152mm reractor vs SDM 16" shootout

[OneCosmos (Chris)]

OK, now that you have all stopped laughing at the silly notion that a puny 6" refractor could possibly provide any kind of competition against such a large mirror, let me explain. Yes, any 16" scope would leave a 6" refractor for dead on just about any celestial object, but the exception are those object bright enough to mitigate the obvious advantage of the big scope. Size not withstanding, I have always maintained that my views of Jupiter and Saturn have been better than any views I've ever had through even very large dobs.



Now I know many of you will put your faith in physics (and rightly so) and argue that 16" resolution must always win out and show more detail than a smaller scope and any other impression is subjective wishful thinking on the part of refractor owners, possibly fooled by the contrast such instruments are famous for.



Well last night I had the SDM 16" f4.5 set up side by side with the APM 152mm APO refractor both looking at Saturn which was about 30 degrees up in the east.



I was able to perform a very good test where as many of the parameters as possible were similar to not completely void the results. I should say though, right up, I don’t yet have a collimator tool but Jonathan Bradshaw bought his over the previous evening and it was collimated at that time and I haven’t moved it since so it should still be OK but I suspect a little tweaking wouldn’t go amiss.

In the SDM I had the dokters 12.5mm – probably one of the finest eyepieces ever made giving 144x whilst in the refractor I had the vastly inferior Meade 8.8 giving 136x. I was not using any planetary filters.

Both were beautiful views but there isn’t a person alive, (not even Jonathan) who could look through both one after the other and not award first prize to the refractor – no ambiguity no caveats at all; the refractor view was hands down many times better. To say it was pin sharp doesn’t do it justice, it looked etched on to the glass with the finest virtual chisel ever not made. Cassini’s division was more than obvious, it looked so sharp and contrasty that you could cut your fingers on its sharp edges – and that goes for the planet too. There was obvious detail on the surface and beautiful colours to boot. There is simply no finer view possible. The physics isn’t wrong it is just that there is more to it than the maths on the page imply - that must be true.


Yes, I do love refractors but am fully aware of their limitations due to size and I absolutely love large dobs and SDMs especially are the Mozart of the telescope world, simply magnificent. The DSO views are second to none and ease of use, pride of ownership etc all come in to play, but for viewing Jupiter and Saturn I defy anyone to come to Nundah and prove me wrong. By the way, the seeing conditions in Nundah - high up as we are with nothing to disturb the air flow is excellent. On thew refractor I can push magnification to its limits.


Whether a refractor is really worth the money for relatively few objects is another question - but since I already have it I can enjoy the views.


Has anyone done a similar test?



Chris

[JA]

Originally Posted by OneCosmos

Both were beautiful views but there isn’t a person alive, (not even Jonathan) who could look through both one after the other and not award first prize to the refractor – no ambiguity no caveats at all; the refractor view was hands down many times better. To say it was pin sharp doesn’t do it justice, it looked etched on to the glass with the finest virtual chisel ever not made. Cassini’s division was more than obvious, it looked so sharp and contrasty that you could cut your fingers on its sharp edges – and that goes for the planet too. There was obvious detail on the surface and beautiful colours to boot. There is simply no finer view possible. The physics isn’t wrong it is just that there is more to it than the maths on the page imply - that must be true.

That's exactly right... If by "The Physics" you mean, the resolution part of the picture that is. Yes the reflector should have 16/6 ~=2.7 times better resolution, BUT there are a number of factors that conspire to even the margin somewhat:

1. The atmospheric seeing.

2. Differences in the (contrast robbing) light scattering in the instruments

3. The presence of a secondary mirror (or camera in its place) and its effect on contrast, scattering and resolution.

4. The fact that we humans perceive sharpness in an image psycho-optically as a combination of BOTH

• the fineness of graduation (resolution) in an image AND
• the image contrast

I don't think it's possible to ever say unequivocally, out-of-hand, "X beats Y", without a real-world test, such as the one you've conducted, and even then, only for the items tested subject to one's own decision criteria. In this case it's image quality - so that'd be a biggie

Thanks for sharing your results.

Best
JA

[astro744]

I have used Astrophysics 6" f12 Super Planetary, Astrophysics 7" f9 a C14 and a 16" IK6 (Newt mode, I think f5) many times and the refractors won every time. I did notice a difference between the f9 and f12 and although contrast was better in the 6" the 7" simply showed more. I have used the 9" f15 Oddie many a time before the unfortunate fires too and it was THE telescope for planets.

I personally own a 2.4" and 4" APO and enjoy them on planets with almost instant cool down. I also have a 10.1" f6.4 Suchting mirror (one of mine refigured professionally) and first light through it was the 2003 opposition of Mars and I ran out of magnification with my 6mm Clave Plossl and 2x Clave Barlow; the image was simply that good (as was the seeing that night).

I then viewed Saturn one night a few years ago through a friends 16" f5 Suchting mirror with a relatively small secondary (2.6" I think) and it simply trumped everything previously seen through any of the refractors. The telescope was cooled and the seeing was so still that night and I could see more bands and colours on Saturn than ever before through any telescope. I just wish Mars had been around then.

I do enjoy my refractors and they provide excellent views more often but given excellent seeing (and of course collimation, cool down, quality of mirror both primary & secondary, obstruction etc.) a larger reflector will beat a smaller refractor. I do like the images I see through my refractors but I can no longer directly compare a 6 or 7" refractor against a 16" f5 Newtonian but the images I saw through the two Suchting mirrors have etched themselves deeply and the telescope of choice for this years Mars opposition will be my 10.1". I just hope I get a night of exceptional seeing to give me that memorable view once again.

[AstroJunk (Jonathan)]

Yet on Saturday night, the SDM blew away the refractor, especially at higher magnifications. My guess is that Saturn was just too bright in the 16" and hence the perceived lack of contrast at such a low power. But I wasn't there...

[clive milne]

It's all about thermal management.

Refractors will consistently work straight out of the box..

A large Newtonian requires one to take care of convective plumes in the light path, both from the thermal mass of the optic and the ground upon which it sits. Once you get that right, there is only one thing that will beat a large Newtonian for planetary and that is a large binocular Newtonian.

And yes, I used to own a 7" AP so I have that as a reference point.

~c

[OneCosmos (Chris)]

Originally Posted by AstroJunk

Yet on Saturday night, the SDM blew away the refractor, especially at higher magnifications. My guess is that Saturn was just too bright in the 16" and hence the perceived lack of contrast at such a low power. But I wasn't there...

But on that Saturday the only thing we really compared was the moon and the refractor was none too shabby! What I'm more encouraged by is the fact that on the Friday night out at Samford the views of Jupiter then through the SDM were equisite. Such a shame I couldn't have had the refractor there for the direct comparison on that night.

[OneCosmos (Chris)]

Originally Posted by astro744

I then viewed Saturn one night a few years ago through a friends 16" f5 Suchting mirror with a relatively small secondary (2.6" I think) and it simply trumped everything previously seen through any of the refractors. .

That would have been awesome to see. I'm hoping the Hughes' will be at Astrofest this year with their often talked about 14" Suchting mirror. I've seen through it before but this year I can properly compare it to the 16" - which has a modest GSO by comparison, but I think Jonathan would admit is no bad performer nevertheless.

[OneCosmos (Chris)]

Originally Posted by JA

... BUT there are a number of factors that conspire to even the margin somewhat:

1. The atmospheric seeing.

2. Differences in the (contrast robbing) light scattering in the instruments

3. The presence of a secondary mirror (or camera in its place) and its effect on contrast, scattering and resolution.

4. The fact that we humans perceive sharpness in an image psycho-optically as a combination of BOTH

• the fineness of graduation (resolution) in an image AND
• the image contrast

It sounds to me as though the consensus is that given perfect seeing, a very high quality mirror, a small secondary and good eyepiece the DOB should always win over the refractor. The fact that it doesn't is because the refractor just copes better with compromises on any or all of those constituent parts so is more likely to produce a good image more of the time.

[gregbradley]

I have a CDK17. I have only used it visually at home which is semi rural skies. It was hopeless. So affected by the seeing it was not sharp at all.
But I imagine at a dark site allowed to thermally equalise it could be very good as it can do some very sharp imaging.

I have also used:

AP140
FSQ106
FS102
FS152
TEC180
TEC110.

The FS152 gave the most astounding views. A nice doublet does not take long to cool off. Fluorite is great for visual.

A 4 inch APO is too dim really for my taste. The TEC180 was quite good as well but the FS152 is more memorable. Perhaps I just used it more often for visual. Coupled with a Nagler 24mm it was magic.


I have looked through some large dobs and they definitely can show the dimmer objects very well. I also liked my Celestron 11. But the FS152 was the best.

Greg.

[OneCosmos (Chris)]

Originally Posted by gregbradley

The TEC180 was quite good as well but the FS152 is more memorable. Perhaps I just used it more often for visual. Coupled with a Nagler 24mm it was magic.


Greg.

Thanks Greg. Interesting that the 152mm was better than the Tec180. I love the views through my APM 152 but at the end of the day, 6 inches is just 6 inches. It excells at the really bright objects though.


Chris

[gregbradley]

A fluorite doublet seems to be a good choice for visual.

Greg.

[Allan]

Chris, welcome to the SDM family, I look forward to seeing SDM#41 at our star party next year.

People like to debate the old refractor vs dob thing, and I find it interesting to hear peoples views. There's no right or wrong answers, as beauty is always in the eye of the beholder. The preference is always largely dependant on a persons situation as well. Someone who owns a dark site and enjoys deep sky won't see as much value in a large refractor. Another person in the city will probably feel the same about a big dob.

Personally, I am a dob fan, but I also see refractors having a place in my lineup. I will describe it like this. Many times over the years I have been incredibly impressed by the images I have seen through refractors, including some beautiful TEC 160's and 180's, and AP180's. But no refractor has given me goosebumps or made my hair stand on end or made me yell out in awe like the images I have seen through big dobs around the world. I have seen things that are so crazy amazing, that they invoke an involuntary physical reaction. Refractors have never done that for me.

As always though, everyone's opinion is their own based on their own experience and situation.

[OneCosmos (Chris)]

Originally Posted by Allan

Chris, welcome to the SDM family, I look forward to seeing SDM#41 at our star party next year.

Thanks Allan. I have to attend because it is imperative to see the universe through a 32” Lockwood-equipped SDM . I don’t think I’ll bother to bring the 6” refractor

No refractor anyone could afford anyway would induce such a physical response on DSO’s, well not a favourable one anyway. This thread, however was specifically about views of Saturn and the refractor is most certainly capable of inspuring at the deepest level in a way that the often slightly softer view through the dob would not. I really, really hope to be proved wrong next year when I look through the best large mirrors in the world although it would be terrible for my bank balance!

[astro744]

I totally agree that the views through a refractor particularly a 6" can be most memorable and whilst I know and have seen the level of detail that larger high quality reflectors can attain, the refractor does deliver a very impressive image more often than not and it takes a night of really bad seeing for a refractor image to be poor.

Last night at the star gazing event I used a TV-101 and a 4.8mm Nagler on Jupiter, Saturn, The Moon (very nicely framed), Antares, Sirius, Jewel Box (9mm Nagler for better framing), Alpha Centauri, Acrux and Zubenelgenubi. The views were simply stunning even through all that fog that was settling and those that had a look were impressed. I find refractors in general more often than not give THE BEST views of planets and only in the best seeing conditions will I reach for my Newtonian which I know will show more.

I would simply love to have a 6" refractor simply for the extra light that a 4" cannot give at the higher powers but I know that is what my Newtonian is for so I am contempt with what I have. In fact one of my most used telescopes is my little TV-60 which simply provides stunning views of planets at the drop of a hat. It has instant cool down and is extremely portable on the Tele Pod.

[Profiler (Profiler)]

Hello

I have just noticed this thread and ironically I have asked a similar but slightly different question on another thread.

http://www.iceinspace.com.au/forum/s...d.php?t=166938

On this thread the question, if I may be permitted to roughly summarise, is which offers the better view between a top quality big refractor and a top quality big Dob.

HOWEVER - my question on the other thread concerns my attempt to help some newbies decide between a very interesting budget refractor - a 4 inch f/13 achro versus a budget 8 inch Dob.

My friends want the telescope for one purpose only which is to get the best bang for their buck when looking at Mars during the pending opposition. I can lend them a tripod (for the refractor) and good eyepieces (for both OTAs) so the real question is which OTA (only) will they get the most from.

The comments on my original thread

http://www.iceinspace.com.au/forum/s...d.php?t=166938

seem to largely favour the Dob. However, on this thread it seems most folks seem to think a good quality refractor beats a good quality Dob - so would the same hold true for the alternate situation I have just outlined.

Any comments would be gratefully appreciated.

[Tropo-Bob (Bob)]

Originally Posted by Profiler

Hello

I have just noticed this thread and ironically I have asked a similar but slightly different question on another thread.

http://www.iceinspace.com.au/forum/s...d.php?t=166938

On this thread the question, if I may be permitted to roughly summarise, is which offers the better view between a top quality big refractor and a top quality big Dob.

HOWEVER - my question on the other thread concerns my attempt to help some newbies decide between a very interesting budget refractor - a 4 inch f/13 achro versus a budget 8 inch Dob.

My friends want the telescope for one purpose only which is to get the best bang for their buck when looking at Mars during the pending opposition. I can lend them a tripod (for the refractor) and good eyepieces (for both OTAs) so the real question is which OTA (only) will they get the most from.

The comments on my original thread

http://www.iceinspace.com.au/forum/s...d.php?t=166938

seem to largely favour the Dob. However, on this thread it seems most folks seem to think a good quality refractor beats a good quality Dob - so would the same hold true for the alternate situation I have just outlined.

Any comments would be gratefully appreciated.

Hmm, not really the same question at all. This thread compares quality scopes.

The other thread asked about budget scopes, with the empathises on the costs being modest. The question stated in the other thread also changed and I had to read this thread to understand that the question was now only to be about OTAs. There is still incomplete information, for example, will the refractor be provided with a quality or standard star diagonal? Such things may change responses in that thread.

I took the mention of a 4inch F13 in the other thread to be wishful thinking, as, I will be surprised if a quality 4inch F13 achromatic can be found at a modest cost in time to follow the rapidly approaching opposition of Mars.

[Profiler (Profiler)]

Hi Bob

Sorry for the confusion as the thread evolved and indeed I originally thought of the skywatcher 102 achro F10 refractor. Subsequently, another post highlighted the existence of an even better 102 F13 refractor made by Bresser which I didn't previously knew existed.

You raise a good point about a diagonal which I could also supply if needed. You raise another good point which is I have no idea as to the quality of the Bresser telescope other than the spec's.

The majority of opinions on the other thread seem to support the Dob whereas here a refractor seems to be the preference. That is, the underlying principle that aperture wins in the other thread seems to be refuted here and the smaller refractor beats the big aperture dob

[RickS (Rick)]

I'm struggling to understand this thread, Chris. Don't you have a camera?

[raymo]

The f/10, and to a lesser extent the f/13, being achromats, will exhibit some CA, especially on the moon, unlike the apochromats discussed in this thread.

[OneCosmos (Chris)]

Originally Posted by RickS

I'm struggling to understand this thread, Chris. Don't you have a camera?

Haha I’m thinking giving a talk at Astrofest to explain that not having a camera on the end doesn’t mean the telescope is broken