Is there an "optimum" field of view?

[LittleSkink]

Our telescope is a little Heritage 100P, 400mm f4 and the plan is to get a budget (c£30) wide angle EP as we are finding it a good way to learn our way round the sky and see Deep Space stuff, but unsure what the most useful visual field angle would be

using:

http://telescope.eyepiece-calculator.co.uk/

I worked out the standard SW MA 25mm has 3.125 deg of visual field - not that it is sharp to the edge by any means - so far that has been useful enough

Longer Plossls seem well liked, cheap and have decent eye relief for glasses so are an obvious choice, but to get 25mm or 32mm for such a short focal length scope?

A 25mm Plossl will give about the same visual field and maybe more clarity to the edge whereas a 32mm Plossl gives over 4 deg - is that what it will really show? and is it that much use?

[ronin]

I think the answer is No.

Generally Optimum is talked of differently.

A plossl has say 50 degree view, an ES 82 has 82 degree view.

Now "optimum" is in this respect about 68 degrees.

I think that a 68 degree view is the widest that the eye can "see" without you having to move your head/eye to see the bit outside of 68 degrees. When people talk of the field of view being 3.5 degrees then that is what the eyepiece takes in and then magnifies.

So in that respect 68 degrees is "optimum" as you see all at once.

In terms of Exit Pupil, I have seen 2 options 1mm and 2mm.

1mm means a magnification equal to the scope diameter, so 100x in yours, which in turn means an eyepiece equal to the f number, 4mm.

I have seen 2mm which means a magnification of 50x and an eyepiece therefore of 8mm.

I tend to use the 1mm exit pupil, very easy to remember and it sounds good.

Magnification I revert to the exit pupil aspect and say equal to the scope diameter.

Field of view depends on the object, not much is 1 degree, M42, M45 are the easy ones, not sure how big the average cluster is, less then the moon I guess. So optimum there is object dependant.

Before you decide on any eyepiece are you intending to change from the Heritage 100 f/4 to anything bigger?

If Yes then consider what next and perhaps bias the purchase with that in mind.

At the moment you are likely looking at shorter eyepieces, if you were to get say a 127 Mak then short eyepieces would be close to useless. Also it may be worth considering something like the BST's for that future use.

[Charic]

.........never heard of 'optimum field of view' before. This surely is just dependent on the requirements of the end user, and ultimately, how much their willing to spend for the wider views. Me I just prefer to frame the subject as best as possible using whatever eyepiece  achieves that result.

I'm aware that the 'Optimum magnification' range for your telescope calculates to somewhere between 25x to 50x power which is a 16mm down to 8mm eyepiece.

Your 'minimum magnification' is about 17x power which equates to a 24mm eyepiece.

The highest power attainable maintaining an exit pupil of 0.5mm would be 200x power with a 2mm eyepiece.

This matches exactly what ronin mentions (  first as usual ) a 1mm exit pupil from a 4mm eyepiece, matching the focal ratio of the telescope, and  the maximum power would be twice your aperture so 100mm x 2 = 200x power giving 0.5mm exit using a 2mm eyepiece, but this is the extreme of the telescopes theoretical use,  under perfect atmospheric conditions?

As for 'optimum field of view', back to you, your choice!

[Moonshane]

To my mind the optimum field of view is the one that allows an object to be seen in as much detail as conditions will allow with a little space around it given the aperture and size of object

[bingevader]

A 25mm Plossl will give about the same visual field and maybe more clarity to the edge whereas a 32mm Plossl gives over 4 deg - is that what it will really show? and is it that much use?

25mm will be about the "optimum" if you are talking widest field of view.

32mm would work, but would no longer be "optimum" because of the larger 8mm exit pupil.

7mm is about as wide as your fully dilated, dark adapted pupil will get, so anything larger than that and the extra light won't reach the retina.

But I'd have to agree, that the optimum FOV for me is the EP that frames the object best.

For different objects, the optimum FOV will be different.

[LittleSkink]

thanks folks, I think what I was asking was the optimum field for star hoping (as a newb maybe I haven't got the right terminology - or approach) even yet

With my 25mm (c 3deg visible f.o.v.) I hop around the sky from star to star and use the alt/az mount on the scope to explore left/right up/down if that isn't working. One reason to look at a 32mm Plossl was to get wider views (of maybe 4deg) to help with the "wandering"

Having read a chunk of Deep Sky Observation over the last couple of nights I see that Neil Bone uses 1.5 deg to star hop but he also suggests having a plan of attack for objects being sought - which I guess is where I need to take my learning next. So I just got me a 'Plan' isphere - now I need a 'plan'

[John]

Personally I find a combination of a red dot type finder and an 8x50mm right angle finder the most effective tools for star hopping. I can back these up if needed with a low power / wide angle eyepiece in the main scope to find objects below the threshhold of the optical finder.

The red dot finder has a field of view as wide as your eye can go of course while it's around 5 degrees in the 8x50 finder.

You might find a combination of a red dot finder and your 25mm eyepiece is enough most of the time.

[Moonshane]

What he said^^ :0)

[rwilkey]

thanks folks, I think what I was asking was the optimum field for star hoping (as a newb maybe I haven't got the right terminology - or approach) even yet

With my 25mm (c 3deg visible f.o.v.) I hop around the sky from star to star and use the alt/az mount on the scope to explore left/right up/down if that isn't working. One reason to look at a 32mm Plossl was to get wider views (of maybe 4deg) to help with the "wandering"

Having read a chunk of Deep Sky Observation over the last couple of nights I see that Neil Bone uses 1.5 deg to star hop but he also suggests having a plan of attack for objects being sought - which I guess is where I need to take my learning next. So I just got me a 'Plan' isphere - now I need a 'plan'

Hi there, you will not get 4 degrees AFoV with any Plossl ep.  It is hard enough to get that with any ep, if at all.  That includes the best out there including TV's offerings.  The average best is around the 2 degree mark, which you can get with 82 degree ep's and upwards.  On my ES82 30mm I can get 2.7 degrees of actual sky, that's the best I can do with my current equipment.  Exit pupil a pleasing 3.3mm

[Moonshane]

You would get 4 degrees with a heritage 100mm f4 dob and 32mm plossl I think? the exit pupil would be large though at 8mm, albeit usable at darker sites. A more standard 6" f5 would give 2.13 degrees wit the same eyepiece. It all depends on the focal length of the scope. These days I find it more effective to buy a different scope with a shorter focal length than an ultrawide eyepiece.

[swamp thing]

I have read (could be wrong) that the 'clear' vision range of the human eye is about 40˚ so one could argue that any wider an AFOV is kinda wasted on us. However I like to move my eye about (most people do) so the wider the AFOV the more pleasing I find the view. I find wide fields kinda offer a more natural balance of vision in that they cater not only for the 'clear' vision area but some extra area too. 

It could also be argued on this that we shouldn't worry too much about edge correction as we aren't really looking at the edges with our 40˚ clear area of vision so why worry?

Obviously though if one wants to move the eye about its nice to have a corrected field right across. 

[Arminovski]

So how big are the galaxies and other bigger objects on the sky? How many degrees do I need to get in to the eyepiece to be able to "frame" them?

I haven't seen many yet.

[Moonshane]

for all but a handful the common objects are less than a degree in expanse, some a lot less than half a degree. your 120mm f8 will need a 20mm plossl for the bulk of objects and a little bit more power for many of them. 

[Paul73]

For star hoping. The 9x50 RACI (like John says) is great. I have found that a pair of binoculars helps as they have a really nice wide FOV. By getting my bearings with the Binos, then getting to the right bit of the sky with the finder is much easier.

Hope that this helps.

Paul

[John]

Yep - binos are a useful finding tool too