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is 4 inch enough for serious planet observing?

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#1 michiel

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Posted 16 November 2020 - 12:20 PM

I sold all my scopes, except for my questar, which is just 89mm but very good. I live near the coast with usually very unstable seeing. I was wondering, is a good 4 inch refractor enough for nice planetary observing. I want a reasonable high quality scope. I was considering amongst others the 4 inch f11.2 ED refractor from TS telescope service in Germany.


https://www.teleskop...ularauszug.html

 

I have two questions:

1) would this be considered a high quality refractor?

2) would it be enough step up from my questar to keep me happy.?

I must note that I love refractors especially long focal length ones. so I am tempted.

I also consider their 5 inch f7.8 ED doublet  but that one is about 1500 euros and the 4 inch F11 is only 700 euros. they also have a 4 inch f8 ED for 1000 euros. 

Actually I get more and more excited about the 4 inch f11.2....... but does it make sense ??



#2 25585

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Posted 16 November 2020 - 12:31 PM

From your Questar, a 120mm would be a better jump. The Sky Watcher ED120 F7.5 doublet has great optics, & may be cheaper than the TS, though less refined.


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#3 t.r.

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Posted 16 November 2020 - 12:39 PM

For solid planetary and maybe as an only scope, I recommend a 6ā€ ED doublet. Can be carried on a cheaper CGEM mount and will have enough resolution and light gathering to do it all. If cost is an issue stick with the Questar.

Edited by t.r., 16 November 2020 - 12:41 PM.

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#4 Jsquared

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Posted 16 November 2020 - 12:49 PM

120mm would be a much better jump. Iā€™d save my money and use the questar till u have money saved for the 120.
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#5 Ihtegla Sar

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Posted 16 November 2020 - 12:49 PM

A 4" is enough if you don't mind floaters and if the optics are high quality. I've seen amazing detail on Mars this year in my FC100DL, but if you already have an 89mm a 4" won't be as big of a jump as a 5" or a 6"
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#6 gwlee

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Posted 16 November 2020 - 12:54 PM

As an unserious planetary observer, I find small refractors convenient for planets, but a 6-8ā€ scope works much better for me even in poor seeing conditions.
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#7 photomagica

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Posted 16 November 2020 - 12:59 PM

My vote is for a 120mm. I've been watching Mars with a new-to-me 127 and it has really been good in periods of good seeing. That said, if you choose a high quality 100mm for size reasons, I don't think you will be disappointed. For planets look for one with a longer than average focal length.

Bill


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#8 Rutilus

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Posted 16 November 2020 - 01:05 PM

I sold all my scopes, except for my questar, which is just 89mm but very good. I live near the coast with usually very unstable seeing. I was wondering, is a good 4 inch refractor enough for nice planetary observing. I want a reasonable high quality scope. I was considering amongst others the 4 inch f11.2 ED refractor from TS telescope service in Germany.


https://www.teleskop...ularauszug.html

 

I have two questions:

1) would this be considered a high quality refractor?

2) would it be enough step up from my questar to keep me happy.?

I must note that I love refractors especially long focal length ones. so I am tempted.

I also consider their 5 inch f7.8 ED doublet  but that one is about 1500 euros and the 4 inch F11 is only 700 euros. they also have a 4 inch f8 ED for 1000 euros. 

Actually I get more and more excited about the 4 inch f11.2....... but does it make sense ??

"Is a good 4 inch refractor enough for nice planetary observing"  Yes.

"Is a 4 inch enough for serious planet observing"   No.

 

A 4 inch refractor will and do give nice views of the planets,  but for serious observing  you want something bigger,

and that scope is most likely not going to be a refractor in the sizes that the vast majority of us can afford to buy or

even house in a  purpose built observatory. 


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#9 Reid W

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Posted 16 November 2020 - 01:23 PM

I recently acquired an AzPro and over the weekend tried a side-by-side direct comparison of my Vixen Ed81s and a nice Celestron C90. The C90 has very good optics.

What I noted in Mars comparisons is that the C90 showed very slightly more detail, just about in consequential, while the contrast of the color variations was better in the refractor. The C90 was better in tight doubles but showed fewer faint stars when observing open clusters.

So in all, the refractor was just slightly better.

Last night, I compared the 81mm to my 90mm (f9) fluorite. Here, the comparison wasn't nearly as close. The 90 was superior.... except in splitting a pair of 1.8" doubles. ... could be seeing conditions.

To your question, yes, a fine 100mm optic should show more than the Q. As others have suggested, a 120ed would be a nice choice too.

Finally, if your Q can support them, a set of binoviewers will make a significant impact to your planetary views.


Edited by Reid W, 16 November 2020 - 01:25 PM.

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#10 emilslomi

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Posted 16 November 2020 - 02:34 PM

Thread title: yes

1) yes

2) no

I agree with the people that suggested 120. I would go for a 125 f8 (ok, 7.8)ed doublet (I think one is comming up at Astronomis). Nice and long! 1k for a 100 f8 ed? Have to look at the TS pages, but a 100 ed f7 can be had for less without (in theory) sacrificing visual observing quality.

 

Emil


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#11 BillP

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Posted 16 November 2020 - 02:53 PM

4" is plenty good enough.  Just optimize it with a really high performing "planetary" diagonal (i.e., not a dielectric, use a prism for best contrast transfer), good contrast eyepieces, a Powermate over a Barlow if you intend to use longer focal length eyepieces, and a Baader Contrast Booster filter (or a precision ND or Polarizing filter to attenuate the light; or some good single coated optics instead).

 

I have a 10" Dob, 6" Apo, and 4" Apo.  Even when seeing is stable it does not necessarily mean the views will be good as particulate and water vapor levels in the atmosphere can still kill the contrast and make the view look washed out.  So the 4" will often do just as well as the 6".  The 10" Dob will of course get way more fine details, like the eddies around the GRS and more detailed internal structure in the GRS, but not worth the effort to get the mirror thermally stable enough if it is a scope that goes from inside to outside and the view is many times bright enough to wash out the finer details if the seeing is not letting you get to a small enough exit pupil to tone things down.  But the 4" shows wonderful details and is just plain fun also.  So even with my 6" Apo, I still take the 4" out way more for planetary and get lots and lots of satisfying views.  Don't get me wrong, the 6" is wonderful and gets more planetary details when the conditions allow, but compared to a 4" where it really shines being substantially better is on DSO, particularly galaxies -- galaxies that are a struggle for the 4" are just right there in your face in the 6".

 

This view was with the 6" Apo.  But comparison a few weeks later with the 4" the view was substantially the same, with only the mottled shadings in the southern hemisphere not being evident and the desert regions not having this sandy/grainy textural appearance that the 6" was showing. So the detail loss was not to the major features, but just some of the internal shadings.

MARS 10-5-2020 2300.jpg
 
In these two views note the difference in contrast the eyepiece can do.  The 1st pic is using a singlet sphere eyepiece, whereas the 2nd one is using a wide field that has 8 elements in 4 groups.  The more glass, the less contrast (everything else being equal).
Mars 2010-01-31 (CN).jpg 6429917-Mars 2010-02-07 (TSA-UWA).jpg
 
This view of Jupiter is with the 4" Apo and binoviewing with 28 RKEs and Barlow.  Note that even with a 4" one can see structure inside the GRS.
Jupiter in TSA.jpg

Edited by BillP, 16 November 2020 - 03:01 PM.

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#12 rkelley8493

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Posted 16 November 2020 - 02:57 PM

A 4" is enough if you don't mind floaters and if the optics are high quality. I've seen amazing detail on Mars this year in my FC100DL, but if you already have an 89mm a 4" won't be as big of a jump as a 5" or a 6"

I agree. I can resolve some pretty fine detail using a 4" Fluorite doublet, but I do have to battle with floaters when I get above 200x. There's surprisingly good detail at ultra-high magnification though. For example, this cellphone image was taken using my 4" doublet, 4.5mm Delos eyepiece + 2x Powermate for a magnification of 329x.

IMG_20201112_185739 4.5+2xpm.jpg


Edited by rkelley8493, 16 November 2020 - 02:58 PM.

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#13 Astrojensen

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Posted 16 November 2020 - 03:00 PM

Whether a telescope of any given size is enough for serious observing depends more on the observer than the aperture... 

 

 

Clear skies!

Thomas, Denmark


Edited by Astrojensen, 16 November 2020 - 03:00 PM.

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#14 BillP

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Posted 16 November 2020 - 03:16 PM

Whether a telescope of any given size is enough for serious observing depends more on the observer than the aperture...

 

Hmmm...what was that sound?  Oh, it was a nail being hit on the head waytogo.gif


Edited by BillP, 16 November 2020 - 03:16 PM.

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#15 PKDfan

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Posted 16 November 2020 - 03:31 PM

Just a few days past opposition with Mars transiting very high I had several seconds of perfect seeing and with a 2x 2"ed barlow and 6.5(6.7actual)Morpheus for ~270x with my F/9 100ed SkyWatcher and had a short window of hubble type clarity and even now I'm still a bit stunned how good my scope is! So absolutely a 4inch scope works magnificently IF your seeing is great.

Clear skies & Good seeing
Edit:typo

Edited by PKDfan, 16 November 2020 - 10:28 PM.

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#16 Jon Isaacs

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Posted 16 November 2020 - 04:04 PM

Whether a telescope of any given size is enough for serious observing depends more on the observer than the aperture... 

 

 

Clear skies!

Thomas, Denmark

 

:waytogo:

 

One can always argue bigger is better and there's a lot of truth in that.

 

But serious observing is about the observer and not the equipment and some amazing planetary sketches are done with ~60  mm refractors. 

 

I think Thomas has made some from his island in the Baltic.

 

Jon


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#17 N-1

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Posted 16 November 2020 - 04:53 PM

 

4" is plenty good enough.  Just optimize it with a really high performing "planetary" diagonal (i.e., not a dielectric, use a prism for best contrast transfer), good contrast eyepieces, a Powermate over a Barlow if you intend to use longer focal length eyepieces, and a Baader Contrast Booster filter (or a precision ND or Polarizing filter to attenuate the light; or some good single coated optics instead).

 

I have a 10" Dob, 6" Apo, and 4" Apo.  Even when seeing is stable it does not necessarily mean the views will be good as particulate and water vapor levels in the atmosphere can still kill the contrast and make the view look washed out.  So the 4" will often do just as well as the 6".  The 10" Dob will of course get way more fine details, like the eddies around the GRS and more detailed internal structure in the GRS, but not worth the effort to get the mirror thermally stable enough if it is a scope that goes from inside to outside and the view is many times bright enough to wash out the finer details if the seeing is not letting you get to a small enough exit pupil to tone things down.  But the 4" shows wonderful details and is just plain fun also.  So even with my 6" Apo, I still take the 4" out way more for planetary and get lots and lots of satisfying views.  Don't get me wrong, the 6" is wonderful and gets more planetary details when the conditions allow, but compared to a 4" where it really shines being substantially better is on DSO, particularly galaxies -- galaxies that are a struggle for the 4" are just right there in your face in the 6".

 

This view was with the 6" Apo.  But comparison a few weeks later with the 4" the view was substantially the same, with only the mottled shadings in the southern hemisphere not being evident and the desert regions not having this sandy/grainy textural appearance that the 6" was showing. So the detail loss was not to the major features, but just some of the internal shadings.

 
 
In these two views note the difference in contrast the eyepiece can do.  The 1st pic is using a singlet sphere eyepiece, whereas the 2nd one is using a wide field that has 8 elements in 4 groups.  The more glass, the less contrast (everything else being equal).
 
 
This view of Jupiter is with the 4" Apo and binoviewing with 28 RKEs and Barlow.  Note that even with a 4" one can see structure inside the GRS.

 

That's all well and good, but it's not serious planetary observing, see post #8 tongue2.gif



#18 helpwanted

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Posted 16 November 2020 - 05:07 PM

Bill:

 

a small enough exit pupil to tone things down

 

explain please.



#19 MalVeauX

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Posted 16 November 2020 - 05:23 PM

Heya,

 

Observer experience with observation and atmospheric seeing are more important than fussing over 102mm or 120mm optics of any kind.

 

You could get the 120mm aperture and visually not notice anything more than what you'd see in the 102mm if the seeing was not better and supportive and if your planetary experience observation wasn't well tuned.

 

It also matters if your goal is binoviewing at high magnification, as things can get dim fast, so your preference to viewing brightness and exit pupil goals matter, so aperture plays into this.

 

What do you consider serious?

What are your expectations?

 

There are serious planetary observers that use long 60mm fracs.

 

Very best,


Edited by MalVeauX, 16 November 2020 - 06:02 PM.

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#20 Bomber Bob

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Posted 16 November 2020 - 06:11 PM

4" is plenty good enough.  Just optimize it with a really high performing "planetary" diagonal (i.e., not a dielectric, use a prism for best contrast transfer), good contrast eyepieces, a Powermate over a Barlow if you intend to use longer focal length eyepieces, and a Baader Contrast Booster filter (or a precision ND or Polarizing filter to attenuate the light; or some good single coated optics instead).

 

Amen.

 

Last night was my most recent observing Jupiter, Saturn, & Mars with a very used 1980s Celestron (Vixen) C-102 F10 -- it has a string of tiny clam-shells along the edges of the flint.  I also have a custom-made 4" F10 that is absolutely the very best of its aperture & type that I've used in 50+ years of observing (the Dakin 4).  I try not to refer to it too much as it's a one-off, while Celestron sold 100s / 1000s of the Japan-made 102s (including an incomparable Fluorite doublet version).  Even with its defects, my C-102 is sharp at 200x in 7 / 10 or better planetary seeing.  The views of Jupiter & Mars were sketch-worthy; and, Saturn showed 3 belts, jet-black Cassini, and Titan + 2 moons.

 

I like my 1958 Questar Standard, but no way it can compete with the views of double stars & open star clusters in my C-102 -- the Double Cluster @ 36x with an RKE 28mm Space Walk eyepiece is but one glaringly obvious example of what a well-made 4" achromatic can do.  My 4" F10 refractors are my workhorses, and they get the lion's share of my observing time -- Moon-less or Moon-full nights...


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#21 Jim Curry

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Posted 16 November 2020 - 06:47 PM

The 4" scope can open a world of wonders for you. 


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#22 BillP

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Posted 16 November 2020 - 07:14 PM

That's all well and good, but it's not serious planetary observing, see post #8 tongue2.gif

Seriously? Is that really a serious statement about being serious about something.  So need to go bigger to be serious?  Seems to be some seriously wrong logic there!  I am serious when I say that serious planetary observation not only can be done with 4" scope, but is done with a 4" scope! How do you know?  Easy, when the observer is serious about their viewing.  Of course there are some who need (i.e., crutch.gif ) larger apertures to be serious.  But seriously they have some serious issues.  But that's ok, we just pat them on the head and let them know, yes dear, you are being serious, while we go back to our 4" scopes lol.gif


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#23 BillP

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Posted 16 November 2020 - 07:24 PM

Bill:

 

a small enough exit pupil to tone things down

 

explain please.

i.e., leveraging exit pupil to reduce the brightness so subtle features are not washed out of the view.  So the smaller the exit pupil the dimmer the view and also of course the more image scale from magnification.  Depending on the planet, its particular features, and how close it is to us at the time, even a <1mm exit pupil can be a little too bright so that the bright regions of the planet wash out details in adjacent darker regions that may have subtle details.  So attenuating the brightness can bring those subtle details back into view.  To reduce brightness you have several options to leverage: add magnification to reduce the exit pupil, switch to a single coated eyepiece that has less transmission, add a ND or Polarizing filter.  You can also put a bright light on so your eyes are not so dark adapted, but for me that trick with a refractor is not as effective as the others because need to make sure that bright light is visible while observing.  With a Dob I just shine the light down the OTA to brighten the background which is easier.  But my arms are not long enough to do that with a refractor lol.gif  IME need to do this with Jupiter and Mars in particular to catch some of the boarder details best of some features. So depends on what you are focusing on when observing the planet as to whether it is needed or not.


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#24 N-1

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Posted 16 November 2020 - 07:24 PM

Seriously? Is that really a serious statement about being serious about something.  So need to go bigger to be serious?  Seems to be some seriously wrong logic there!  I am serious when I say that serious planetary observation not only can be done with 4" scope, but is done with a 4" scope! How do you know?  Easy, when the observer is serious about their viewing.  Of course there are some who need (i.e., crutch.gif ) larger apertures to be serious.  But seriously they have some serious issues.  But that's ok, we just pat them on the head and let them know, yes dear, you are being serious, while we go back to our 4" scopes lol.gif

You have me convinced. My own TS 102 f/11 ED expected to arrive any day now...


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#25 BillP

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Posted 16 November 2020 - 07:39 PM

You have me convinced. My own TS 102 f/11 ED expected to arrive any day now...

A 5mm, 6mm, and 7mm will be your planetary workhorses!  Or a 12mm, 15mm, 18mm with a TV 2.5x Powermate grin.gif


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