Refractor for planets, mostly visual, 4" or 5"

[vlaiv]

41 minutes ago, mikeDnight said:

An experience I had some time back may be of interest. I'd been loaned a lovely 8" SW Dob from paulastro to use alongside my then SW 120ED Equinox. I'd set both scopes up alongside eachother around 3pm on a spring afternoon so as to let each acclimatise. Around 6pm, another friend paid me a visit, and we just sat chatting for about three hours. As my friend got up to leave, I suggested we both have a look at the Moon and Saturn, both of which were high in the sky. I aimed the 8" Dob at the Moon and was in awe at the superb view of a detailed lunar landscape that was as steady as can be imagined. It was a terrific view, and in my mind I thought the 120ED had no chance of coming close to replicating such a view. When I aimed the ED at the Moon, I was again stunned, not by greater detail, but by the sharpness of the view. The superior sharpness of the 120 ED more than compensated for the greater resolution of the Dob. Neither of us could believe the difference would be so obvious or that the scales would tip so greatly in favour of the refractor.   Next it was the turn of Saturn, and again the 8" Dob gave a view that was nothing short of spectacular. Again I thought to myself that the ED wouldn't stand a chance of matching, let alone beating the view through the 8".  And again I was utterly stunned by the 120ED's planetary punch.  In the 8" Saturn was rock steady, showing all the major ring components with ease, so that the A ring, Cassini division, B ring and Crepe ring were all immediately obvious. The globe too showed the EQ and temperate belt along with dark polar hood easily.  The 120ED again delivered a significantly sharper view, which became most obvious when comparing the ring detail. The refractor hinted quite forcefully at fine grooves in the B ring that were not visible in the 8" Dob. The A ring easily showed the Enke minima and even hinted at a razor thin dark Enke gap close to the outer edge of the A ring. Cassini's division also showed a soft outer edge that almost bled into the A ring. Radial spokes were seen in the refractor but not in the Dob, which is another indication of the better definition of the refractor. The 8" Dob was a superb scope, but the 120ED had the edge on definition. 

I'm don't quite understand the comparison between two scopes, could you be more specific about that - in particular point about refractor being sharper but not having greater detail than in 8" dob and that it compensated for greater resolution of the dob by this sharpness.

I'm trying to imagine the scene - but failing. How can one thing be sharper while other has greater detail and greater resolution? This could be issue with terminology.

What exactly do you mean by:

a) sharpness

b) detail

c) resolution

You also mention that there was hint of razor thin Encke gap. What magnification was this at? As far as I know - Encke gap is only 325km and when Saturn is closest to the earth - that makes ~0.0559". Airy disk size of 120mm aperture is 2.14" or about x40 that of Encke gap when Saturn is closest to earth.

In above marble test, for anyone wanting to try this - it would be similar to trying to observe 16µm slit placed next to a 1cm marble at 60m distance.

[JeremyS]

1 hour ago, mikeDnight said:

An experience I had some time back may be of interest. I'd been loaned a lovely 8" SW Dob from paulastro to use alongside my then SW 120ED Equinox. I'd set both scopes up alongside eachother around 3pm on a spring afternoon so as to let each acclimatise. Around 6pm, another friend paid me a visit, and we just sat chatting for about three hours. As my friend got up to leave, I suggested we both have a look at the Moon and Saturn, both of which were high in the sky. I aimed the 8" Dob at the Moon and was in awe at the superb view of a detailed lunar landscape that was as steady as can be imagined. It was a terrific view, and in my mind I thought the 120ED had no chance of coming close to replicating such a view. When I aimed the ED at the Moon, I was again stunned, not by greater detail, but by the sharpness of the view. The superior sharpness of the 120 ED more than compensated for the greater resolution of the Dob. Neither of us could believe the difference would be so obvious or that the scales would tip so greatly in favour of the refractor.   Next it was the turn of Saturn, and again the 8" Dob gave a view that was nothing short of spectacular. Again I thought to myself that the ED wouldn't stand a chance of matching, let alone beating the view through the 8".  And again I was utterly stunned by the 120ED's planetary punch.  In the 8" Saturn was rock steady, showing all the major ring components with ease, so that the A ring, Cassini division, B ring and Crepe ring were all immediately obvious. The globe too showed the EQ and temperate belt along with dark polar hood easily.  The 120ED again delivered a significantly sharper view, which became most obvious when comparing the ring detail. The refractor hinted quite forcefully at fine grooves in the B ring that were not visible in the 8" Dob. The A ring easily showed the Enke minima and even hinted at a razor thin dark Enke gap close to the outer edge of the A ring. Cassini's division also showed a soft outer edge that almost bled into the A ring. Radial spokes were seen in the refractor but not in the Dob, which is another indication of the better definition of the refractor. The 8" Dob was a superb scope, but the 120ED had the edge on definition. 

One thing puzzles me about this story Mike: you sat around for 3 hours chatting to a friend while the sky was clear and the planets out? 🤔

[dweller25]

Here is a comparison my wife and I did some years ago between a 5” APO and a Skywatcher 8” F/6 Newt, Mars gave one clear result, Saturn gave the opposite result....

 

[Robindonne]

Maybe those guys had that night 8 of these

 

and 120 of these

 

And suddenly all scopes looked equally sharp

Edited January 30, 2021 by Robindonne

[johninderby]

Or 😁

[JeremyS]

50 minutes ago, Robindonne said:

Maybe those guys had that night 8 of these

 

and 120 of these

 

 

And suddenly all scopes looked equally sharp

Is that Albariño on a Dobsonian mount? 🤣

[mikeDnight]

1 hour ago, JeremyS said:

One thing puzzles me about this story Mike: you sat around for 3 hours chatting to a friend while the sky was clear and the planets out? 🤔

Yep, I know it sounds strange, but you haven't met my friends - yet! 😨

[mikeDnight]

3 hours ago, HollyHound said:

I'm finding exactly the same now that I routinely use two Hyperion Zoom eyepieces on my Mak 127 lunar/planetary setup... much more detail and the floaters all but disappear at high magnifications 👍

 

Pineapples are the answer to no more floaters!

[John]

Meanwhile, back at the original question asked ......

[Deadlake]

I have a Vixen SD103S scope. It’s been a delight to use, however the one thing I regret is not going for the SD115S version as that extra aperture would of brought the resolution of the scope within the usual maximum resolution of the UK skies and given extra reach on planetary viewing. The vixen scopes are lightweight like the Tak, which neglects the usual step up in weight with other manufacturers.

The other item is that replacing the Vixen diagonal with a Baader prism and using Vixen HR EP’s has enabled really high resolution in the scope, x495 which I thought was the preserve of premium APO’s. The point being it’s all the visual train, that I would consider. Of course there are some Tak owners out there who like barlowing the Vixen HR’s as well as say it’s perfectly useable. 

Here is a good thread running at the moment on CN discussing quality of a lense verses aperture.

https://www.cloudynights.com/topic/752583-does-aperture-outway-strehl-ratio/page-3#entry10846933
 

The answer is predictably optics quality can offset aperture within a certain difference, however as ever it’s a payoff between in this case a premium 4” will cost more then a good 5” which will show a similar view. 

Given the models you first linked to I’d go with aperture, I don’t think the weight increase will limit the use of the scope. Also doublets will cool down very quickly which always helps.

 

 

[Deadlake]

11 hours ago, vlaiv said:

I'm don't quite understand the comparison between two scopes, could you be more specific about that - in particular point about refractor being sharper but not having greater detail than in 8" dob and that it compensated for greater resolution of the dob by this sharpness.

I'm trying to imagine the scene - but failing. How can one thing be sharper while other has greater detail and greater resolution? This could be issue with terminology.

What exactly do you mean by:

a) sharpness

b) detail

c) resolution

You also mention that there was hint of razor thin Encke gap. What magnification was this at? As far as I know - Encke gap is only 325km and when Saturn is closest to the earth - that makes ~0.0559". Airy disk size of 120mm aperture is 2.14" or about x40 that of Encke gap when Saturn is closest to earth.

In above marble test, for anyone wanting to try this - it would be similar to trying to observe 16µm slit placed next to a 1cm marble at 60m distance.

Does any of a to c cover contrast, I think that’s more important to the human eye when looking at planets?

[johninderby]

I think it’s the combination of all the factors including seeing and not so simple as just resolution or contrast. On some nights one may suit the seeing conditions better than another.

Is that why most own more than one scope. 🤔😁

Edited January 31, 2021 by johninderby

[Deadlake]

27 minutes ago, johninderby said:

I think it’s the combination of all the factors including seeing and not so simple as just resolution or contrast. On some nights one may suit the seeing conditions better than another.

Is that why most own more than one scope. 🤔😁

Indeed refractors are about being outside and setup in 15 minutes, most useable scope. No need to wait for acclimating or collimation overhead. It’s been pointed out before with the UK skies being able to use large apertures can be limited to a few nights a year.

[Stu]

2 hours ago, Deadlake said:

I have a Vixen SD103S scope. It’s been a delight to use, however the one thing I regret is not going for the SD115S version as that extra aperture would of brought the resolution of the scope within the usual maximum resolution of the UK skies and given extra reach on planetary viewing. The vixen scopes are lightweight like the Tak, which neglects the usual step up in weight with other manufacturers.

The other item is that replacing the Vixen diagonal with a Baader prism and using Vixen HR EP’s has enabled really high resolution in the scope, x495 which I thought was the preserve of premium APO’s. The point being it’s all the visual train, that I would consider. Of course there are some Tak owners out there who like barlowing the Vixen HR’s as well as say it’s perfectly useable. 

Here is a good thread running at the moment on CN discussing quality of a lense verses aperture.

https://www.cloudynights.com/topic/752583-does-aperture-outway-strehl-ratio/page-3#entry10846933
 

The answer is predictably optics quality can offset aperture within a certain difference, however as ever it’s a payoff between in this case a premium 4” will cost more then a good 5” which will show a similar view. 

Given the models you first linked to I’d go with aperture, I don’t think the weight increase will limit the use of the scope. Also doublets will cool down very quickly which always helps.

 

 

Good post there Deadlake.

The useability argument (as mentioned in your previous post) shouldn’t be under estimated. I’ve owned a couple or three 120EDs and have really enjoyed them. However my Tak FC100DC gives similar performance under my skies and I’m much more likely to use that due to its much smaller size.

Strangely I mostly use the Tak for white light solar these days, and the Vixen FL102S for lunar and planetary because of the longer focal length which makes it easier to reach higher powers. If travelling though, it is the Tak every time.

[Carl Au]

20 hours ago, jetstream said:

I personally don't crank up the mag excessively most of the time.

Or use that scope most of the time either I imagine 

Edited January 31, 2021 by Carl Au

[vlaiv]

4 hours ago, Deadlake said:

Does any of a to c cover contrast, I think that’s more important to the human eye when looking at planets?

There are all related to contrast. In fact, when we speak of telescope resolving power - each of these terms is contrast in a certain way.

[Deadlake]

1 hour ago, Carl Au said:

Or use that scope most of the time either I imagine 

JetStream is the kind of Tak owner who Barlows up Vixen HR's on a TSA120.
😀
I'd hope 600x is enough for most planetary observations....

[Alan White]

8 hours ago, Deadlake said:

I have a Vixen SD103S scope. It’s been a delight to use, however the one thing I regret is not going for the SD115S version as that extra aperture would of brought the resolution of the scope within the usual maximum resolution of the UK skies and given extra reach on planetary viewing. The vixen scopes are lightweight like the Tak, which neglects the usual step up in weight with other manufacturers.

The other item is that replacing the Vixen diagonal with a Baader prism and using Vixen HR EP’s has enabled really high resolution in the scope, x495 which I thought was the preserve of premium APO’s. The point being it’s all the visual train, that I would consider. Of course there are some Tak owners out there who like barlowing the Vixen HR’s as well as say it’s perfectly useable. 

Here is a good thread running at the moment on CN discussing quality of a lense verses aperture.

https://www.cloudynights.com/topic/752583-does-aperture-outway-strehl-ratio/page-3#entry10846933
 

The answer is predictably optics quality can offset aperture within a certain difference, however as ever it’s a payoff between in this case a premium 4” will cost more then a good 5” which will show a similar view. 

Given the models you first linked to I’d go with aperture, I don’t think the weight increase will limit the use of the scope. Also doublets will cool down very quickly which always helps.

 

 

I have the same scope and feel it performs on equal terms with similar sized 100mm Taks etc.
This is the opinion of several others on this forum too.

The HR eyepieces are wonderful with this scope, but my one stops at 3.4mm, not due to lack of desire for more, but funding.

In answer to the OP, a 100mm Refractor of good quality will not disappoint, cools quickly, gives reasonable view even before cooled in not much time and
when on the right night, which at this size if often compared to an 8-10" scope is more often, gives stunning Lunar and planet views.
Its not big image scale, but a quality image, that is smaller and clearer, rather than bigger and smeared.
I honestly believe the 4" APO / ED scope is the ideal UK one to own.
A 5 would be great, but how much will it reduce the seeing nights?

My 2p worth...

[Fedele]

For visual purposes, or imaging with webcam, for planets you need  diameter and focal lenght.  There is not story. 

in smaal refractors the planet disks are very small at hight  x.  A SC or a MAK are best for this work. 

[KP82]

On 31/01/2021 at 08:20, Deadlake said:

I have a Vixen SD103S scope. It’s been a delight to use, however the one thing I regret is not going for the SD115S version as that extra aperture would of brought the resolution of the scope within the usual maximum resolution of the UK skies and given extra reach on planetary viewing. The vixen scopes are lightweight like the Tak, which neglects the usual step up in weight with other manufacturers.

The other item is that replacing the Vixen diagonal with a Baader prism and using Vixen HR EP’s has enabled really high resolution in the scope, x495 which I thought was the preserve of premium APO’s. The point being it’s all the visual train, that I would consider. Of course there are some Tak owners out there who like barlowing the Vixen HR’s as well as say it’s perfectly useable. 

Here is a good thread running at the moment on CN discussing quality of a lense verses aperture.

https://www.cloudynights.com/topic/752583-does-aperture-outway-strehl-ratio/page-3#entry10846933
 

The answer is predictably optics quality can offset aperture within a certain difference, however as ever it’s a payoff between in this case a premium 4” will cost more then a good 5” which will show a similar view. 

Given the models you first linked to I’d go with aperture, I don’t think the weight increase will limit the use of the scope. Also doublets will cool down very quickly which always helps.

 

 

I agree with your comments on usability.

I find myself go out with my APM 107 way more often than my SW 8" dob for visual despite the fact the 8" dob is pretty easy to set up. Open clusters, star fields, doubles and even the Moon look more pleasing in the refractor. For planets the 8" only edges out when the seeing condition permits and it's fully acclimatised. I keep my dob in the corner of the living room as I don't trust the shed or the garage due to humidity and dust. The only times I use the dob exclusively are those moonless nights when I specifically want to look at some faint fuzzies. But more than often I use those moonless nights for imaging with my APM 115 or camera lenses.

Edited February 1, 2021 by KP82

[alex_stars]

Hi all and once more a big "thank you" for all the contributions.

My mind is set, I will get the 125mm F/7.8 Doublet (FPL-53 & Lanthan). For me, that appears to be the best bet. A doublet to cool fast, as I mostly do visual (so not really a need for a triplet) and the largest APO I can currently afford. So there we are. Ah yes and a set of really nice marbles to place across the meadow behind the house to observe something (thanks @vlaiv). Then I am "seeing" independent and gonna have a lot of fun 😉

I'll report back when I got the scope.Will take until spring though cause I want to see some planets before I form some opinion on the quality of the scope.

CS,

Alex

[johninderby]

That’s a scope I have been curious about and will be interested in how you get on with it. 

Edited February 1, 2021 by johninderby

[Deadlake]

See if you can get from a UK importer, then you will not get hit with Brexit handling fees. TS does not charge UK tax, so ok on that front. 

[jetstream]

12 minutes ago, alex_stars said:

I will get the 125mm F/7.8 Doublet (FPL-53 & Lanthan

Excellent and don't forget one of these

https://www.teleskop-express.de/shop/product_info.php/info/p7951_Baader-2--BBHS-Sitall-Diagonal-Mirror-with-1-10-lambda-surface.html

[Deadlake]

2 minutes ago, jetstream said:

Excellent and don't forget one of these

https://www.teleskop-express.de/shop/product_info.php/info/p7951_Baader-2--BBHS-Sitall-Diagonal-Mirror-with-1-10-lambda-surface.html

Or the Baader prism one? 😀😆

Edited February 1, 2021 by Deadlake