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What can you fit in the FOV of your 1200mm scope?

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#1 stargazer193857

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Posted 06 August 2014 - 10:26 AM

This question goes out to everyone with a 6" f8, 8" f6, 10" f5, or 12" f4, using a 32mm Plossl or 24mm 68* eyepieces, basically the largest field stop in a 1.25" eyepiece.

 

I've seen several pictures of the Pleiades being 1 degree wide, and I have a distant memory of them looking 1.5 degrees in my 10x50 last winter, and I've read in references in Wikipedia stating they are 2 degrees wide. I think Stellarium shows them as 1.3 degrees.

 

I'm just wonder what large DSO can be seen in this size 1200 fl family of scope without resorting to 2" eyepieces and large central obstructions. I know someone in my club with a 10" dob can't see the whole Pleiades, but he also does not own a 32mm Plossl. I think he has a 27 or 28m as his largest eyepiece.

 

 

 

Besides the Pleiades, other targets I'd like to be able to fit are M33, M32/M110, Orion Nebula, M82/M81, Leo Triplet. Does anyone have experience viewing each of those in a commercial medium sized dob and 32mm eyepiece and know how well they fit?


Edited by stargazer193857, 06 August 2014 - 10:32 AM.


#2 David Knisely

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Posted 06 August 2014 - 10:42 AM

With the standard maximum 27mm field stop on 1.25" eyepieces, you are limited to a 1.29 degree true field of view.  This is barely enough for the main 7 stars in the Pleiades to show up.   Things like the Rosette Nebula, M8, the main arc of the Veil (NGC 6992), and some of the larger open clusters can all fit in that field of view.  Clear skies to you.  



#3 Starman1

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Posted 06 August 2014 - 10:42 AM

The Pleiades are really more of a binoculars object.  I've seen the Pleaides going to the edge of a 2.5 degree field and we weren't certain all its stars were within the field.

If you want to use only 1.25" eyepieces, you are limited to a 27mm field stop (by and large).

That's a focal length in the scope of:

619mm for a 2.5 degree field

774mm for a 2 degree field

1031mm for a 1.5 degree field

1547mm for a 1 degree field



#4 MitchAlsup

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Posted 06 August 2014 - 08:12 PM

This question goes out to everyone with a 6" f8, 8" f6, 10" f5, or 12" f4, using a 32mm Plossl or 24mm 68* eyepieces, basically the largest field stop in a 1.25" eyepiece.

 

In my opinion, the 8", 10", and 12" scopes SHOULD have 2" focusers, obviating most of the question.



#5 beatlejuice

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Posted 06 August 2014 - 09:46 PM

 

Does anyone have experience viewing each of those in a commercial medium sized dob and 32mm eyepiece and know how well they fit?

 

M33 - yes

M31/32/110 - barely but tend to say no due to the wide expanse of M31

M81/82 - yes

Leo triplet - yes

Orion nebula - yes

 

But there is so much more that a widefield 2" eyepiece can give you on other targets not mentioned.

 

Eric


Edited by beatlejuice, 06 August 2014 - 10:07 PM.


#6 stargazer193857

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Posted 06 August 2014 - 10:32 PM

Well I don't want to get too close to a 7mm exit pupil in a reflector, so I guess that would mean I'd have to go for a wider AFOV to get the wider field of view. 40mm would be as big as I'd want to go. A 70* 38mm has a field stop that fills the tube.

 

I agree now that a 2" is necessary to see the scope's potential.


Edited by stargazer193857, 06 August 2014 - 11:07 PM.


#7 havasman

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Posted 06 August 2014 - 11:31 PM

123 arc minutes



#8 gnowellsct

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Posted 07 August 2014 - 01:48 AM

You have to remember that the size of an object is in function of (a) aperture and (b) whether or not the catalog/database/map measurements were made photographically.  A large scope using a camera will record a much bigger galaxy (any galaxy, really) than a smaller scope operated by a visual observer.  The large photographic instrument will detect not only fainter stars, but a lower density of stars, which means a larger measurement will go on to the map.   In terms of its actual boundaries you *can't* see the entire Andromeda galaxy and m32 and 110 in the same field of view.  

 

Incidentally, you also get perceived variability depending on sky transparency.  Go high into the Sierra and you'll get a bigger Andromeda galaxy.

 

The aperture is not  a trivial issue.  I *cannot* fit the Perseus double cluster into a single field of view on my C14.  But when I center the C14 on the "empty space" (NOT!) between the two main clusters I actually see more group members than I see in both the main clusters in my 4" refractor at 17x in a four degree field.

 

While we all understand what you're talking about, your discussion of these issues will gain ENORMOUS clarity if you learn to think in terms of degrees of arc, arc minutes and true field of view, which is a function of the eyepiece focal length and the scope's focal length, etc.  It might take a day or two to get the hang of it but then instead of  writing:

 

I know someone in my club with a 10" dob can't see the whole Pleiades, but he also does not own a 32mm Plossl.

 

You might write:

 

I know someone with a 10" f/4.5 dob who can't see the whole pleiades, but his widest eyepiece is [for example] a 20mm plossl"

 

Then the busy minds here might get down to it:

 

Ten inches is 254 mm

Times 4.5 is 1143  (or 1200 fl family as you wrote)

The plossl is probably around 50 degrees "afov" 

 

So we're talking about 1200/20=60 x magnification.  For true field 50 degrees / 60x = 0.83 degrees

converting that to arc minutes: 60 arc minutes * .83 = 48 arc minutes

 

Now taking that known parameter of the eyepiece-scope combination, we look at a Deep Sky Atlas 2000 or software of any variety, and we see that this is inherently smaller than the object, never mind whether photographic, binocular, or whatever.  In fact some software lets you plug in your eyepiece and scope combo and plop a circle of the correct radius on your target so you can see what you will see in the scope.

 

Now if your friend breaks down and spends thirty bucks on an inexpensive 32mm plossl, he is going to have 1200/32=37.5x, and a plossl will give approximately 50/37.5x or 1.33 degrees, or 60*1.33=120 arc minutes which is very close to what havasman just posted above.  

 

And that is in fact the limit of the hypothetical f/4.5 with a 1.25" focuser.  Change the focuser (and probably the secondary too), or give a scope with a longer or shorter focal ratio, and all these factors will change; and also as a function of aperture, with which *focal length* changes even if *focal ratio* does not.  A thirty inch f/4.5 Newtonian telescope has a field of view almost as tight as an f/11 C14 (notorious for its narrow field of 40 arc minutes, tho' I love mine).

 

Now I don't mean to drown you in these arithmetic ratios which you seem to be more or less aware of because of your stipulation of 48 to 50 inches of focal length in your various ratios and mirror diameters.

 

But it remains the case that a 6" f/8 is gong to bring in A LOT LESS ANDROMEDA galaxy than your 12" f/4, so there is gong to be less there to see, but maybe framed better.  The owner of the 12" f/4, in measuring the length of the galaxy, will come up with a *longer galaxy* than the f/8 6".  

 

If you put the six inch scope in the Sierra and the 12" scope in the Adirondacks with 95% relative humidity, all bets are off.  The 6 inch may bring in *more* Andromeda compared to the 12".  

 

The question of framing is also somewhat distinct from a good view.  First of all, there are many more objects available to the larger scope.  The Perseus Double cluster can't be squashed into a c14 but the C14 provides a wonderful tight framing of NGC 7789.  

 

NGC 7788 and NGC 7790 are as gratifying in an f/11 c14  (at 100x with 40 arc minute tfov) as the Perseus Double cluster is in a four or five inch f/6 refractor with a 3.5 to 4.5 inch fov.

 

There are so many gorgeous open clusters out there that the question of how to frame the Pleiades loses interest, really, once you know your way around the OCs suitable to your aperture.  

 

There is the additional issue of learning-what-the-scope can do.  I can frame the Veil Nebula in my 4"  f/6.5 refractor and see the whole thing.  It is quite the sight to see the circumference of that supernova remnant.  But in my personal experience the intricacy of the part of the Veil known as NGC 6992 (aka the "Network Nebula") is much more engrossing, with the added bonus that when you get tired of taking it in you can use your paddle to scoot along the full length of that portion of the Veil, and then move on to the other one.

 

For these reasons I see in your query a wide variety of technical and aesthetic issues that are, as it were, unspoken, and I hope you don't mind my going on at length.

 

Greg N 



#9 Jon Isaacs

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Posted 07 August 2014 - 05:01 AM

Well I don't want to get too close to a 7mm exit pupil in a reflector, so I guess that would mean I'd have to go for a wider AFOV to get the wider field of view. 40mm would be as big as I'd want to go. A 70* 38mm has a field stop that fills the tube.

 

I agree now that a 2" is necessary to see the scope's potential.

 

If you want the widest possible field of view, you definitely need a 2 inch focuser.  But you also need to keep in mind that in scopes as fast as the ones you are considering, F/4-F/6, the quality of the eyepiece as well as the exit pupil are important considerations.. If you are trying to fit the Pleiades in the field of view, at least aesthetically, they ought to be reasonably clean and round because they are going to be bright..  

 

As far as the Pleiades go.. I find a 2.5 degree TFoV is sufficient though more can be a help. A 1200mm focal length with a 46mm field stop provides a 2.2 degree TFoV, sufficient but not ideal. 

 

Jon



#10 stargazer193857

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Posted 07 August 2014 - 10:15 AM

The sketch in this link is with a 1200mm focal length and 32mm Plossl, but seems to contradict Wikipedia.

 

http://www.google.co...d=0CHoQ_B0wCgMy

 

Stellarium shows the bright 7 stars extending just over a degree, and the fainter stars around it extending about 1.6 degrees.

 

Sketches I saw in a Google image search back up Stellarium.

 

Wikipedia says 110 arc minutes, which is larger.

 

My 900mm with 32mm Plossl has a 2 degree FOV. I aimed it at the Pleiades once, but the sky background was bright enough, and so many fainter stars in the cluster were shown, that I could not make out which ones were the seven sisters, or know for sure if I got all seven sister.

 

In my 25x70, which has a 2.4 degree FOV, the fainter stars are not as prominent, and the sky background is darker. The 7 sisters stood out nicely with room to spare, but I don't remember exactly how much room there was to spare. In either my 10x50 or 16x50, I don't remember which, the seven sisters filled about 1/4 of the field of view. The 16x has a 4.2 degree FOV, and the 10x 6.5 degree.

 

The guy with the 10" dob uses a 28mm Plossl. I asked him yesterday, but I don't remember if he said he just got the main bowl or if he got 6 stars in view. So he had 1.36 degrees.

 

 

 

As for M31, I don't need to fit the whole disk. I just wanted to fit M110, the core of M31, and also M32. My main target is the dust lanes of M31, with M110 and M32 just visible as reference points. Maybe the core of M31 would ruin my night vision so I can't see the dust lanes, so maybe it is a futile goal.



#11 stargazer193857

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Posted 07 August 2014 - 11:51 AM

Well I think this thread has just about accomplished its purpose. With a 1200mm tube, I can see most of what I want in a 1.25" eyepiece. It then just becomes a question of whether I'd want to spend $300 on a 2" eyepiece or on a an extra scope such as a 6" f5 or smaller.

 

I won't be buying anything for at least a year, but I enjoy thinking about this sort of thing. If I just go with the 6" f5, a barlow is needed for high power, it has less light for DSO, it has a larger %CO, and the f5 collimation must be more exact though I can get good at that.


Edited by stargazer193857, 07 August 2014 - 11:53 AM.


#12 David Knisely

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Posted 07 August 2014 - 12:07 PM

Some of the cheap 32mm 1.25" barrel plossls have the internal barrel of the eyepiece as the field stop.  This is about a 28mm figure, which using the field stop equation, gives a maximum true field of view of about 1.34 degrees in a telescope with a 1200mm focal length.  That would be a field on the Pleiades similar to the drawing you linked to.  However, some of these eyepieces are often not threaded for filters and the edge performance is sometimes not quite as good due to reflections off of the insides of that barrel and the "pushing" of the apparent field of the Plossl to where its outer edge performance tends to decline.  There was once a 35mm Ultrascopic from Orion that had a wider field stop in an 1.25" barrel, but it suffered from vignetting on the outside edges of the field.   In any case, if you want to get all of the Pleiades in the field with room to spare, you are indeed looking at an eyepiece with a 2" barrel.  Clear skies to you.



#13 gnowellsct

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Posted 07 August 2014 - 01:20 PM

 

Well I don't want to get too close to a 7mm exit pupil in a reflector, so I guess that would mean I'd have to go for a wider AFOV to get the wider field of view. 40mm would be as big as I'd want to go. A 70* 38mm has a field stop that fills the tube.

 

I agree now that a 2" is necessary to see the scope's potential.

 

If you want the widest possible field of view, you definitely need a 2 inch focuser.  But you also need to keep in mind that in scopes as fast as the ones you are considering, F/4-F/6, the quality of the eyepiece as well as the exit pupil are important considerations.. If you are trying to fit the Pleiades in the field of view, at least aesthetically, they ought to be reasonably clean and round because they are going to be bright..  

 

As far as the Pleiades go.. I find a 2.5 degree TFoV is sufficient though more can be a help. A 1200mm focal length with a 46mm field stop provides a 2.2 degree TFoV, sufficient but not ideal. 

 

Jon

 

 

 

It may be that your exit pupil limits how wide you can go but the converse is when you drop a 40mm 70 degree two inch eyepiece into a modern telescope you are getting the *most* field of view that physics allows for contemporary designs.  I have used 15" and 14" fast dobs at very wide exit pupils and the views are gorgeous.  Put it this way, people pay many thousands for wide field views in refractors and you can do the same with a Newt for only a few hundred bucks--the eyepiece.  If your eight inch is cropped to six inches by exit pupil so what?  People pay $20,000 for six inch apos.  GN








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