Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

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

  • Please log in to reply
42 replies to this topic

#26 Redbetter

Redbetter

    Cosmos

  • *****
  • Posts: 8,537
  • Joined: 16 Feb 2016
  • Loc: Central Valley, CA

Posted 02 January 2020 - 06:19 PM

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.

 

 

While not inherent to the focal length itself, there are some aspects that tend to coincide with longer focal length that can make some difference to contrast surrounding bright objects.  This has more to do with the baffle design and layout for the intended use of the scope.  A longer ratio (and focal length) "planetary" refractor is likely to be more tightly baffled since it is unlikely to be designed to fully illuminate much of a 1.25" field stop, let alone 2". 

 

This becomes most noticeable at the extremes:  e.g. an AT60ED with a 2" focuser at f/6.  There isn't a whole lot of baffling that can be done that impacts the contrast of center of the field for such a short system without impacting the ultra wide field capability of the scope.  Whereas a 60 f/15 achro tends to be very aggressively baffled, so that only a few millimeters of the central field are fully illuminated.  I don't know that the effective difference is actually noticeable for planetary detail, but I have had some indication of it for things like threshold detection of Saturn's moons in small instruments in town.

 

There is some sacrifice involved in being able to do it all.



#27 daquad

daquad

    Apollo

  • *****
  • Posts: 1,412
  • Joined: 14 May 2008

Posted 02 January 2020 - 07:15 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.  

I think you left out the last digit in your focal lengths.


  • BFaucett likes this

#28 mikeDnight

mikeDnight

    Viking 1

  • -----
  • Posts: 869
  • Joined: 19 Apr 2015
  • Loc: Lancashire, UK

Posted 03 January 2020 - 08:19 AM

Despite its apparent limited resolution, a good 4" refractor can often keep pace with larger aperture scopes because of a quality that refractors tend to excell at, and that is definition. Resolution and contrast are important but definition is paramount in a planetary scope, and optical quality is a determining factor. Resolution is effectively set by the aperture, but with planetary detail the resolution limit goes well beyond that set for stellar point sources. As regards contrast - well that can be increased with the use of increased magnification - most obviously noticed when observing deep sky. The use of good eyepieces play an important roll in getting the best out of all three qualities.

 

As regards longer focal lengths being best for planetary, well I think this stems from the era of the the achromats, where the longer scope controlled the colour spread better and so gave the superior definition. Today there's no real need for long focal length refractors as colour spread can be matched or even superseded using modern ED glasses. Using an F7 refractor with a 2X or 3X barlow will allow a longer focal length eyepiece to be used for high magnification views, retaining the comfort and eye relief of the eyepiece. 

For a time I used a Takahashi 1.6X extender Q amplifier with my Tak FC100DC, giving me an F11.8 focal ratio. The views were simply outstanding in that the Q was invisible in the light path and provided a perfect view. On one evening when I was observing the Moon with the Q attached, and at the same time using a 2X barlow in my binoviewer with paired 16.8mm orthoscopics, I swept across the appenine mountains. As I crossed the cliff edge of the mountains in 3D, my stomach rolled and I gave out a slight cry, until I realised I wasn't actually going to fall. However, when removing the Q, the textbook star images of the FC100DC and its superlative planetary definition remaind the same. The only real advantage for me was that the Q allowed the use of very higher powers, especially in conjunction with a barlow. I now only use my scope at its native F7.4, but with a 2X barlow in my binoviewer. At F7.4 I get outstanding planetary views, yet i have the added advantage of having a beautiful rich field refractor for sweeping the milkyway. 

Interestingly, I also have a 10" F6.3 Dob which gives great planetary views, but I still prefer using my refractor for planetary observation.


  • daquad, Sasa and Ihtegla Sar like this

#29 leonard

leonard

    Apollo

  • *****
  • Posts: 1,284
  • Joined: 19 Oct 2007
  • Loc: West Virginia

Posted 03 January 2020 - 09:41 AM

Despite its apparent limited resolution, a good 4" refractor can often keep pace with larger aperture scopes because of a quality that refractors tend to excell at, and that is definition. Resolution and contrast are important but definition is paramount in a planetary scope, and optical quality is a determining factor. 

 

               Yes optical quality is all important with a planetary scope weather reflector or refractor .

 Seems contrast and definition are inextricability linked .



#30 Gregory Gross

Gregory Gross

    Ranger 4

  • -----
  • Posts: 313
  • Joined: 13 May 2017
  • Loc: Pacific Northwest

Posted 13 January 2020 - 03:53 PM

I've been reading this thread with great interest, as I've been pondering the acquisition of a lightweight long-focus refractor with superior optics for some time. I've been learning that finding such a thing is a lot harder that I would have thought.

 

The balance I'm trying to strike is having a scope with better optics and better performance than what the typical 80mm f/10 achromat has but with the benefits that a smaller aperture brings in terms of more forgiving mounting requirements.

 

The needle I'm trying to thread is finding something along the lines of f/10 or f/11 with a good amount of aperture, superior optics to eliminate as much false color as possible, but still small and light enough to be carried on my Orion AstroView mount, which is far less cumbersome (and is thus far more used) than my heavier mounts. My main interest for such a scope is for full-disk lunar and, with Herschel wedge, solar observing.

 

Planetary work would be an added bonus but not an essential task for this scope. I have a 4" Mak and C8 that both perform very well for planetary observing, the former being well suited for those grab-and-go planetary sessions and the latter providing bright, well-resolved, and satisfying views. So I feel like my bases are covered on the planetary side.

 

Not too long ago, I had the opportunity to try out a Herschel wedge on a variety of refractors: a 72mm f/6 ED doublet, an 80mm f/6 ED triplet, a 100mm f/6 achromat, and a 90mm f/11 achromat. The performance of the 90mm f/11 achromat easily blew away the views that those other scopes offered even in spite of their better optics or larger aperture. Standing at the eyepiece, the difference was immediately obvious. The Sun's photospheric granularity in the 90mm f/11 achromat just crackled and popped with detail and contrast that I simply could not pick up in the other scopes. False color was there, to be sure, but there was a certain je ne sais quoi that was evident in the 90mm f/11 achromat and that was missing in the other scopes. Hard to describe.

 

I know that others on this thread have argued in favor of the faster but larger aperture scopes over a slower 80mm scope. But given the specific applications I have in mind, my mounting requirements, and my own real-world experience, I'm tending towards smaller aperture and longer focus.



#31 aa6ww

aa6ww

    Vanguard

  • *****
  • Posts: 2,285
  • Joined: 23 Oct 2011
  • Loc: Sacramento, Calif.

Posted 13 January 2020 - 04:24 PM

This is only true if the seeing conditions limit the size of a larger refractor to not reach is potential.

Seeing conditions are by far the most important factor in the quality of any refractor or any telescopes performance, not how great it's optics are. Again, as I always do, I turn to Jupiter, the most difficult object to scrutinize in searching for optical performance.

On nights of near perfect to perfect seeing conditions, bigger always performs better. One look at Jupiter with a 4" vs a 6" APO clearly shows why bigger is better. The dark colors are darker, the light colors are lighter, much more details become obvious on the surface details. Colors become more obvious as apertures increase. Salmon and blue colors come through nicely.

It will no longer be about sharpness, or magnification, it will be about "details."

Little 4" refractors are excellent for back yard fun. They are more forgiving in less then Ideal seeing conditons, and of course excellent in excellent conditions.

Under very good to excellent seeing conditions, a large APO will give a crushing blow to any 4" refractor, regardless of the name on the label.

Try spending a full night observing the GRS transit across the entire Jovian surface. It takes a few hrs. Using a 6" APO for example, in the course of those two hrs, you can watch the entire surface dance and move, not just rotate. You can watch festoons change shape. You can't see this visually in a 4" refractor, there's not enough resolution to see these types of details. In a 4" refractor, you hit a point were more magnification just becomes a bigger number, but the details seen don't increase as magnification increases. You may top out at 300x and hold onto the details, but if the seeing conditions are excellent, you can push the magnification to 400x and above, with amazing details on the larger scopes. These are definitely rare days, but they happen every year. On days like this, you wake up your family to see what you see also. You stay up all night and take the day off if you can, since they are fun times.


Everything where more aperture wins, a larger refractor will win, when the playing field is equalized, when the seeing conditions allow the larger scope to perform to its size.

By the middle of winter, we can usually convince ourselves that the small scopes we can still use in winter are really all we need. They are untouchable.

Once the weather warms up and we start getting out more, the big scopes start to come out and with a sigh of relieve, we remember why we bought them and continue to own them and want them.

On predicted nights of excellent seeing conditions, a large quality reflector will also give the death blow to any 6" APO also on almost everything you can look at.

When the playing field is equal and the seeing conditions are there, when it maters, the small scopes can try, but they cant keep up, and its really not that close also.

...Ralph


 

Despite its apparent limited resolution, a good 4" refractor can often keep pace with larger aperture scopes because of a quality that refractors tend to excell at, and that is definition. Resolution and contrast are important but definition is paramount in a planetary scope, and optical quality is a determining factor. Resolution is effectively set by the aperture, but with planetary detail the resolution limit goes well beyond that set for stellar point sources. As regards contrast - well that can be increased with the use of increased magnification - most obviously noticed when observing deep sky. The use of good eyepieces play an important roll in getting the best out of all three qualities.
 
As regards longer focal lengths being best for planetary, well I think this stems from the era of the the achromats, where the longer scope controlled the colour spread better and so gave the superior definition. Today there's no real need for long focal length refractors as colour spread can be matched or even superseded using modern ED glasses. Using an F7 refractor with a 2X or 3X barlow will allow a longer focal length eyepiece to be used for high magnification views, retaining the comfort and eye relief of the eyepiece. 
For a time I used a Takahashi 1.6X extender Q amplifier with my Tak FC100DC, giving me an F11.8 focal ratio. The views were simply outstanding in that the Q was invisible in the light path and provided a perfect view. On one evening when I was observing the Moon with the Q attached, and at the same time using a 2X barlow in my binoviewer with paired 16.8mm orthoscopics, I swept across the appenine mountains. As I crossed the cliff edge of the mountains in 3D, my stomach rolled and I gave out a slight cry, until I realised I wasn't actually going to fall. However, when removing the Q, the textbook star images of the FC100DC and its superlative planetary definition remaind the same. The only real advantage for me was that the Q allowed the use of very higher powers, especially in conjunction with a barlow. I now only use my scope at its native F7.4, but with a 2X barlow in my binoviewer. At F7.4 I get outstanding planetary views, yet i have the added advantage of having a beautiful rich field refractor for sweeping the milkyway. 
Interestingly, I also have a 10" F6.3 Dob which gives great planetary views, but I still prefer using my refractor for planetary observation.


Edited by aa6ww, 13 January 2020 - 04:46 PM.

  • Sasa and Lookitup like this

#32 barbie

barbie

    Apollo

  • *****
  • Posts: 1,050
  • Joined: 28 Jan 2013
  • Loc: Northeast Ohio

Posted 13 January 2020 - 04:59 PM

I get much enjoyment and see much detail in my sensibly perfect 4inch apos than the larger mediocre 6" apos I've viewed through. Aperture means nothing without excellent optical figuring.  I would much rather view with a high quality 4"(or even 5") aperture than a mediocre 6" aperture and the 6" apos I've viewed through left much to be desired. The spherical correction was not good at all in these units and they couldn't achieve clean splits on close doubles that were well within the limits for a 6" aperture. I won't cite brand names here but will say that they were of the recently released (within the last couple of years vintage) and mass produced with very poor to mediocre optical figuring.  A sensibly perfect 4" would be much preferred by me to a mediocre 6". Then there's the problems with ease of setup or transport. Not everyone can lift a heavyweight 6" Apo up to an even heavier mounting which is needed to support a larger scope this size!! They simply aren't portable for us older folks with bad backs and knees!!


Edited by barbie, 13 January 2020 - 05:05 PM.

  • GlenM, Sasa and Lookitup like this

#33 Bonco2

Bonco2

    Ranger 4

  • *****
  • Posts: 321
  • Joined: 01 Jun 2013

Posted 13 January 2020 - 05:00 PM

I have a Carton 80mm f/15 and a 4 inch Genesis f/5. For high power planetary views the Genesis is slightly better but the 80mm is close behind.  Same for double star observations. But for DSO's and wide field observing the Genesis wins hands down. I enjoy owning both of them.

Bill



#34 Jared

Jared

    Fly Me to the Moon

  • *****
  • Posts: 6,152
  • Joined: 11 Oct 2005
  • Loc: Piedmont, California, U.S.

Posted 13 January 2020 - 06:36 PM

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.
 

I suspect it’s a combination of several factors:

 

* Polychromatic Strehl is higher in slower scopes

* With a given eyepiece powers are higher with the slower scope, darkening background often creating the perception of higher contrast if making comparisons with the same eyepiece.  Yes, some people do that.

* Till fairly recently, short focal length eyepieces had ridiculously little eye relief, making viewing a less comfortable experience with faster scopes thus increasing eye strain

* Optical quality is typically higher with slower scopes, not just sphero chromatism but also other aberrations, since the curves are less aggressive and alignment less finicky

* Back in the day of single speed focuses, achieving precise focus with a fast scope was not all that easy, so less experienced viewers in particular may not always have been looking at the best image the scope could produce

* I have always suspected that longer optical tubes placing the objective an extra foot or two in the air improves the seeing a bit as well—enough to notice on some nights and times where ground effects dominate over high altitude seeing

 

Some of these factors are historical only and don’t really apply any more.  Others would be true even today.  My guess is that even now a slower refractor will typically yield a better image all other factors being equal.  Doesn’t stop me from buying faster scopes due to convenience and astrophotography requirements.


  • m9x18 likes this

#35 drd715

drd715

    Mariner 2

  • -----
  • Posts: 252
  • Joined: 07 Jan 2015
  • Loc: Fort Lauderdale

Posted 13 January 2020 - 11:19 PM



I get much enjoyment and see much detail in my sensibly perfect 4inch apos than the larger mediocre 6" apos I've viewed through. Aperture means nothing without excellent optical figuring. I would much rather view with a high quality 4"(or even 5") aperture than a mediocre 6" aperture and the 6" apos I've viewed through left much to be desired. The spherical correction was not good at all in these units and they couldn't achieve clean splits on close doubles that were well within the limits for a 6" aperture. I won't cite brand names here but will say that they were of the recently released (within the last couple of years vintage) and mass produced with very poor to mediocre optical figuring. A sensibly perfect 4" would be much preferred by me to a mediocre 6". Then there's the problems with ease of setup or transport. Not everyone can lift a heavyweight 6" Apo up to an even heavier mounting which is needed to support a larger scope this size!! They simply aren't portable for us older folks with bad backs and knees!!


The best description of the dilemma concisely presented in an relatable way.

Sent from my SM-N950U using Tapatalk

#36 MalVeauX

MalVeauX

    Fly Me to the Moon

  • *****
  • Posts: 6,194
  • Joined: 25 Feb 2016
  • Loc: Florida

Posted 14 January 2020 - 08:37 AM

The 4". More aperture. Easier to mount. Overall better planetary image into your eyeball.

 

The 80mm F10 FPL53 is really just a niche scope for folk wanting a "classic refractor" experience, with a rather silly price tag on this thing, but with modern optics, focuser, rings, etc.

 

If you want a long focus refractor with no CA and minimal SA, look at the 102mm F11 ED. It's bino-viewer ready, good focuser, no CA, planet buster and it's inexpensive.

 

One thing though, short stubby fast scopes are easier to mount than physically long scopes, regardless of weight. For example a mount may handle a C6 (6" F10 SCT) but won't handle a 4" F7 frac, let alone something F10~F11 at any aperture frac likely. You have to have a substantial mount to have a physically long refractor well mounted if you want to use higher magnification for planets. Keep this in mind. This is also another reason the 4" F7 is a much better candidate overall if you take this into consideration.

 

Very best,


Edited by MalVeauX, 14 January 2020 - 08:39 AM.


#37 Paul Sweeney

Paul Sweeney

    Explorer 1

  • -----
  • Posts: 87
  • Joined: 19 Feb 2018
  • Loc: Heidelberg, Germany

Posted 16 January 2020 - 06:23 AM

All else being equal, aperture wins. But that is rarely the case. Optical quality is what really makes the difference. How often do we hear of small refractors beating Newts? Or bigger refractors? The better the quality, the more details will be visible. On most nights, my Vixen 80mm F15 will give the clearest image with the best detail. The optical quality is outstanding. But when the air is stable, my 120mm f8 achro will start winning, but with a lot of color. On really good nights, the 12" dob blows everythin out of the water. But how often is that? Go with whichever scope has the better lens quality. That way, the major limiting factor will be the seeing , not the scope.

Paul


  • Bonco2 likes this

#38 Gregory Gross

Gregory Gross

    Ranger 4

  • -----
  • Posts: 313
  • Joined: 13 May 2017
  • Loc: Pacific Northwest

Posted 16 January 2020 - 11:59 AM

I'd add to Paul's comments by saying that one should go with whichever scope is the best one for the job. Sometimes one is in the mood for a little of everything, so a jack-of-all-trades scope works best. Other times, one is in the mood for a quick-look session at the Moon. A small scope with good optics and little if any false color is best for that. It all depends.

 

To tie this back to the topic of this thread, I'd say that a long-focus 80mm refractor with good glass will perform well precisely for those quick-look lunar or, with proper filtering, solar sessions. Aperture will clearly offer someone a more immersive experience but at the cost of increasing mounting requirements.



#39 astro42

astro42

    Vostok 1

  • *****
  • topic starter
  • Posts: 191
  • Joined: 07 Jan 2019
  • Loc: Alberta,Canada

Posted 16 January 2020 - 12:15 PM

Thanks everyone for all the comments and suggestions its been very informative.



#40 Gregory Gross

Gregory Gross

    Ranger 4

  • -----
  • Posts: 313
  • Joined: 13 May 2017
  • Loc: Pacific Northwest

Posted 16 January 2020 - 01:33 PM

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.

Just curious, astro42: After the multitude of responses this thread got, do you still feel the same way about what an 80mm f/10 ED refractor with FPL 53 lanthanum glass would offer you? Or do you think that a 4" refractor is the way to go?

 

I'm not looking for a particular answer. Just curious about where your priorities are and what particular scope would suit those priorities best. It's cliché to say that there is no one perfect telescope for all applications.



#41 astro42

astro42

    Vostok 1

  • *****
  • topic starter
  • Posts: 191
  • Joined: 07 Jan 2019
  • Loc: Alberta,Canada

Posted 16 January 2020 - 02:28 PM

Just curious, astro42: After the multitude of responses this thread got, do you still feel the same way about what an 80mm f/10 ED refractor with FPL 53 lanthanum glass would offer you? Or do you think that a 4" refractor is the way to go?

 

I'm not looking for a particular answer. Just curious about where your priorities are and what particular scope would suit those priorities best. It's cliché to say that there is no one perfect telescope for all applications.

After reading through the thread a few times I have kind of change my thinking from my previous response.

I'm getting close to buying the 80 F10 scope just to have something a bit different than the norm 80mm  f7 short focus scope that most everyone buys.

I guess like the guys that buy a $5000  3 1/2" Questar or the $5000 Takahashi 4" refractor.

You get the best optics and build quality but a $900 12" Dob will blow away the very expensive small refractors 95% of the time.

Of course the 80 F10 is not on the same level as a Questar or Takahashi but its a cool unique little scope that I'm going to enjoy.


Edited by astro42, 16 January 2020 - 09:07 PM.

  • Tyson M likes this

#42 Gregory Gross

Gregory Gross

    Ranger 4

  • -----
  • Posts: 313
  • Joined: 13 May 2017
  • Loc: Pacific Northwest

Posted 16 January 2020 - 02:40 PM

I'm finding it rather difficult to find sub-100mm long-focus refractors with premium optics in production. I would think that there would be more of a market for them, and I'm surprised there aren't more of them out there.

 

If you decided to execute on your thinking, don't hesitate to share your thoughts with us. I'm very curious about the scope you have your eye on and would welcome your thoughts on it.



#43 astro42

astro42

    Vostok 1

  • *****
  • topic starter
  • Posts: 191
  • Joined: 07 Jan 2019
  • Loc: Alberta,Canada

Posted 16 January 2020 - 09:05 PM

I think I'm going to order the 80 F10 tomorrow.

Ill let you know my impression once I receive it.


  • Tyson M likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics