective and refractive surfaces does not affect their performance and their long focal length becomes an advantage due to the larger image scale.

Cheers,
John B

If you read some of the posts I made in this recent thread you will see exactly what I think of SCT's as visual instruments. Here are some of my posts replicated below

Originally Posted by mental4astro View Post
Marc, you make SCT's sound like poison !

SCT's are fine visual instruments.

Originally Posted by SkyWatch View Post
I assume that when John talks "physics", he is referring to the large secondary of the SCT's that causes a loss of contrast and therefore slightly poorer view to an equivalent newt or refractor. This however would only be if the newt had an optimised (minimum size) secondary

Here is the table I posted in the other thread.

Now, if I don't really like SCT's, particularly as a visual instrument, why would I make the comment "If you want one scope for both visual and imaging they are a great choice and probably the best choice."

They are "reasonably" portable, transportable and storable. I live on an acre of land with a 3 car garage as a storage shed. I have no problems storing 18", 14" and 10" dobs. If I lived in a 2 bedroom unit in Sydney I doubt that would be the case. SCT's do a "reasonable" job of both planetary and deep sky visual astronomy and planetary and deep sky imaging. While not the best at anything they can do a reasonable job on all fronts. Because of their short tube length and thus moment arm they can be used for imaging on a medium sized EQ mount. Dobsonians make easily the best visual instruments IMO, for both planetary and deep sky, but they are a poor choice for imaging. IMO an EQ table doesn't help. Just ask Paul Haese who spent about 20K on an 18" SDM dob for planetary imaging and sold it 6 months later and bought a SCT. Refractors give aesthetically nice images but because of their limited aperture they cannot match a medium / large aperture high quality Newtonian as a visual instrument. Ant that is how I came to make the comment I did, nothwithstanding that I would never own a SCT.

Cheers,
John B

[AG Hybrid (Adrian)]

Interesting read John. I also agree with a 10" goto dob over an 8" sct for value and performance.

On a side note:

Isn't it your birthday? Why are you trippin about SCT's on IIS instead of chilling out and enjoying some cold ones?

[ausastronomer (John Bambury)]

Originally Posted by AG Hybrid

On a side note:

Isn't it your birthday? Why are you trippin about SCT's on IIS instead of chilling out and enjoying some cold ones?

Hi Adrian,

Because at the moment I am actually at work pouring all the cold ones for my patrons to enjoy

I made a decision yesterday afternoon, when the weather forecast was good, to go fishing this morning and work tonight instead of working during the day today.

Cheers,
John B

[Amaranthus (Barry)]

If you don't believe me, check what Thierry Legault has to say about SCTs and collimation.

Thanks Bratislav, I'd read that article by Legault quite a while ago so it was good to revisit it. But I think we need to be clear here about expectations! I use my SCT as a flexible visual workhorse, from DSOs through to planetary. For me, high magnification is 350x (and I rarely go above this) and I typically work in the 80-140 magnification range. So my absolute tolerance for slight miscollimation is moderate, and I rarely see my 8SE requiring an stage-3 'in-focus' collimation. I check this maybe once per month and may need to do slight adjustments a few times a year. I call that robust!

It is, like anything, a matter of trade-offs, and I appreciate that if you are willing to spend a LOT of time CONSTANTLY collimating, then you might just squeeze a little extra resolution out of your SCT. Or, you can take the attitude that 'near enough is good enough' and keep collimation at a reasonable 'fit' (based on diminishing returns), and so have more time for observing rather than fiddling. Or, buy a larger aperture

I guess I follow Rob Mollise's philosophy on this, rather that Legault's perfectionism: http://skywatch.brainiac.com/collimation.pdf ... But each to their own.

[ausastronomer (John Bambury)]

Originally Posted by Camelopardalis

I'd love a look through your scope, John, should you wish to let me if our paths ever crossed, seriously curious to see what I'm missing, but then there's the inevitable wanting for a cherry-picked, unique, or at least very expensive scope

Hi Dunk,

This is the scope I use almost exclusively for the outreach work I do with 3RF. It's ideal for this as the eyepiece height is perfect for all adults and kids only need a single step.

I have always made this scope available for anyone to look through.

Last year I had it at a new outreach event called "Stars on The Fly" which was an event that I put together with the management of the Illawarra Fly Treetop Walk. Sadly, their Marketing Manager, who I coordinated the event with left at Xmas; and the event will not run this year. I have also had it up at Lostock (IISAC 2010) and Border Stargaze (2009) and a big public event with about 500 people at Armidale in 2010. I will probably take it to the Parramatta Park Public event which will be held in September or October this year.

I am hoping to be able to attend the Pony Club (Mangrove Mountain) observing session for the new moon in May. I will be taking the scope to that event if I can attend. If I attend you are more than welcome to come and have a look through it.

Word of fair Warning

A couple of people have subsequently ordered an SDM telescope after looking through this scope. One US visitor to our annual 3RF Coonabarabran trip ordered an SDM telescope about 15 minutes after first using my scope. He could have ordered a scope from any one of a number of US manufacturers when he returned home, but he wanted one " just like that only bigger".

Cheers,
John B

[bratislav (Bratislav)]

Originally Posted by ausastronomer

Bratislav,

I don't think you actually took the time to correctly read and comprehend what I was implying. At no time did I imply that a SCT was the best at anything, either visual or imaging.

Is this your quote then ?

"If you want one scope for both visual and imaging they are a great choice and probably the best choice." (my italics)

That was the only statement I argued with. SCTs are not 'great' at visual, nor imaging, nor they are the best choice.
They are convenient (for both application), but that is all.

"IMO an EQ table doesn't help. Just ask Paul Haese who spent about 20K on an 18" SDM dob for planetary imaging and sold it 6 months later and bought a SCT."

As you know, SDMs are not equatorials, but AltAz scopes. They suffer from field rotation, which is hard to combat, but they could still be reasonable imaging scopes. I am not sure why Paul went Dob -> SCT route. I am in the process of building a platform for my 16" Dob, I guess I will find out soon enough.