What is a true "Rich Field" instrument?

[Ricochet]

1 hour ago, hughgilhespie said:

Amateur Telescope Making, Advanced (Book Two) Albert G. Ingalls, Editor.  'The Richest-Field Telescope - a Plea for Low Magnification. Page 623 (in my edition)

A simple formula, the magnification should be 3.39 per inch of aperture. That will show the maximum possible number of Milky Way stars when used visually.

Regards, Hugh

Presumably this is for a fixed AFoV? If two eyepieces show the same TFoV but at different powers then the higher power eyepiece will have a darker sky background and hence show fainter magnitude stars. 

[PeterW]

Stellar magnitude depends on aperture, the greater magnification will darken the sky and make fairer stars easier to see, but also reduce the exit pupil and make fuzzies harder to see. Rich field scopes are best for good skies where they can effectively work at low power. Interesting 3.39x/in from Unk Ingalls, 7x50 is a perfect fit. Way off the 50x/in the optical nuts like to test their scopes at!

PEyer

[Ags]

2 hours ago, hughgilhespie said:

Amateur Telescope Making, Advanced (Book Two) Albert G. Ingalls, Editor.  'The Richest-Field Telescope - a Plea for Low Magnification. Page 623 (in my edition)

A simple formula, the magnification should be 3.39 per inch of aperture. That will show the maximum possible number of Milky Way stars when used visually.

Regards, Hugh

...assuming a dilated pupil of over 7mm, I think. Under light pollution, I think an exit pupil around 3mm would be better - narrower field, but fainter things will stand out better and the sky won't look so washed out.

[hughgilhespie]

2 minutes ago, Ags said:

...assuming a dilated pupil of over 7mm

There wasn't so much light pollution in  America in 1910!

 

[Astrobits]

As I understand it, Richest Field refers to the maximum number of stars you can see in the field of view. I am not sure that DSO's are considered.

As aperture increases the number of stars visible increases, and to see the widest field the lowest magnification is needed. However, as the magnification decreases so the exit pupil increases and when the exit pupil exceeds your eye's pupil then the effective diameter of the objective decreases thus limiting the number of faint stars visible. Thus everything is interlinked and the result is, as far as I am aware, the Richest Field instrument has been determined to be at 9X63 for a 'normal' 7mm eye pupil. If you have an eye pupil of only 5mm then 9 x 63 will only be working at 9 x 45.

Therefore your own Richest Field instrument depends on you and your own eye's pupil dia. Multiply that with the magnification you want and you get the maximum diameter objective that you can use.

Nigel

[jetstream]

3 hours ago, PeterW said:

the greater magnification will darken the sky and make fairer stars easier to see,

I find this true up to a point, with decreasing star visibility (faint) past a certain point(mag). The advantage these fast large aperture newts have on stars is readily apparent vs smaller aperture (fracs) scopes. Not to say the smaller aperture scopes can't work well.

[Peter Drew]

It also depends on where you look, not all areas of the sky have the same star density. A 30" telescope, although is has a small field of view even at its lowest practical magnification, would probably with enough magnification for M13 to fill the field, show more stars than any lesser aperture.    ? 

[jetstream]

6 hours ago, Grumpy Martian said:

I have Explore Scientific 30 mm, Nagler 22 mm. This telescope has given me a magical view of The Andromeda galaxy. 

I will search out M24. Can M 24 be seen in the Northern hemisphere? 

As we age our pupil dilates less as we know. Your 30mm ES 82 is a VG eyepiece but gives a large 6.7mm exit pupil in your f4.5. The Nagler 22mm a 4.9mm, which is the one I would try on star field first and is also the one I would use on nebula.

M24 is low and more susceptible to sky conditions but I would pan around Cassiopeia for some nice views to start with.  You have a near perfect rich field telescope in the 200mm f4.5 IMHO and the worse the sky conditions are the more this scope will beat the smaller aperture ones, again just my opinion. These scopes really respond to 100 deg EP's such as the Lunts and try upping the mag with them on the star fields.

Naglers "Majesty Factor" rings true here.

[jetstream]

10 minutes ago, Peter Drew said:

It also depends on where you look, not all areas of the sky have the same star density. A 30" telescope, although is has a small field of view even at its lowest practical magnification, would probably with enough magnification for M13 to fill the field, show more stars than any lesser aperture.    ? 

Exactly true and this is why my 15" waxes my other scopes view on certain areas of the sky..

[Stu]

26 minutes ago, Peter Drew said:

It also depends on where you look, not all areas of the sky have the same star density. A 30" telescope, although is has a small field of view even at its lowest practical magnification, would probably with enough magnification for M13 to fill the field, show more stars than any lesser aperture.    ? 

Do you know what Peter, I read the title ‘Rich-field’ and automatically translated it to ‘wide-field’, which of course it doesn’t necessarily mean, as you rightly point out!

[Guest]

Last Friday night was a lovely and clear. The Milky Way was clearly visable. I enjoyed sitting down and scanning the sky with my 7 x 50, 7 degree field of view binoculars. The apature only being two inches.

I mentioned that my eight inch

f4. 5 Newtonian has given some great views through the 30 mm Eyepiece. But the field of view was approx 2.7 degrees. Somewhat down from 4 or 5 degrees for a proper rich field instrument. 

So it would appear that there are no hard and fast rules as to what a rich field telescope is. It is down to personnel choice. 

One thing is that despite the limited apature of my 7 x 50's and enjoying hand held. I in would like to know if I could get a set of binos that would offer improved views with larger lenses, but still be hand held. I was thinking of 11 x 70's. I know that the 15 x 70's are differcult to hold and get a steady image. 

[Merlin]

In light-polluted areas ( which means practically everywhere! ) the RFT is at a disadvantage, as it emphasises  the skyglow more than a longer focal length 'scope with its higher magnifications.

[refractordude]

At a bortle 4/dark yellow or better location, this is what I use https://www.amazon.co.uk/Celestron-71198-50-Cometron-Binocular/dp/B00DV6SI3Q/ref=cm_cr_arp_d_product_top?ie=UTF8. This is the tripod I use https://www.amazon.co.uk/Orion-Tritech-Field-Tripod-Fluid/dp/B01MSMRDLJ/ref=sr_1_1?ie=UTF8&qid=1534460599&sr=8-1&keywords=orion+tritech+tripod. Just got the Celestron Skymater 15x70 today.