Long focal length newtonians

[N3ptune]

Hello!

I was reading about William Herschel and his amazing life has a composer and has an astronomer, It's really incredible to succeed in both these disciplines, the man was a genius. In the same page I also saw something else that caught my attention, there were a few images images of telescopes Herschel or Gallileo were using at the time, they were newtonians but with odd dimensions like:

6 inches diameter by 7 foot long

https://en.wikipedia.org/wiki/File:HerschelTelescope.jpg

or 18.5 inches diameter by 6 meters long.

It seems like the size ratio of Newtonians is different nowadays, why is that?

--> Why can I use a 8 inches by 1 meter now and then it was a 6 inches by 7 foots!!  (Huge difference.)

Or

--> What benefit would I get for visual observation if today I had a 8 inches by a few meters long newtonian? (Against my actual instrument, has a benchmark)

I would like to know that if by chance someone knows.

Thanks (:

[spaceman_spiff]

My guess is that back then increasing the F number of the telescope (increasing the focal length for a given aperture) would reduce the effects of optical aberrations caused by mistakes in mirror manufacture. The old reflectors may have used a spherical primary (easy to make) which at large F numbers will also give a good image. Im not sure about this though.

Nowadays mirrors are made with much better precision and we can get far more aperture without errors such as astigmatism, spherical aberrations and other defects. There are some long tube reflectors on the market, and because of their higher F numbers they will have less coma (the main aberration caused by a parabolic mirror), a smaller central obstruction (possibly more contrast) and will be less demanding on eyepieces.

Also having long reflectors like Herschel's is really not practical for most people!

Dan

[N3ptune]

Thanks Dan for this reply, it makes sense, a longer tube to amortize mirror defects of the time. I have a little bit of coma in my telescope but not much now, it's true, some said to me because it was too short.

[YKSE]

Spherical mirrrors were surely part of the reason, another reason should the eyepieces available at that time. 

According to Chris Lord's "Evolution of the Astronomical Eyepiece" eyepieces available then were Keåöer. Huyghenian, Ramsden, Herschel, Dollond, etc, all have critical focal ratio f12 or slower, meaning the scopes need to be f12 or slower to have satifactory correction for chromatical aberration (axial colours and lateral colours) and spherical aberration.

Nowadays eyepieces have much better colour and shperical aberration corrections, therefore the focal ratio can be quite shorter.

[N3ptune]

Ok it's solely a question of technology regarding eyepieces and mirrors. 

--> it would be pointless to build a longer newtonian with the mirror fabrication technology we have today and use a ratio like it was used at Herchel's time.

Thanks for your time here!

[Peter Drew]

I have a few long focus reflectors and there is no doubt that provided you can mount them adequately the reduction in secondary obstruction and other optical aberrations results in excellent contrast and high resolution performance. A couple of other reasons that the reflectors of Herschel et al were long focus are that many did not have secondaries, just the primary tipped to bring the focal point to the top edge of the tube. Without a long FL astigmatism would have been unacceptable. Also, early mirrors were made of speculum metal which tarnished relatively quickly and required frequent repolishing, much easier to retain a spherical surface rather than a parabola!  

[mikeDnight]

Herschel 6" 7foot long reflector was a genuine Newtonian and it was reportedly his favourite telescope. He was so fond of this particular telescope that over the years he made several new primary mirrors as well as many more new flats.

 It's a shame long Newtonians are not readily available. Short Newts have serious aberration issues when even fractionally off axis and just aren't in the same league as long ones! I'd love an 8" F10! 

[Astrobits]

Don't forget that the techniques of mirror testing "in the lab" were developed after Newton's and Herschel's time. They had to rely on the star image when their mirrors were used -- a much more tedious affair and one that didn't necessarily indicate where the error was and how to correct it.

Nigel

[andrew s]

1 hour ago, mikeDnight said:

 It's a shame long Newtonians are not readily available. Short Newts have serious aberration issues when even fractionally off axis and just aren't in the same league as long ones! I'd love an 8" F10!

You could get a 16" F5 mirror and stop it down to 8".

Regards Andrew

[Putaendo Patrick]

I wonder how common reflector telescopes were before the development of glass mirrors in the mid 19th Century. Herschel's telescopes were by all accounts of exceptional optical quality for their time, despite the problems of speculum which suffered from tarnishing and also distortion due to temperature changes. Nevil Maskelyne, the astronomer royal, for example was initially very sceptical of Herschel's reflectors!

I guess longer focal length was sought mainly to increase magnification - Johannes Hevelius probably came close to the record with a 47 meter long refractor in the second half of the 17th century! Historically it seems, planetary and lunar observation was more important in this period, for example Hevelius on the Moon, Huygens with Titan, Cassini on Saturn's belts, Herschel with Uranus as well as two more of Saturn's moons etc.

Also, before the end of the 19th century, there was no standardisation of EPs, and I imagine they were often a rather hit and miss affair!

 

 

[Peter Drew]

Magnification, other than too much was not the main problem, it was the reduction of CA and other aberrations of single element objectives that could be mitigated by long focal lengths. Herschel made short focus single element eyepieces by dropping molten glass into water which formed spherical "lenses".