Reflector vs Refractors

[Dannomiss]

8 hours ago, Timebandit said:

Also a great grab and go and very portable option is a pair of binoculars

I have some Olympus 10 x 50s and some of the best times have been laying back on sun lounger looking up at night with them

[cloudsweeper]

54 minutes ago, Barry-W-Fenner said:

I now have the joy of owning 3 reflectors and am a big fan of them. I haven't had the opportunity to try a refractor and k ow very little about them so can't comment. However I would certainly love to look at some doubles through a refractor and experiance the views they have to offer. 
 

IMO any opportunity you get to take in the wonderful views of our universe should be jumped on! Be it via reflector or refractor 😀

 

You're right there Barry.  

Concerning doubles, they are generally better/clearer with a frac.  

Doug.

[Stu]

2 hours ago, cloudsweeper said:

Says a lot John!  I've had 1 Mak, 1 SCT, 2 Reflectors, and 5 fracs!  Current audit: 1 SCT, 1 Dob, 3 fracs.

Having a decent selection means you have all bases covered.

Doug.

I seem to have seven fracs at the moment and be temporary custodian of an eighth. It’s an affliction I tell you! One dob, two newts and one Dall-Kirkham (just to be different! 🤣)

[Tommohawk]

10 hours ago, mikeDnight said:

Im not following your point regarding the 300mm vs 60mm. As far as light grasp goes, a 300mm reflector gathers considerably more light than a 60mm refractor and will go far deeper and show far more detail, including colour in many deep sky targets because it has more than 25X the surface area of the 60mm. Apart from giving beautiful rich star fields and pleasing lunar views, i really cant see any advantage of a 60mm scope over a 300mm. It's resolution is too limited for a planetary advantage and its light grasp would not be enough to show the majority of DSO's well.

I guess the point I was trying to make is that whilst greater aperture has to be better in itself, you need to realise that from a practical perspective a bigger aperture usually means a longer focal length. So as you say, a 300mm aperture scope will give an image 25x brighter than a 60mm scope having the same focal length. That would be fabulous. However, the reality is that the 300 mm instrument is likely to also have 5x the focal length (it would if it had the same F value, so you have 25x more photons spread over an image which is 25x larger (in area). 

My error was to put an imager's slant on this. With larger targets, such as Andromeda galaxy, a 60mm instrument with a FL of say 300mm would probably just cover a typical sensor. A 300mm instrument with the same F value would gather 25x more photons, but it would produce an image 25x larger (by area) and you would therefore get no extra relevant photons on the sensor. The sensor would collect the same number of photons as with the 60mm scope (actually thats not quite true because of course the core is brighter than the periphery, but the point is made hopefully.

Presumably with visual work you switch to a longer FL eyepiece to avoid having too narrow a field of view. And of course you can scan around. 

In any event, I was simply trying to make the point that aperture isn't everything. But maybe it is?

[Stu]

12 minutes ago, Tommohawk said:

In any event, I was simply trying to make the point that aperture isn't everything. But maybe it is?

I think I’m both instance aperture is probably about image scale. For imaging you would choose a larger, longer focal length instrument for imaging small galaxies for instance, and for visual the benefit of large dobs is that you can magnify targets more whilst maintaining image brightness.

[Alien 13]

2 minutes ago, Stu said:

I think I’m both instance aperture is probably about image scale. For imaging you would choose a larger, longer focal length instrument for imaging small galaxies for instance, and for visual the benefit of large dobs is that you can magnify targets more whilst maintaining image brightness.

Totally agree, there are views that you can see with a 60mm refractor that are impossible with a reflector no matter how big it is and vice versa..

Alan

[andrew s]

You have to wonder why professional astronomers seek larger and larger telescopes given how unimportant aperture seems to be.

Maybe it's because amateur astronomers focus largely on relatively large and relatively bright objects and when they don't  they seek (let me speculate) aperture. 

Regards Andrew 

[andrew s]

16 minutes ago, Alien 13 said:

Totally agree, there are views that you can see with a 60mm refractor that are impossible with a reflector no matter how big it is and vice versa..

Alan

I am struggling with this. Can you give some examples? Are you thinking visual and or imaging?

Regards Andrew 

[Stu]

10 minutes ago, andrew s said:

I am struggling with this. Can you give some examples? Are you thinking visual and or imaging?

Regards Andrew 

If you read my ‘A tale of two Veils’ post Andrew you will see the example for a 72mm frac vs a 350mm dob. One gives up close, detailed views of the Veil, the other shows it in its entirety, set in context which is a very different, but equally rewarding experience and one of my favourite visual views.

[andrew s]

5 minutes ago, Stu said:

If you read my ‘A tale of two Veils’ post Andrew you will see the example for a 72mm frac vs a 350mm dob. One gives up close, detailed views of the Veil, the other shows it in its entirety, set in context which is a very different, but equally rewarding experience and one of my favourite visual views.

Ok thanks, but that's not what I thought was being proposed. 

I was being too absolutist thinking what could be done rather than what is commonly is done. 

Regards Andrew 

[Alien 13]

29 minutes ago, andrew s said:

I am struggling with this. Can you give some examples? Are you thinking visual and or imaging?

Regards Andrew 

Was thinking visual, no matter how big or small a scope is you can only get 6mm worth of light in..

Alan

[Stu]

9 minutes ago, andrew s said:

Ok thanks, but that's not what I thought was being proposed. 

I was being too absolutist thinking what could be done rather than what is commonly is done. 

Regards Andrew 

I guess if we had f1 20” dobs with amazing coma correctors and we all had pupils the size of bush babies then things might be different 😉. As it is, smaller, short focal length apo refractors are about the only way to get widefield visual views of the larger objects; amazing what they can show under a dark sky.

[cloudsweeper]

10 hours ago, Stu said:

I guess if we had f1 20” dobs with amazing coma correctors and we all had pupils the size of bush babies then things might be different 😉. As it is, smaller, short focal length apo refractors are about the only way to get widefield visual views of the larger objects; amazing what they can show under a dark sky.

Interesting theoretical concept Stu, a focal ratio of unity.  Apart from any other considerations, it would probably produce a background sky like a beacon, I reckon!

Doug.

[Stu]

56 minutes ago, cloudsweeper said:

Interesting theoretical concept Stu, a focal ratio of unity.  Apart from any other considerations, it would probably produce a background sky like a beacon, I reckon!

Doug.

Indeed, I’m not saying it wouldn’t be without its problems; huge secondary required for a start (probably the same size as the primary 🤣🤣🤣), massive exit pupil, extreme coma etc etc. You would probably be able to use very tight photographic line filters though which might be fun.

My intention though was to illustrate the point that smallish, short focal length apos are the best way to get widefield views.

[cloudsweeper]

55 minutes ago, Stu said:

My intention though was to illustrate the point that smallish, short focal length apos are the best way to get widefield views.

Very true! 

(I hope I didn't come across as being in any way critical - I certainly had no such intention!  It's just that my theoretical/scientific interest was piqued by the notion of an extremely low focal ratio!  🙂)

Doug.

[andrew s]

I am happy to accept that if you can have two or more telescopes then a small apo refractor for bright wide field observing and a large reflector for dimmer smaller objects is ideal. 

However, that is not the point I have been trying to counter. I have just been looking at the intrinsic comparison of reflectors and refractors.

Say you are looking for your first telescope and you go to the FLO website what might you chose for visual work. 6mm max pupil as per @Alien 13.

Sky_watcher for price performance. You might consider:

Heritage 150 P 150 mm diameter f/5 750 mm fl £199

Heritage 130P 130 mm diameter f/5 650 mm fl £142

Evostar   80ED  80 mm diameter f/7.5 600 mm fl £379 without mount

Evostar 100ED 100 mm diameter f/9 900 mm fl £757 without mount.

I would submit that there are eyepieces available to allow the maximum possible field stop for all these in a 2"  and that for all of them the magnification can be such that the exit pupil is 6mm or less. The reflectors can be equipped with a coma corrector for the cost of the refractor mounts / field flateners.

I conclude that at the two ends of the spectrum the small refractor and large Dobsonian are givens but there is an overlap where things are not so clear cut.

On @Stu f/1 Dob it might be worth considering that the ESO VLT 8m mirrors are f1.8  and while it might not do for visual that f/1 Schmidt Cameras were made.

Indeed as a teenager I visited the Cambridge astronomy department where the were trying out an all mirror telescope design which is only now being build as the LSST (3.5 degree field). The main mirror was just 200mm in diameter. 

So extremes are possible for specific use just like the Scopetech 80mm f/12.5 or LSST.

Regards Andrew

PS In the end I don't think the technology matters that much. Some of the best telescopes are hybrids. 

 

 

 

Edited July 5, 2020 by andrew s

[markse68]

1 hour ago, Stu said:

Indeed, I’m not saying it wouldn’t be without its problems; huge secondary required for a start (probably the same size as the primary 🤣🤣🤣), massive exit pupil, extreme coma etc etc. You would probably be able to use very tight photographic line filters though which might be fun.

My intention though was to illustrate the point that smallish, short focal length apos are the best way to get widefield views.

Mel Bartels takes it to extremes with his high  etendue scopes  25” f2.6  😳 https://www.bbastrodesigns.com/25/25 inch F2.6 Telescope.html 

Sounds like you could almost go for his philosophy Stu- one eyepiece, multiple scopes 😂

[Tommohawk]

1 hour ago, andrew s said:

Heritage 150 P 150 mm diameter f/5 750 mm fl £199

Heritage 130P 130 mm diameter f/5 650 mm fl £142

Evostar   80ED  80 mm diameter f/7.5 600 mm fl £379 without mount

Evostar 100ED 100 mm diameter f/9 900 mm fl £757 without mount.

I think we can definitely agree that reflectors like this are incredible value, and offer a versatile solution for most users needs, especially where budget is an issue. 

Hows that? Not too contentious hopefully? 😉

[Macavity]

Late to this, but the words of Sir Patrick: "Inch for inch reflectors are less efficient than
refractors"etc., are permanently etched into my brain. [lol]. Idem "small" 2" refractors
being "useless"? This stuff has to be put into (historical) context... yet also intrigues!
(Not wishing to add it here... I have some ideas re. what SPM was thinking/doing...) 😎

But then Amateur Atronomy (science generally?) is significantly influenced by ADAGE
and (even) the "status" of the people who say it? I just mention that for anybody who
believes that "peer review" (even "Popular Scientist" opinion ) is completely infallible! 🥳

On the other hand, my way of deciding (on any subject with "polarized" opinion) is to
read a LOT of posts... observe the "concensus"... Check out what folk, whose opinion I
tend to value, say? Best of all (though sadly a bit LOT more expensive) try it yourself! 😛

As a beginner (to practical Astronomy) - confronted by the plethora of scope types,
I started out with an ST102 & MAK90. Maximise/study the (non-aperture) difference. 😉
I quickly learned that both have their merits...

This made me chuckle recently:
https://www.facebook.com/StarToonsbyAdamMaund/photos/a.194320497876114/605422446765915/

 

Edited July 5, 2020 by Macavity

[Tenor Viol]

The lengthy (sorry!) post I made earlier I pointed out how fashions have changed in amateur astronomy in the last 40 years, driven basically by the technology forces of making precision optics and digital photography.

Regardless of type of optic, resolution is firmly dependent on aperture. Image scale is affected by focal length, and brightness of image is a function of aperture and focal ratio.

   

[Timebandit]

45 minutes ago, Macavity said:

Late to this, but the words of Sir Patrick: "Inch for inch reflectors are less efficient than
refractors"etc., are permanently etched into my brain

 

 

This may be the case, and I certainly am not going to question the late great Sir Patrick.

But from a amateur astronomer . I can afford a great 4" to 5" refractor. I can afford to buy a mount to use this refractor. 

But I cannot afford to buy 14" refractor, I cannot afford to buy a Mount for a 14" refractor and I cannot afford to build an Obs to keep this in, and enable it use. 

I can afford to buy a 14" reflector on a dob mount .

So inch for inch a quality refractor may be better than a quality reflector, But practically wise and cost wise a reflector is a must for most of us that want great light gathering ability that we can practically use and afford. 

 

 

 

[DaveL59]

3 minutes ago, Timebandit said:

 

 

But I cannot afford to buy 14" refractor, I cannot afford to buy a Mount for a 14" refractor and I cannot afford to build an Obs to keep this in, and enable it use. 

 

 

 

 

don't forget too, being able to LIFT the thing onto the mount, unless you've an engine crane handy of course  

[jetstream]

The best scope regardless of type is one that will get used often and is easy to take to dark skies. IMHO.

[markse68]

i’ve just spent a very pleasant afternoon wondering around the gardens at Wisley with my mum on the first sunny day in what seems like ages! I was wearing sunglasses and the colours were so saturated and contrasty- very beautiful and kodachrome-like. When I lifted them the view was obviously brighter but the colours were more muted- everything looked greyer. It struck me that there was a similarity to the difference between my dob and the only refractor i’m familiar with- my little telementor. The telementor seems to present things in a richer more contrasty kodachrome way. But is it real? Or is it like wearing sunglasses? Does it matter lol- I have a uhc for the dob

 

[Stu]

6 hours ago, cloudsweeper said:

Very true! 

(I hope I didn't come across as being in any way critical - I certainly had no such intention!  It's just that my theoretical/scientific interest was piqued by the notion of an extremely low focal ratio!  🙂)

Doug.

No, not at all Doug 👍👍😁