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

[alex_stars]

Dear fellow stargazers,

I am pondering the question which refractor to buy for observing the planets (Moon included) and maybe some globular clusters as well as double stars. But the focus is on visual observations of planets. Might occasionally pop my planetary camera on the scope, but not my main interest.

Currently I have a Skymax 180, which is nice, but the cool down time is a challenge for me even with an insulated scope. Since I recently learned that it actually has a 34% obstruction (MTF plots here), I am on the lookout for a new scope. My current contenders are:

• TS-Optics 102 mm F/11 ED refractor (details here)
• TS-Optics 125 mm Doublet F/7.8 FPL-53 Apo (details here) or similar like the Explore Scientific Triplet ED Apo 127 mm F/7.5 (details here)

I could stretch my budget to get one of these 5" refractors but won't be able to afford a luxus class scope like the APM LZOS 130 mm F/9.2 (details here). My mount is an Celestron AVX.

So I was contemplating if the 5" is a worthwhile investment for visual planetary work  or if the 4" F/11 is the better choice given its long focal length. This would be my first refractor so I would appreciate any input on the matter.

Clear skies,

Alex

Edited January 29, 2021 by alex_stars

[Mr Jones]

There are many experienced folk here who will be able to offer a far greater level of insight than me (especially on the specific scopes you mention).

The 4" refractor will be lighter, faster to cool, and less sensitive to poor seeing than the 180 SM. So you can nip out and get on with observations within short windows of time. But they are somewhat compromised vs your Skymax when solely considering planetary observation, especially the F7 scope, as the 180 SM is built for planetary work.

As we are in a planetary 'dry period' right now, that refractor compromise might actually work in your favour, opening up other observing opportunities?

If you want a great planetary refractor, go as long as large as you can. You'll be more content with 125 F11 than 100 F7, that's for sure. You could also consider a used Intes Micro 715 with cooling fans (but probably little more advantage than the 180 you currently have in reality).

The image enclosed shows the compromise faced by the 100mm refractor vs your 180. Note eyepiece requiremets of f7 vs f11 to get a similar fov on Saturn. Your eyes may find long observation periods using a 4mm ortho tiring vs the 12.5mm you can get away with on the 180 SM. 

[Carl Au]

I just bought the Starwave 102EDR which is the UK version but with FPL 53 glass. I am delighted with my new purchase. You don't need to buy from Germany, Altair 

[johninderby]

The main reason I sold my Skymax180 was the long cool down time. Now replaced with an 8” Classical Cassegrain that delivers great lunar / planetary views and has has similar cool down time to a refractor.

Perhaps a SW ED120 would be a good choice as it’s not too expensive? 🤔

Also have Big Red the 100mm f/13 planetary scope with Japanese optics but needs a solid mount but amazing planetary views.

 

Edited January 29, 2021 by johninderby

[Carl Au]

[John]

My fracs range from 100mm to 130mm. 120mm is a real "sweet spot" in terms of combining performance potential with relative ease of mounting and portability. They all deliver superb lunar, planetary and double star performance. The 130 is a bit more of a handful in terms of mounting needs but it is one of the 130mm F/9.2  triplets that you refer to. I tend to favour alt-azimuth mountings but I have owned and used an AVX equatorial and it did a good job even with the 130mm F/9.2:

 

[michael.h.f.wilkinson]

I am not sure any of the refractors you mention could beat the views with your SkyMax 180, and in terms of imaging the 180 will win hands down. I have a Celestron C8, which has a planetary performance similar to the SkyMax 180, and to complement it I got myself a wide field refractor (APM 80mm F/6 triplet) for viewing larger DSOs, and DSO imaging in general. For planetary imaging, aperture is definitely king, and the lower contrast caused by the central obstruction of the SkyMax is really not such a big deal. The SkyMax is also much easier to set up than the bigger refractors (even though it is quite a hefty OTA). To get an idea what your SkyMax OTA could do on e.g. Jupiter under good conditions, here is one of my best shots of Jupiter

I should note I have looked through a 4" F/10 achromat at Jupiter, with my C8 right beside it, and the view through the 4" scope didn't come anywhere near the level of detail seen through the C8

[Saganite]

Were it not for the chance to buy the 5" refractor that I have, I intended to and would have bought the Skywatcher ED120.  A very fine and capable all rounder for not too much money.

Edited January 29, 2021 by Saganite

[Alkaid]

5 minutes ago, michael.h.f.wilkinson said:

I should note I have looked throud a 4" F/10 achromat at Jupiter, with my C8 right beside it, and the view through the 4" scope didn't come anywhere near the level of detail seen through the C8

+1 

[John]

I think it is the cooling time for the 180 mak that is the OP's main concern ?

 

[michael.h.f.wilkinson]

13 minutes ago, John said:

I think it is the cooling time for the 180 mak that is the OP's main concern ?

 

That is of course an issue. I store my C8 OTA in an unheated garage, and that ensures cooling time is relatively short. Not always possible, of course

[dweller25]

5 hours ago, alex_stars said:

Dear fellow stargazers,

I am pondering the question which refractor to buy for observing the planets (Moon included) and maybe some globular clusters as well as double stars. But the focus is on visual observations of planets. Might occasionally pop my planetary camera on the scope, but not my main interest.

Currently I have a Skymax 180, which is nice, but the cool down time is a challenge for me even with an insulated scope. Since I recently learned that it actually has a 34% obstruction (MTF plots here), I am on the lookout for a new scope. My current contenders are:

• TS-Optics 102 mm F/11 ED refractor (details here)
• TS-Optics 125 mm Doublet F/7.8 FPL-53 Apo (details here) or similar like the Explore Scientific Triplet ED Apo 127 mm F/7.5 (details here)

I could stretch my budget to get one of these 5" refractors but won't be able to afford a luxus class scope like the APM LZOS 130 mm F/9.2 (details here). My mount is an Celestron AVX.

So I was contemplating if the 5" is a worthwhile investment for visual planetary work  or if the 4" F/11 is the better choice given its long focal length. This would be my first refractor so I would appreciate any input on the matter.

Clear skies,

Alex

The TS 125 looks like a great planetary scope to me.

A bit like a modern day FS128 perhaps

Edited January 29, 2021 by dweller25

[alex_stars]

27 minutes ago, John said:

I think it is the cooling time for the 180 mak that is the OP's main concern ?

 

Absolutely true. That is one of the main issues I have with Skymax 180. Thanks @John

@dweller25 I agree. After reading through all the posts here (a big thank you to all who have contributed so far) I tend towards the 5" class with the following contenders:

• TS-Optics 125 mm Doublet F/7.8 FPL-53 Apo
• Explore Scientific Triplet ED Apo 127 mm F/7.5
• Skywatcher ED 120

The TS seems to be the same as the Altair Wave Series 125 EDF F7.8 APO (info here) and the Tecnosky AP 125/975 ED (info here). And there are probably more to find. Does anybody know of any obvious differences on those re-branded scopes or are they all from the same factory?

CS,

Alex

[Alkaid]

48 minutes ago, John said:

I think it is the cooling time for the 180 mak that is the OP's main concern ?

 

Yes, you're quite right - sorry guys. 

The 5" frac, whichever you chose, will solve all that.

 

[John]

The triplet will need more time to cool than a doublet. Not as much as a 180mm mak-cassegrain but a bit more none the less.

 

[Carl Au]

My old ED120 was a very good telescope. Optically not quiet as good as my previous ED100 but certainly gave a slightly bigger and brighter picture. It will I imagine be the lightest of the 120 scopes you mention, but much of that is down to build quality, which is rather basic. You pay for the FPL 53 glass not the tube. The ED120 is a bigger scope than you might imagine. You might want to budget for an extension to give you the additional eyepiece height you are going to need for comfortable view. I think that the others also have rotatable focusers which the ED 120 doesn't have. 

I don't know if that helps you at all or not.

[Louis D]

I picked up a 90mm TS APO FPL-53 triplet in 2019 just to see what they're like.  I was shocked to find out it takes about 30 minutes to cool down or warm up just 10 degrees F.  That's as slow as my very open 8" Dob!  Mars views were very sad compared to the 8" Dob as well, to say nothing of resolving GCs.  Aperture is king.

I'd spend the money on an 8" or 10" Newt with a mirror hand figured to 1/20th wave (think Zambuto or similar) if you want the finest planetary details.

[mikeDnight]

I think a 5" refractor hits a real sweet spot for planetary. But there are a couple of things to seriously consider before taking the plunge. The bigger the scope the more effort will be needed to set up and strip down. That can at times be a negative when you're frozen to the bone and your scope and mount is caked in ice, even though the scope is good. A 5" refractor needs a strong mount if its being used for planetary observing, as tremor will wash out finer detail, and unless its being mounted in an observatory the scope will be subjected to strong breezes and wind's at times. 

A lot of people underestimate the ability of a good 4" refractor to cut through the seeing and pull out impressive planetary detail. It's not all about aperture or exotic glass types. The quality of the figure is massively important, so although aperture is often said to be "king", quality is "queen", and as in chess the queen is of far greater importance. And, although an 8" or 10" reflector will show good planetary views some of the time, a good 4" refractor will show good planetary views most of the time. Plus the refractor will give a clearer, cleaner, sharper view that often outweighs the softer views that most other scope designs show.  For visual planetary observing a quality doublet can be preferable to a triplet simply because it will acclimate more rapidly. You can also get even more impressive views by using a binoviewer and a pair of cheap orthoscopic's rather than hugely expensive multi element designs.  Below are a few pencil sketches that illustrate the visual appearance of some planets through a 4" refractor. (The lunar sketches are very basic and the actual view through the telescope was vastly more detailed). (( Note also the first two Mars sketches. The top sketch was made using an 8" F6 Newtonian, while the lower was made using a 100mm F8 apo refractor)).

Edited January 30, 2021 by mikeDnight

[paulastro]

Some valid points there Mike.  I would add though, you are a very talented observer and artist and many observers will probably need a bigger scope than a four inch (sorry, 3.9 inch) to see the detail you can record.

By the way, I'm impressed with your chess knowledge 😉.

[RobertI]

12 hours ago, Carl Au said:

I just bought the Starwave 102EDR which is the UK version but with FPL 53 glass. I am delighted with my new purchase. You don't need to buy from Germany, Altair 

I also recently bought this scope and used it during the Mars opposition. I have been totally amazed by its planetary abilities, and have also used it to view the moon with really excellent results. Although its limited aperture is apparent for deep sky, for planetary it has so far been a revelation. Although I haven’t managed a back to back test with the C8 on planets/moon (which will probably turn out to be better when cooled and the seeing is right) it has been so good that I haven’t wanted to get the C8 out. It’s so easy to use (if stored in a garage it actually doesn’t need cooling) and at F7 is very much more manageable than my 100mm F10 achro. Personally I would find an 100mm F11 too unwieldy. I use it with binoviewers which have also been a revelation.....but that’s another story. Of course I have never tried an ED120. 🙂

[Voyager 3]

20 hours ago, alex_stars said:

Dear fellow stargazers,

I am pondering the question which refractor to buy for observing the planets (Moon included) and maybe some globular clusters as well as double stars. But the focus is on visual observations of planets. Might occasionally pop my planetary camera on the scope, but not my main interest.

Currently I have a Skymax 180, which is nice, but the cool down time is a challenge for me even with an insulated scope. Since I recently learned that it actually has a 34% obstruction (MTF plots here), I am on the lookout for a new scope. My current contenders are:

• TS-Optics 102 mm F/11 ED refractor (details here)
• TS-Optics 125 mm Doublet F/7.8 FPL-53 Apo (details here) or similar like the Explore Scientific Triplet ED Apo 127 mm F/7.5 (details here)

I could stretch my budget to get one of these 5" refractors but won't be able to afford a luxus class scope like the APM LZOS 130 mm F/9.2 (details here). My mount is an Celestron AVX.

So I was contemplating if the 5" is a worthwhile investment for visual planetary work  or if the 4" F/11 is the better choice given its long focal length. This would be my first refractor so I would appreciate any input on the matter.

Clear skies,

Alex

Is the APM 140 SD apo in your budget/consideration ? 

[RobertI]

11 hours ago, mikeDnight said:

You can also get even more impressive views by using a binoviewer and a pair of cheap orthoscopic's rather than hugely expensive multi element designs

I would second that, binoviewers have transformed my enjoyment of the moon and planets. They make it more comfortable to view for longer periods, are more immersive and the result is you end up seeing more. It's worth thinking about factoring binoviewers into your budget IMO. Interesting video here:

 

 

Edited January 30, 2021 by RobertI

[mikeDnight]

That's a good video!  From my understanding though, the only light loss with a prism occurs at the prism surface, and that 100% of the light that enters the prism also leaves the prism. Obviously with multiple prisms there will be losses at each air to glass entrance, but otherwise I find the views virtually as bright with as without when it comes to Moon & planets. Whatever the reasons, looking back at my observational sketches I made while using my FS128 and single eyepiece, and comparing what I see through my FC100D, I see significantly greater detail through the binoviewer & 100mm scope. 

[Paz]

I think with the choice being between a 4 and 5 inch refractor the physical set up considerations are as significant as consideration of the views.

I originally had in mind getting a 120mm f7 or so refractor. I went to an astronomy show to check them out and it took me onky a few moments to realise 120mm was going to be too big. In my case it wasn't that i couldnt handle such a scope but rather that the set up effort would match or exceed that of my 14" reflector which a 120mm scope is not going to be able to compete with, so I concluded I would never use it after the novelty wore off. Instead I went for 102mm and have a good set up that doesnt compete with the 14".

One  other thought is I do think ones eyesight can influence scope choice. Getting the most out of a small aperture high quality scope requires good eyesight. 

I find bigger aperture is hugely beneficial and think this is down to my eyesight not being the best.

[CraigT82]

Yes, everything mikeDnight says is true...And Im a die-hard newtonian user (though that's more to do with my disposable income rather than a particular love of the telescope design :D)

When it comes to visual observing, my opinion is that the sky will be the limit for the vast majority of the time... even on the best nights the seeing blur will be around 1" , and most of the other times it will be around 2" so is there much point in having a scope that can resolve much finer detail than that visually? In other words a 4" scope could provide you with all the detail that the sky is capable of delivering 99% of the time, and so what you may want to focus on is getting the best 4" scope you can afford; one that will present to your eyes that detail in the sharpest and most defined manner. The image will be dimmer that it is through the 180 mak but you will get used to that, (and that will even be an advantage on some targets). 

If I were you, out of the list in your OP I'd go for the 102 ED f/11 ( though I'd also be taking a good look at a SW ED100 f/9) and spend the remaining budget on some nice planetary eyepieces and a decent binoviewer. The AVX should handle it nicely. I'd recommend a motorised focuser for it as that makes focusing a piece of cake with no wobble at all. I haven't heard many reviews about them, though I used to have the achromatic version which was a real peach of a scope. I only moved it on as I needed more aperture as I image mostly (Imaging the planets is a whole different ball game).

Edit: here is a pic of my old 102 f/11 achro and a lunar phone snap taken at 375x mag. The eyeball view was actually sharper and more detailed than this. There is a violet wash (it is an achromat after all) but the ED version shouldn't show this at all. 

Edited January 30, 2021 by CraigT82