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Any advantage to a long focus 80mm refractor over a short focus 4" refractor?

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#1 astro42

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Posted 01 January 2020 - 11:48 AM

Would there be any advantage of a 3" F10 refractor that has slightly better optics over 4" F7 refractor for planetary observing.

Note both of these are ED refractors.

The 3" has FPL 53 lanthium glass.

The 4" has just FK-61 glass.


Edited by astro42, 01 January 2020 - 12:06 PM.


#2 sg6

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Posted 01 January 2020 - 12:05 PM

I would go for the 4" f/7.

Neither are fast and a 4" f/7 should be well corrected. not a lot less or more then the 3" f/10.

The extra aperture would win therefore.

 

Also a 3" f/10 may still give a poorer focus, the depth of focus likely comes into play and again the 4" likely better for that "snap to" focus effect.



#3 Astrojensen

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Posted 01 January 2020 - 12:08 PM

Depends on a lot of things, such as how well they're made, the level of color correction (you can't judge it accurately just based on one of the glass types), etc. 

 

Generally, I'd guess that the 3" f/10 would have better spherical correction and much less false color, but whether this is enough to outperform the 4" f/7 depends on how well the latter is made. 

 

 

Also a 3" f/10 may still give a poorer focus, the depth of focus likely comes into play and again the 4" likely better for that "snap to" focus effect.

Personally, I've always found the opposite to be true.

 

 

Clear skies!
Thomas, Denmark


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#4 slepage

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Posted 01 January 2020 - 12:10 PM

Hi, For planetary you want largest aperture possible for increased resolution but you also want a longer focal length for increase contrast and longer focal length.  In your case the focal length of the 3 inch is 76mm and the focal length of the 4 inch is 71 mm. So really it is a toss up, but I would go for the larger aperture of the 4 inch for the better resolution.  This assumes that both pieces of glass are comparable.  


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#5 aeajr

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Posted 01 January 2020 - 12:12 PM

4", definately



#6 Jond105

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Posted 01 January 2020 - 12:29 PM

Truthfully they’d probably both be nice to have. But if only one, 102ED. The 80 may not show any color whatsoever. But the view will be much more dim in the 80 as opposed the 102 at the same mag. The 102 seems best bet. 


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#7 BillP

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Posted 01 January 2020 - 02:05 PM

Assuming the color correction between the two is in the same ballpark (can't tell unless you know the mating element), at the end of the day the only difference that matters will be the aperture, and the 4" will get you 28% more resolution and 63% more light.  IME I find that a 0.65mm exit pupil is a working max for planetary that retains very good contrast and brightness, dimmer than that and you start losing low contrast details.  At that exit pupil the magnification in the 80mm is 123x and in the 102mm is 157x.  So the 4", in addition to more resolution, will also allow you to go to higher magnification at same brightness/contrast level.


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#8 aa6ww

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Posted 01 January 2020 - 02:24 PM

So who sells a 3" F/10 with FPL-53 glass?  Never head of that combo in a scope.

 

...Ralph



#9 Astrojensen

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Posted 01 January 2020 - 03:04 PM

So who sells a 3" F/10 with FPL-53 glass?  Never head of that combo in a scope.

 

...Ralph

SharpStar (China) makes them. Kasai sold them a short moment and they're currently sold by a Canadian vendor, whose name I can't recall at the moment.

 

 

Clear skies!
Thomas, Denmark



#10 JIMZ7

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Posted 01 January 2020 - 03:08 PM

4" will always win aperture vs. 3".

Long focal length better on double stars & planets. Had a 70mm f/13 refractor better on double stars than a 80mm f/11.4 refractor. The 70mm China the 80mm Japan. Figure that out?

Jim



#11 BillP

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Posted 01 January 2020 - 04:25 PM

...and when you mask the 102 f/7 to 80mm it is an f/8.9 so darn close to f/10 like the other.


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#12 Jon Isaacs

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Posted 01 January 2020 - 04:46 PM

Hi, For planetary you want largest aperture possible for increased resolution but you also want a longer focal length for increase contrast and longer focal length.  In your case the focal length of the 3 inch is 76mm and the focal length of the 4 inch is 71 mm. So really it is a toss up, but I would go for the larger aperture of the 4 inch for the better resolution.  This assumes that both pieces of glass are comparable.  

 

There's nothing inherent about a longer focal length that increases contrast.

 

Increasing the aperture, assuming equal optical quality, will increase the contrast, resolution and light gathering.

 

As Thomas said, there's more to this than just aperture, glass type and focal length/focal length.

 

But, it's difficult to make up that aperture.

 

Jon


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#13 SteveG

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Posted 01 January 2020 - 05:02 PM

Would there be any advantage of a 3" F10 refractor that has slightly better optics over 4" F7 refractor for planetary observing.

Note both of these are ED refractors.

The 3" has FPL 53 lanthium glass.

The 4" has just FK-61 glass.

After owning a 3" ED refractor for a while, I found the aperture was too limited. I can happily use a 4" refractor all night though, especially at a dark site.


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#14 BillP

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Posted 01 January 2020 - 05:21 PM

There's nothing inherent about a longer focal length that increases contrast.

 

This is true.  But curious where this common contention came from?  Wonder if it was maybe a byproduct of the classic achromat days where of course a longer focal ratio would have meant better color correction, so more in-focus light, and also maybe that the longer OTA probably meant more effective baffling so less susceptible to contrast robbing stray light.
 


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#15 drd715

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Posted 01 January 2020 - 06:15 PM

Planets like longer physical focal length. Longer fl will allow a given eyepiece to perform better, the light cone angle isn't as steep. You may also get better eye relief using a longer eyepiece with a long fl scope and still get better power.

While both of your posted scopes are nearly the same fl, the 4 inch will be brighter for visual use.

However - the best 4 inch scope (for planets) at a very good value price is the F-11ed altair astro or TS version of it. Sharp pinpoint stars on a dark background - less CA and easy depth of focus.

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#16 mikeDnight

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Posted 01 January 2020 - 06:26 PM

I too would go for the 4"". You can after all use a barlow to increase the effective focal length, while still retaining the physical length of an F7. It's a win win, as you'll get the best of both worlds with wide field of an F7 and the higher powers afforded by an F14, without having to use very short fl eyepieces.


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#17 Jon Isaacs

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Posted 02 January 2020 - 03:21 AM

Planets like longer physical focal length. Longer fl will allow a given eyepiece to perform better, the light cone angle isn't as steep. You may also get better eye relief using a longer eyepiece with a long fl scope and still get better power.

 

 

What the planet's like is aperture. There are inherent advantages to a larger aperture, contrast, resolution, image brightness.. 

 

Some eyepieces perform better in slower scopes, some don't.  Modern eyepieces have plenty of eye relief in the shorter focal lengths.. In this case, the 4 inch requires a 3.5 mm focal length to reach 200x, the 3 inch, a 4.8 mm to reach 160x. Orthos and the like will have short eye relief, modern negative-positive eyepieces will have a comfortable amount.

 

Jon


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#18 aa6ww

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Posted 02 January 2020 - 04:17 AM

Both scopes would be good (ie...fun)  for planets.  They are both very small aperture for serious planetary observing. For back yard fun, just on any random night,  both would be lots of fun and be fun to use in the 130x to 200x  magnification range on average to better seeing conditions.

 

For Mercury, my money would be on the 80mm F/10 with FPL-53. More then likely you would be chasing the sun so it would generally be low in the sky. Atmospheric turbulence would  probably equalize the size difference between the two, I'd give a slight edge to the 80mm F/10 with its superior glass. Basically you are trying to observe a star like object by observing Mercury. All you would really be doing was trying to get a tiny sphere out of Mercury and the 80mm F/10, like this one for example:

 

http://www.longperng...act=view&no=227

 

would probably punch a smaller hole near the Horizon and may acclimate  faster to give you a cleaner shot of Mercury before it dips below the Horizon. It would be close compared to your AT-102ED but still, Id give the slight edge to the 80 F/10 with FPL-53 and Lanthanuma glass for its small hole needed and superior glass.

 

For Venus, again, I'd go with the 80 F/10, Venus is mostly about toning down its brightness. It also chases the sun so its usually never very high in the sky. 

Venus is one of the toughest tests for chromatic aberration but eyepieces and its position in the sky are also a crucial part of getting a clean pristine view of Venus. I've seen some shading on Venus's body but it was using an AP 152 Triplet F12 Super Planetary refractor. 

I think the 80 F/10 FPL-53 would deliver a slightly crisper view, easier to find that sweet spot for focus and may tone down any slight signs of Chromatic abberation on Venus.  I had an AT-102ED, I dont remember any CA on Venus. So any advantage of the 80 or the 102 would be very slight.

 

For all the other planets, aperture should rule and the 102 F7 should provide more planetary surface details. 

 

Should be able to pick up the polar caps on Mars easier with the 102ED, maybe some markings on the surface during the Mars close approaches and if seeing conditions are better then normal.

 

On Jupiter, the 102 F/7 should win because of more aperture.  Its still just a 102, so even on ideal seeing condtions which are extremely rare, you may be able to see festoons on Jupiters body if you use a barlow and push the magnification in the  250x range, but again, that would a be very rare night. You should be able to see Moon transits across the surface with the 102ED also, but again, if would be a struggle to see a full lunar disk and its shadow traveling across the surface. Seeing black dots would be more feasible for its moons across the surface.Should be able to count the bands better with a 102 vs 80 mm if the seeing condition's were very good.

 

On Saturn, you would have a better chance of seeing more details on the Ring with the 102. Maybe seeing some ring divisions and surface shading details, maybe able to pick up more than just Cassini division with very clear seeing conditions. 

 

On Uranus and Neptune, you need aperture and magnification. The 102 wins for Aperture and you'd have to barlow that scope and push the magnification to 200x plus to start making out and recognizing a blue disk on Uranus. Probably close to 300x  to see a tiny dim blue disk on Neptune. 

 

The Moon and Sun have their favorite scopes also, but they aren't planets.

 

 

.....Ralph

 

 

 

 

 

 

 

 

 

 

 

 

 

you can see plenty of details on Jupiter for example, in General everyday use you'dd probably hit the atomospheric cei

Would there be any advantage of a 3" F10 refractor that has slightly better optics over 4" F7 refractor for planetary observing.

Note both of these are ED refractors.

The 3" has FPL 53 lanthium glass.

The 4" has just FK-61 glass.



#19 Jon Isaacs

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Posted 02 January 2020 - 05:43 AM

For Mercury, my money would be on the 80mm F/10 with FPL-53. More then likely you would be chasing the sun so it would generally be low in the sky. Atmospheric turbulence would  probably equalize the size difference between the two, I'd give a slight edge to the 80mm F/10 with its superior glass. Basically you are trying to observe a star like object by observing Mercury. All you would really be doing was trying to get a tiny sphere out of Mercury and the 80mm F/10, like this one for example:

 

 

Mercury is a favorite of mine and I love viewing the crescent Mercury as it bobbles about sinking in the western skies.

 

For Venus and Mercury, only the outline is of interest in scopes like these and an aperture mask is sufficient to address the CA even in a ST-80 so I think these two scopes are a toss up on the two inner planets.

 

Venus is quite amazing in an ST-80 with only the center cap removed, It's a 42 mm F/9.5 and provides color free views of a white Venus against a black sky.

 

Jon


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#20 Sasa

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Posted 02 January 2020 - 05:58 AM

Would there be any advantage of a 3" F10 refractor that has slightly better optics over 4" F7 refractor for planetary observing.

Note both of these are ED refractors.

The 3" has FPL 53 lanthium glass.

The 4" has just FK-61 glass.

If both are of excellent optical quality, the only advantage for 3" that comes to my mind is thermalization in short winter planetary sessions.There are not many clear nights during winter in my location. Typically cloudy, misty nights, with only small holes in between clouds (both in space and/or in time). You would not take large telescope out under such conditions at all. At the end, I found myself using AS80/1200 in winter much often than larger ED100/900 simply because it was cooling down more quickly.

 

Otherwise, as others wrote, 4" offers more in terms of resolution. It is still light setup. In my view, 4" doublet is close to the optimal balance between portability, size, readiness to observe and power (capability). This is of course very observer depended.



#21 GlenM

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Posted 02 January 2020 - 06:47 AM

I believe that Long Perng are producing a 80mm F/10 FPL 53 Doublet Refractor. Should be out for testing in the UK this year.

 

Glen.



#22 25585

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Posted 02 January 2020 - 08:10 AM

SharpStar (China) makes them. Kasai sold them a short moment and they're currently sold by a Canadian vendor, whose name I can't recall at the moment.

 

 

Clear skies!
Thomas, Denmark

365astronomy too, recently.



#23 drd715

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Posted 02 January 2020 - 08:13 AM

What the planet's like is aperture. There are inherent advantages to a larger aperture, contrast, resolution, image brightness..

Some eyepieces perform better in slower scopes, some don't. Modern eyepieces have plenty of eye relief in the shorter focal lengths.. In this case, the 4 inch requires a 3.5 mm focal length to reach 200x, the 3 inch, a 4.8 mm to reach 160x. Orthos and the like will have short eye relief, modern negative-positive eyepieces will have a comfortable amount.

Jon

Yes aperture is the answer. Greater ability to resolve fine detail. Probably why I have a152 - 1200. About the biggest refractor i can handle for visual viewing. My old eyes (even with new lenses) don't do very well with short eyepieces - under 7mm, probably the smaller exit pupil. Now eyepieces are a separate complex subject.

A person can always go to a 9.25 sct with long fl in a short tube as a viable option for planets. But something hard to define makes the refractor image appealing. In the long run it seems that more than one scope is desirable. Just depends upon the type and size of the targeted subject. Everyone has their favorites and their personal favorites may change over time. The 4 inch (80-115mm broadly) class is a very sweet spot for refractors. Even with in this size range F ratio, physical focal length and desired width of view will point to different criteria in design.

Bigger and longer (for planets) is nice, but at a cost in both portability and ease of use.

Between the 80mm and the 102mm I would choose the 102mm as it can be masked down to the 80mm equivalent size and F number.

However more important in my view would be the optical quality of the objective first and the quality of the focuser also. I would want as color free and sharpest image as a first priority. Then a larger size solid dual speed smooth focuser.

So many choices, so much money (cost) and limited space. A quandary!

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#24 astro42

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Posted 02 January 2020 - 10:54 AM

I like the idea of the 80mm F10 but it is a niche scope that is also pricey about $1250 CDN.

I'm think if I want a 80mm refractor I would just pick up one of the mass produced (Skywatched EVO) ones that have a slightly shorter focal length for about half the price of the 80 F10.


Edited by astro42, 02 January 2020 - 01:05 PM.


#25 BillP

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Posted 02 January 2020 - 04:32 PM

Planets like longer physical focal length. Longer fl will allow a given eyepiece to perform better, the light cone angle isn't as steep. You may also get better eye relief using a longer eyepiece with a long fl scope and still get better power.

That is an interesting take!  And yes, eyepieces, the more classic varieties, do perform better when the light cone is not so aggressively short.  This is why I mostly have f/8 scopes.  And the eye relief thing is also true, although today there is no shortage of long ER eyepieces from inexpensive like the Paradigms and Hyperions to the premiums like XW and Delos.  However, back in the day before long ER eyepieces were very common, I used to do exactly what you say and use 25-32mm eyepieces for planetary, and had a custom Barlow made for me that did 5x-8x.  Using 32mm, like my 32mm Konig II eyepiece with its nice and big eye lens, with an 8x Barlow to get to 4mm was wonderful.  Eye relief was so very long and comfortable and newbies never ever had any issues using it and finding the view.  Views of planets also always seemed so much more resolute with this combo than with native 4mm eyepieces.  It was a nice combo.


Edited by BillP, 02 January 2020 - 04:33 PM.

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