Calculate AFOV Using Fieldstop Diameter?

[Damo636]

Can anyone tell me how to work out the above. For example, the 28mm UWAN has a measured field stop of 43mm (WO claim 43.5mm) and 31mm Nagler has a 42mm. Does this mean that the shorter fl Uwan has a slightly wider TFOV in practice?

I have always used the AFOV/MAG = TFOV and using this method would give the Uwan a 1.53* TFOV and the Nagler a 1.69* TFOV with my 12" scope. I just don't get how the larger field stop of the Uwan produces a smaller TFOV

[AndyH]

To get the True field of view

(eyepiece field stop diameter ÷ telescope focal length) x 57.3

Andy.

[Damo636]

Thanks Andy. So the 28mm UWAN does have a slightly wider TFOV than the 31mm Nagler, 1.64 degrees v 1.60 degrees with my 12" dob.

[cs1cjc]

AFOV/MAG = TFOV

is an approximation, bacause of eyepiece distortion. All widefield eyepieces have distortion because they are mapping what is small and approximately planar onto a large angle which is apparently spherical. The designer has to choose to correct either rectilinear or angular distortion or perhaps as a compromise neither, but hopefully somewhere between.

[Damo636]

Thanks Chris. Using the AFOV/MAG I incorrectly assumed that the 26mm Nagler would give me almost the same TFOV as the the UWAN when in fact its 20% smaller. Not my finest decision to date

[spaceboy]

But one that could easily be rectified. 26mm Nags do not come up very often and soon sell when they do. There is of course the option of a swap + ££ for a 31mm Nag. The 31mm Nag is some piece of kit and would be by far my most missed eyepiece if the clouds ever cleared that is.

[Damo636]

I have toyed with the idea of swapping to the Terminagler to give me slightly more tfov but haven't been able to commit, yet!! The problem i have is that the 26mm is just superb and in all honesty my favourite eyepiece.

[Don Pensack]

It depends on your age and pupil diameter as to whether the 31 Nagler would be better for you. A slightly wider field of view, yes, but since the exit pupil of the scope is the focal length of the eyepiece divided by the f/ratio, for a lot of viewers older than 50, the 26 is actually a better choice with a typical f/5 telescope.

The 31 may produce an exit pupil larger than the pupil of the eye.

Why is that important?

1) the larger shadow of the secondary mirror at lower power may become a large enough portion of the view as to become obtrusive.

2) the image may be less bright because some of the light of the scope will not enter the eye.

When I went from a 35 Panoptic to a 31 Nagler many years ago, I noticed a distinct increase in brightness of the image. At 60, I'm wondering whether the 26 Nagler might not be a better choice. But since I own a 21 Ethos, which has the same field of view as the 26 Nagler, what I'm really saying is that I'm wondering whether or not I should sell the 31.

Then I look through it again and the idea of selling it simply vaporizes.

[spaceboy]

There is also the huge gap you would have to fill between the 13mm Ethos and 31mm Nag I guess. 21mm Etho springs to mind

[spaceboy]

LOL you read my mind Don

[Damo636]

Thanks guys, all good advise. I have considered the exit pupil Don and at a sprightly 40 years old i reckon i would be ok with a 6mm exit pupil.

As lovely as a 21 Ethos would be i wouldn't really be any better off than i am now (tfov) wise. Plus, i would most definitely be divorced if her indoors ever found out i shelled out £700 on an eyepiece

[cs1cjc]

I would not look at the graphs of age against average pupil size and think that they apply to you! It is easy to check.

I am 60 and my partner measured my fully dark adjusted pupil for me. After viewing one evening I held my eye closed while she put a ruler to my eye. This as not a precision measurement, but she estimated my pupil as being between 7 and 8mm, which was a surprise. However, I now understand why the 38mm in my Newtonians is not a problem for me, with the secondary shadow becoming unobtrusive as my eye adapts to the dark.

It was nice to find my eyes are good at something...

[Moonshane]

personally, I used the afov/mag=tfov calculation. it's near enough and I find it easier to remember.

re exit pupils etc, I sold a 35mm panoptic to partly fund a 26mm Nagler when TV had their 20% off sale about a year ago. I had plans then for the 16" f4 I have now and am really glad I made the change. in the big dob it's my favourite eyepiece and unless I am planetary/lunar observing, I just use two eyepieces, that and the 13mm Ethos. I found though at a dark site that the 31mm Nagler that I borrowed was also superb and provided a very different experience to the 26mm Nagler so potentially there's room for both if you have the cash (I don't).

the best view I ever had of M13 was through a mate's 22" f5 dob and his 31mm Nagler - just awesome - like a blue jewelled ball in the sky.

[Damo636]

I reckon my dark adapted pupil is around 7mm give or take range also Chris. I did a rough measurement a while back in front of the bathroom mirror with the lights off and just enough light so as i could read the ruler.

As much as I would love to own both the 26 & 31mm Naglers Shane i couldn't afford them both at the minute. The 26mm is superb and I cannot see me selling it but somewhere down the line when i'm a little flusher i would love to add the 31mm to the collection.

[Moonshane]

I know what you mean Damo. it's all about choices and compromise. If I sold my two smaller scopes I could afford a used 31mm Nagler to go with the 26mm but I just don't want to so I'll keep saving......

[Adz]

What do people think about using the antares 1.6x barlow with the 31 Nagler or 26mm?

I tried it with the 35mm panoptic and the eye position was impossible, but the eye relief is already long without it.

[Damo636]

I reckon the Nagler would perform only very slightly better barlowed as the eye relief to begin with is a little shorter. Ideally a Powermate would be the weapon of choice Adam for the longer focal length ep's. I have tried my 26mm Nagler with the 2x PM and i can honestly say if I didn't already own the 13mm Ethos I would be more than happy with this combo. The beauty of the PM is they don't alter the eye relief like a traditional barlow.

[spaceboy]

I would not even ascertain the thought of a 2" PM & 31 T5 in anything other than a feather touch or moonlite. I was lucky to have been on the ball the night the sheer weight of the 2.2LB 31 T5 undone the thread on my WO diagonal. Had my heart a pounding that night believe me. Sure makes you realise how demanding large EP's can be on focusers and other equipment that are placed in them.

[Adz]

Yep noted thanks guys. I am thinking of going for that 31Nagler, I have young eyes

[spaceboy]

You will not regret it. I personally felt the 31 T5 is the best Nagler there is, closely followed by the T4's. The T6's are a very nice EP, compact and well corrected but the short eye relief and small eye lens made it almost impossible to make out a defined field stop with out excessive effort. With the 31 T5 you almost have to stop yourself falling in to space.

[Adz]

Spaceboy you know how to increase ones anticipation

I didn't like the t6's when I had one, eye relief ruined the viewing for me.

[Damo636]

The 26mm Nag/Powermate is about as much as i would like to hang out the end of a focuser (equal in weight to a stand alone 31mm or there abouts) but agree the 31mm/Powermate would be pushing the boundaries somewhat!

[Moonshane]

yet another advantage of dobs.....I could hang a bag of spuds on my focuser without the eyepieces falling out as the focuser is inclined slightly upwards for comfort.

[spaceboy]

You still wouldn't find me risking it Shane not with out a quality focuser well bolted to the OTA. Equal to near £700 hanging out of a synta £50 focuser ....... gives me the shivers thinking about it.

[Damo636]

yet another advantage of dobs.....I could hang a bag of spuds on my focuser without the eyepieces falling out

:D