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Grumpy Martian

What is a true "Rich Field" instrument?

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Observing details on planets and the moon is great. But these objects are not always visable. The most pleasure I get from observing is low magnification, wide field views. I have an eight inch f4. 5 Newtonian and a 120 mm Ed refractor that give approx 2 to 2.7 degree fields of views which are bright and pleasing (These with 22 & 30 mm  82degree eyepieces) . I have owned an 80mm f 6.2 Skywatcher Equinox which gave very good views indeed.But the low power wide field views were not particularly bright. But some of the best wide field and bright views were with my 7 x 50, 7 degree binoculars. I was wondering how these views could be replicated and improved upon with a larger apature instrument? I understand that Skywatcher's ST 80 refractor at 400 mm focal length would with long focal length eyepieces give views of around 6 degrees with my Explore Scientific 82 degree 30 mm eyepiece. 

There are some rich field telescope Web sites that show quite large apature Newtonian's with very short focal lengths. The bright wide field views must be fantastic with these telescopes. 

So does the term "Rich Field" means different things to different people? 

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I am quite happy with my ST80 and an ES24/68 EP, giving almost exactly 4 degrees FOV.  This is great from a dark site but the big exit pupil picks up a ton of light pollution, so I am considering going to about 3 degrees of FOV with a 16/82 or 14/82 degree eyepiece. 

To use 2" EPs in an ST80 requires replacing the focusser; in this case you might just consider getting a different telescope instead?

In terms of definition, I would consider any 1.25" telescope with focal length under about 420mm a "rich field telescope". At that focal length you can get to 4 degrees of FOV with 1.25" accessories. Aperture is important, so you need a fast focal ratio of around 5. You want the stars to look pretty, so it's got to be a refractor. It's a bit specialized so you don't want to spend a ton of money. For me the ST80 ticks all the boxes.

Edited by Ags
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8 minutes ago, Grumpy Martian said:

Observing details on planets and the moon is great. But these objects are not always visable. The most pleasure I get from observing is low magnification, wide field views. I have an eight inch f4. 5 Newtonian and a 120 mm Ed refractor that give approx 2 to 2.7 degree fields of views which are bright and pleasing (These with 22 & 30 mm  82degree eyepieces) . I have owned an 80mm f 6.2 Skywatcher Equinox which gave very good views indeed.But the low power wide field views were not particularly bright. But some of the best wide field and bright views were with my 7 x 50, 7 degree binoculars. I was wondering how these views could be replicated and improved upon with a larger apature instrument? I understand that Skywatcher's ST 80 refractor at 400 mm focal length would with long focal length eyepieces give views of around 6 degrees with my Explore Scientific 82 degree 30 mm eyepiece. 

There are some rich field telescope Web sites that show quite large apature Newtonian's with very short focal lengths. The bright wide field views must be fantastic with these telescopes. 

So does the term "Rich Field" means different things to different people? 

My sincere apologies to John. Having posted, I realise that John has a post "Hand held rich field telescopes". 

Would it be possible for the mods to close and put this as a reply in John's post. 

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I think the two topics are different. John's topic is about handheld telescopes primarily.

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I agree, we can keep the topics separate, they are different enough.

My favourite rich field scope was a Televue Genesis f5. These are 4” Petzval scopes with a long focal ratio doublet objective (f12 ish I think) and a reducer/flattener rear doublet which gives the reduced focal length and a lovely flat field ie no field curvature. Most refractors scopes with short focal lengths do show some field curvature, especially at low powers with widefield eyepieces.

The Objective is an achro really, and the scope did show CA at higher powers, but that was missing the point. With a 31mm Nagler in it, you get a lovely 5 degree field, perfect for trawling up the Milky Way. Wish I hadn’t had to sell it.

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Richfield is most stars in a single field of view.... could be any size, but typically smaller scopes feature. You need a fast scope with an ultrawide apparent field of view with an exit pupil as big as you can take. Mel Bartels has a good summary https://www.bbastrodesigns.com/rft.html he is an expert mirror polisher who can achieve some crazy results... he hunts down “galactic cirrus” type nebulae that no one else has ever tried to find. I have a 13” f3 mirror for this purpose that wants a scope putting round it. Binoculars are another option, but 7mm exit pupil binoculars with >70degree apparent field of view are not made anymore and you need to do a lot of hunting to find them... but as mentioned they give a good view.

Of course you could just use a light pollution filtered image intensifier..... stupid numbers of stars.

 

PEter

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16 minutes ago, Ags said:

I think the two topics are different. John's topic is about handheld telescopes primarily.

I agree too - this one is wider in it's scope (good choice of words there !) :smiley:

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My F/6.5 Vixen ED102 is my "rich field scope" I guess. With the Nagler 31 I can get a 3.8 degree true field but lately I've been enjoying a 40mm 68 degree Aero ED clone which shows 4.1 degrees and is a lightweight 2" eyepiece compared with the big Nagler.

I was observing the double cluster in Perseus the other night with this combination and the views were wide enough not only to show both clusters but also the chains of stars around them which I'd not paid much attention to before. Lovely sight :smiley:

Don't know if there is a proper definition of what a "rich field" scope is though :icon_scratch:

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I am not sure I would call anything with less than 6 degrees FOV rich field, widefield maybe...

Alan

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The 'Rich Field' is the dry one near the loos where the 'frac owners set up, while the 'other field' is the boggy one at the bottom, of the hill where the dob-mob are exiled :evil4:

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1 hour ago, Stu said:

With a 31mm Nagler in it, you get a lovely 5 degree field, perfect for trawling up the Milky Way.

As with my TMB 92 SS f5.5 APO.  Superb under dark skies!!

31 Nagler gives about 16x magnification and a FOV of 4.8 degrees.  My other eyepiece for this is 13mm Ethos, about 40x and 2.5 degrees.

Lookig forward to some Milky Way cruising now the dark nights hve returned!

Cheers Paul

 

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9 minutes ago, clarkpm4242 said:

As with my TMB 92 SS f5.5 APO.  Superb under dark skies!!

Lovely scope, always fancied one of those!

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I read years ago, though I'm not sure where, that 9 X 60 is technically the richest richfield instrument. If that's true, then a good  pair of 9 X60 binoculars will show more stars in a general field than anything else.

Personally i think a good 100mm short focus refractor is a force to be recond with. About ten years ago I observed the Orion nebula through a 30" Dob, which you might imagine would give an unbeatable view. Shortly afterwards I fit my 20mm Nagler into my friends Vixen 102 F6.5 ED (exactly the same as John's) and my jaw nearly hit the floor. My friend begged me to sell him the 20mm Nagler. The combination of that scope and eyepiece was a match made in heaven!

Moving up in aperture, the best rich field scope I've ever used was a SW Startravel 150. Granted, its not a planetary scope, but its a truly stunning RFT and will give powers upto X200 on difficult dso's. Around 15 years ago my friend took his Startravel 150 to my local astro club, where many fine scopes were on the field. The ST 150 delivered the most pleasing views of Perseus and the double cluster I've ever seen. Nothing on the field came close to the ST150 when it came to sharp, rich, bright star fields. It's a specialist instrument and well worth considering as a serious RFT.

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4 minutes ago, mikeDnight said:

Moving up in aperture, the best rich field scope I've ever used was a SW Startravel 150. Granted, its not a planetary scope, but its a truly stunning RFT and will give powers upto X200 on difficult dso's. Around 15 years ago my friend took his Startravel 150 to my local astro club, where many fine scopes were on the field. The ST 150 delivered the most pleasing views of Perseus and the double cluster I've ever seen. Nothing on the field came close to the ST150 when it came to sharp, rich, bright star fields. It's a specialist instrument and well worth considering as a serious RFT.

I guess the 152mm f5.9 clones fit that particular bill well too, better corrected than the SW f5s

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29 minutes ago, mikeDnight said:

I read years ago, though I'm not sure where, that 9 X 60 is technically the richest richfield instrument. If that's true, then a good  pair of 9 X60 binoculars will show more stars in a general field than anything else.

I always get a slightly guilty pleasure from the view through my Skywatcher 9x50 finderscope.  I was dumbfounded that the very basic 7.5x finder I made for my dob (basic 50mm x 182mm lens and cheap 25mm plossl) showed almost everything I could see through the Dob, but smaller... it's actually much 'faster' at f3.6 compared to f6.4 so at low mag it pulls out the faint fuzzies - just no detail!

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I guess the Vixen 2.1 x 42 binos are pretty "rich field" having, according to the reviews a 20+ degree true field.

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The view through my Bushnell 7 x 50, 7 degree binoculars when viewing in a dark sky site is extremely bright. I am in the process of understanding the field of views of instruments in degrees. How to calculate the fields with different combinations of scopes and eyepieces. 

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8 hours ago, Stu said:

I guess the 152mm f5.9 clones fit that particular bill well too, better corrected than the SW f5s

I have one of these scopes coming to me :)   I had an ST120 and thought it was okay, but left a lot to be desired. I am trying my hand at this larger, better quality one strictly for deep sky. 

 

I am stoked. 

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I play around with rich field scopes, including the 200mm f3.8.

Here goes...

What are we going to use the scope on? large faint nebulae? Many scopes do well, some excel. The 90-120mm refractors give a nice view but the 200mm f3.8 and the Heritage 130mm dob beat them (the TSA120 is neck and neck here though, on a lot).

Star fields? the 15" dob shows a mind blowing amount of stars with the 100 deg eyepieces.

For those interested in a relatively easy, beautiful faint object try the Pleiades Bubble - I don't see it exactly like Mels sketch but the sketch down plays the beauty of it. Mels sketch is great and I'm not sure anyone can replicate its look, it must be seen.

Rich field scopes need truly dark skies to work properly IMHO.

image.png.c34903348e5996c0092d75a26f3b2775.png

Edited by jetstream
add Mels sketch
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17 hours ago, Grumpy Martian said:

I have an eight inch f4. 5 Newtonian

This scope will show a boggling amount of faint stuff and will work very well on star fields. Have you tried it on M24? What eyepieces do you use with this scope Martin?

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I have the opticstar AR90 F5.6 and that gives super widefield views, its a lovely achro, well corrected and nice to use

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My APM 80 mm F/6 triplet gives stunning views with both the 31T5 Nagler (5.3°) and the Vixen LVW 42mm (5.62°) especially with a 2" Amici prism, which gives upright, correct images.

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3 hours ago, jetstream said:

This scope will show a boggling amount of faint stuff and will work very well on star fields. Have you tried it on M24? What eyepieces do you use with this scope Martin?

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? 

Edited by Grumpy Martian
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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

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11 minutes 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

Interesting, that means the finder I mentioned above at 50-mm and x7 is pretty much spot on.

And it means my 40mm EP is a good match for my dob.

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