Rich Field Telescope.

[Guest]

"The richfield concept is a telescope that shows the most stars in the field of view. This can be a telescope of most any aperture but it will have a comparatively short focal length for it's aperture so it can provide the brightest, widest field of view"

The above is a definition of a rich field telescope. I always thought that a six inch f5 Newtonian would be the ultimate rich field telescope. But it only talks about the amount of stars that an instrument would show. It does  not mention bright wide field of views of deep sky objects. So what is the purpose of having a rich field telescope.

Of my three telescopes using a 22mm 82° eyepiece. The field of view is given;

1. 200mm f4.5 Newtonian 2°

2. 80 mm f6 refractor 3.6°

3. 150 mm f12 Maksutov 1°.

All three of these telescopes give nice bright and pleasing views of the stars and some deep space objects.

So why do some people persue a proper classified richfield telescope?

A 30mm set of binoculars would give a wide field of view teaming with stars.

Yet some people have built 10 inch Newtonians with a focal ratio of f3.5.

There is a GSO 150 mm f5 Newtonian listed as an astro graph. Would it be an acceptable visual telescope?

[vlaiv]

While all three of listed telescopes give nice views - only two of them can easily provide you with large exit pupil.

As for rich field telescope / high etendue telescope see this:

https://www.bbastrodesigns.com/HET.html

Another way to put it was my recent quest:

Give me largest effective aperture telescope that will fit M31 and some surroundings into FOV.

You might think that eyepiece plays a part here - but it does not really - what does play a part is illuminated and corrected field (diameter - up to 47mm - max for 2" eyepieces) and largest aperture that will provide that while still keeping exit pupil at bay so no light is wasted.

A lot of people report that best views of M31 they had was with binoculars - and indeed - you want something like x10-x15 magnification and large exit pupil.

127mm F/5 refractor telescope is capable of providing that low magnification with ~42mm eyepiece while giving you something like 4.3° of FOV.

 

[jetstream]

First off dark skies are a must for rich field viewing, second many scopes can provide it including my 24" f4.1. Many will argue this but whatever. Again getting under dark skies is a must to be able to see what a RFT (or any other) has to offer IMHO.

[Deadlake]

1 hour ago, jetstream said:

First off dark skies are a must for rich field viewing, second many scopes can provide it including my 24" f4.1. Many will argue this but whatever. Again getting under dark skies is a must to be able to see what a RFT (or any other) has to offer IMHO.

DaRk Skies, how dark. Less then or equal to Bortle 3?

[jetstream]

1 hour ago, Deadlake said:

DaRk Skies, how dark. Less then or equal to Bortle 3?

I truly dislike the Bortle scale and no offence to him. I hear "I'm Bortle 3" or "ok I'm Bortle 3/4 but sometimes 5".

I now go by the Milky Way (and SQM-L, NELM) - if you can "just see" the MW RFT obs will be diminished. If it looks bright but with limited structure ( a transparency issue) RFT view will be good. If the MW shows bright jagged structure with its spurs showing- run for the RFT! and every other scope you have!

My skies run from 21.4 SQM to 21.9, NELM at least 6.8. Transparency runs from poor to excellent.

I usually only do reports when conditions are excellent

[Deadlake]

2 minutes ago, jetstream said:

I truly dislike the Bortle scale and no offence to him. I hear "I'm Bortle 3" or "ok I'm Bortle 3/4 but sometimes 5".

I now go by the Milky Way (and SQM-L, NELM) - if you can "just see" the MW RFT obs will be diminished. If it looks bright but with limited structure ( a transparency issue) RFT view will be good. If the MW shows bright jagged structure with its spurs showing- run for the RFT! and every other scope you have!

My skies run from 21.4 SQM to 21.9, NELM at least 6.8. Transparency runs from poor to excellent.

I usually only do reports when conditions are excellent

Fair enough, in my garden around 20.5 SQM using Dark Sky Meter. I should measure some more to see how it varies.

[jetstream]

1 minute ago, Deadlake said:

Fair enough, in my garden around 20.5 SQM using Dark Sky Meter. I should measure some more to see how it varies.

I hope no offence is taken by my dislike for the Bortle scale- I think it says you should be able to see M33 from your skies

naked eye lol! It takes my very best skies to do this... maybe its just my eyes.

[Guest]

Please. My post was regarding rich field telescopes.

Regards.

[jetstream]

8 hours ago, Grumpy Martian said:

So what is the purpose of having a rich field telescope.

They pack as many stars as possible in the eyepiece and some do a (much) better job than others. IMHO.

[Stu]

I guess I’ve always thought of a Rich Field Scope as a widefield scope with a flat field which may be missing the point, not sure? I love my Genesis for this sort of thing.

https://www.televue.com/engine/TV3b_page.asp?id=16
 

Quoted from the above link...

“Low Power Limits”

While the example for a refractor shows that there is no arbitrary limit to how low you can go, that is not true for reflectors or SCTs that have central obscurations. So ironically, a fast SCT is not ideal for low power wide angle viewing. An even better choice would be an f/4.5 Dobsonian or Newtonian with the secondary obscuration limited to about 20%. Remember though to consider a coma corrector such as the Paracorr.

To me, no view is spectacular if the stars look like blobs.

"RFT"

The Rich Field Telescope is simply one that has sufficient field and aperture to provide exciting Milky Way and other rich field views. As for which RFTs are best, of course I am slightly partial to fast APO refractors, particularly if they have flat-fields. Larger aperture f/4 to f/5 Dobsonians with a coma corrector are every bit as good if the mirror is well-made.

http://www.company7.com/televue/telescopes/tv101.html

I don’t think focal ratio ‘speed’ has any impact on star visibility at the same magnification given the same aperture, so it’s all about shortest focal length ie widest field at a particular aperture, correct?

[jetstream]

From TVs site on RFT

"The Sagittarius star cloud. "

M24, in the right scope under dark skies is a superbly beautiful object. Would any RFT show this one the same? No IMHO. I have spent 10's of hours observing this one and yes the 200mm f3.8 is vg, but so is the VX10, 15" etc.

I think that the widest lowest power views describe a traditional view of the RFT. I mentioned dark skies (maybe too much) but to my eyes a very low power, small aperture scope under less than dark skies does not perform well even if it does qualify as a RFT.

Under lighter skies I'll take more aperture any day. Just my 2 cents.

[Stu]

4 minutes ago, jetstream said:

From TVs site on RFT

"The Sagittarius star cloud. "

M24, in the right scope under dark skies is a superbly beautiful object. Would any RFT show this one the same? No IMHO. I have spent 10's of hours observing this one and yes the 200mm f3.8 is vg, but so is the VX10, 15" etc.

I think that the widest lowest power views describe a traditional view of the RFT. I mentioned dark skies (maybe too much) but to my eyes a very low power, small aperture scope under less than dark skies does not perform well even if it does qualify as a RFT.

Under lighter skies I'll take more aperture any day. Just my 2 cents.

I agree with you Gerry that those larger exit pupils don’t go well under lighter skies; my best fun with the Genesis has been under darker skies (for me, I know they are still very light by your standards). The benefits of a shorter focal length/smaller aperture are only really if you want to sweep larger areas of sky or catch the whole Veil or NAN in one field of view to give context. As you say aperture will always give you more detail, but a narrower view.

[Captain Scarlet]

It’s a balancing act, or optimization if you like. Lower magnification gives you wider field. More aperture gives you more light to detect those more stars, but also comes with increased exit pupil, which is fine until you hit your actual pupil’s size after which extra aperture makes no further difference. To keep the pupil size down as you increase aperture, you need to increase magnification.

More independent of other effects is “darker skies” which of course also show more stars so for any sort of scope and any sort of observing darker skies are better.

I hanker after a big dob, and the RFT concept is front and centre of my thinking for it...

Cheers, Magnus

Edited February 12, 2021 by Captain Magenta

[Stu]

27 minutes ago, jetstream said:

I think that the widest lowest power views describe a traditional view of the RFT.

So, to cut to the chase, are we saying that a rich field scope is simply the fastest scope available at that aperture, anything from an f5 4” Genesis up to an f2.8 32” dob? Or is it not that simple?

[John]

http://www.ianmorison.com/rich-field-telescopes-and-wide-field-observing/

[Stu]

4 minutes ago, John said:

http://www.ianmorison.com/rich-field-telescopes-and-wide-field-observing/

Interesting article John, thanks.

[jetstream]

54 minutes ago, Stu said:

So, to cut to the chase, are we saying that a rich field scope is simply the fastest scope available at that aperture, anything from an f5 4” Genesis up to an f2.8 32” dob? Or is it not that simple?

It can be for sure, but like we were talking dark skies allow smaller aperture scopes (all scopes) to show stars very well. My 90mm SV is a great RFT scope up here but if I took it to the edge of town not so much. Take my 200mm f3.8 to the edge of town and it will much better eventhough the focal lengths of the scope are similar, 630mm vs 760mm.

To me under lighter skies my concept of a RFT would include the largest aperture possible at the fastest speed. This same scope performs best under dark skies too.

Its only when the RFT is described as a small aperture, fast scope that the skies become that much more important.BUT lol! on nebula and if we believe the exit pupil theory and discount the "Entendue" theory then both types of scopes should perform similarly on filtered nebula...ie NAN

Any and all thoughts welcome, in agreement, dispute or otherwise!

Edited February 12, 2021 by jetstream

[scarp15]

Can only really comment on dark sky observing and as an example Barnard's Loop. A tricky subject, when observed by traditional, photon gathering, approach yet an expansive portion is visually attainable in optimum circumstances. Detected both in my 14" dob and 85mm refractor. Beginning at a northern point, drifting downward along its curvature, a holistic profile is visually appreciated, registered with the 85mm refractor, that will enable a 4.4 degree field to sweep along, each side revealed by the smaller image scale. 

Certain large dark nebulae are easier to discern at smaller image scale / wide field and I like to observe intensively compacted clusters such as the Wild Duck Cluster, hung within the local star-field, conveying more of the outlying 'geography' that the object is suspended within; is very pleasing, such as with binoculars and a wide field low power refractor.  

[Stu]

1 hour ago, jetstream said:

It can be for sure, but like we were talking dark skies allow smaller aperture scopes (all scopes) to show stars very well. My 90mm SV is a great RFT scope up here but if I took it to the edge of town not so much. Take my 200mm f3.8 to the edge of town and it will much better eventhough the focal lengths of the scope are similar, 630mm vs 760mm.

To me under lighter skies my concept of a RFT would include the largest aperture possible at the fastest speed. This same scope performs best under dark skies too.

Its only when the RFT is described as a small aperture, fast scope that the skies become that much more important.BUT lol! on nebula and if we believe the exit pupil theory and discount the "Entendue" theory then both types of scopes should perform similarly on filtered nebula...ie NAN

Any and all thoughts welcome, in agreement, dispute or otherwise!

Thanks Gerry! I don’t think there is any disagreement! 👍👍 

I guess the only point as said before is that you can fit the larger objects in a smaller scope which is not possible with a larger aperture/longer focal length. I do enjoy being able to frame objects in a larger field of view eg even with ‘only’ a 24mm Panoptic, the Genesis gives over 3 degrees and can easily frame M8, M20 and M21 in one lovely field.

 

[Deadlake]

3 hours ago, John said:

http://www.ianmorison.com/rich-field-telescopes-and-wide-field-observing/

Interesting read John, this part 

t is important that the telescope used gives good star images out to the edge of the field and for a Newtonian a coma corrector such as the Baader MkIII MPCC would be required whilst for a refractor a field flattener such as the Teleskop Service 2-inch TSFLAT2 should be used.  Very few telescopes will give a sharp flat field over that encompassed by the Explore Scientific 30 mm eyepiece and these would be termed astrographs – but few of these have image circles greater than ~44 mm.

Is Prof Morrison suggested to get the richest view, you should consider using a fast scope that’s has a field flatterer and a minimal image circle of 44mm. 

It’s not just a fast telescope that is required, it’s an Astrograph?

Edited February 12, 2021 by Deadlake

[Stu]

10 minutes ago, Deadlake said:

It’s not just a fast telescope that is required, it’s and Astrograph?

Not totally sure of the definition of an Astrograph, but agree that it should have a flat, coma free field. Would a fast dob with a coma corrector be classed as an Astrograph I wonder?

[Deadlake]

13 minutes ago, Stu said:

Not totally sure of the definition of an Astrograph, but agree that it should have a flat, coma free field. Would a fast dob with a coma corrector be classed as an Astrograph I wonder?

I’d say yes for the DOB. Also large imaging circle as well, as discussed in article.

[Deadlake]

On 11/02/2021 at 14:31, vlaiv said:

127mm F/5 refractor telescope is capable of providing that low magnification with ~42mm eyepiece while giving you something like 4.3° of FOV.

 

we only really needed this line to answer this....
 

Televue 127mm/ F 5.5

LZOS 130mm/ F6

Sharpstar 121 DQ / F5.5

I can think of, none are cheap.

 

[vlaiv]

2 minutes ago, Deadlake said:

we only really needed this line to answer this....
 

Televue 127mm/ F 5.5

LZOS 130mm/ F6

Sharpstar 121 DQ / F5.5

I can think of, none are cheap.

 

Well - you can take 6" F/4 Newtonian + coma corrector and you'll get the same thing - well almost. Not sure how well corrected the field will be - but for low power visual - probably good enough.

6" with lower mirror reflectivity and central obstruction will provide you about same light gathering as 5" refractor.

Then, there is real deal, but with a bit of false color

https://www.teleskop-express.de/shop/product_info.php/info/p7788_Bresser-4827635---127-mm-Refractor--f-635-mm--OTA.html

It is 4 lens elements design - although that is probably just to lessen chromatic aberration it could also be that field is fairly flat?

[Deadlake]

9 hours ago, vlaiv said:

Well - you can take 6" F/4 Newtonian + coma corrector and you'll get the same thing - well almost. Not sure how well corrected the field will be - but for low power visual - probably good enough.

6" with lower mirror reflectivity and central obstruction will provide you about same light gathering as 5" refractor.

Then, there is real deal, but with a bit of false color

https://www.teleskop-express.de/shop/product_info.php/info/p7788_Bresser-4827635---127-mm-Refractor--f-635-mm--OTA.html

It is 4 lens elements design - although that is probably just to lessen chromatic aberration it could also be that field is fairly flat?

A newt would work, question around size of central obstruction. Boren-Simon powernewts are rated for this, as would the usual suspects found here:

https://www.teleskop-express.de/shop/index.php/cat/c58_Telescopes-with-field-corrector.html

 

Edited February 13, 2021 by Deadlake