40mm ep FOV

[rightguard]

one thing that has eluded me in my first year of astronomy is the field of view and how to calculate it.

Ive just brought a 40mm ep in the hope that this will increase my success in finding dso's - larger field of view = less time searching.

however, i would like to know just how much the fov will increase from my 25mm ep.

its a 200p on a heq5, FL of 1000.

any help appreciated

[damnut]

Hi Rightgaurd,

With your 40mm the mag you will get is 25X, you are pushing your scopes minimum mag (29X). The FOV will be on the ep, but couple of links below will do all the maths for you, just punch in the numbers in the naa link!

Cheers

Starizona's Telescope Basics

N.A.A. Telescope Math Calculator

[rightguard]

very good link thanks very much

[umadog]

Just remember that the equation that calculates true field from apparent field is an approximation. The real equation for true field requires knowing the eyepiece field stop diameter: Tele Vue Optics: Choosing Eyepieces

[pvaz]

Along with field of view you must also consider the exit pupil. If it's too wide you'll loose contrast and it will be harder to see faint objects against the sky (the sky looks brighter with large exit pupils). You should always try to keep it under 6mm.

Exit Pupil = EP Focal Lengt / Scope focal ratio

So 40mm/f5 = 8mm which is more then a kids pupil fully dilated (average is 7mm and reduces with age).

To counter this you can choose an EP with wider AFoV and shorter focal length, which will show the same amount of true FoV while keeping the exit pupil controlled and increasing contrast. Something like a Televue panotic 24mm in the 1,25" format, or a 16mm Nagler(Explore Scientific and Skywatcher Nirvana have similar options at about half price) will show the widest FoV possible with 1,25" barrel. In 2" format a Televue Nagler 31 will give you the widest possible view, the Skywatcher Nirvana 28mm comes pretty close at a better price.

[rightguard]

basically im not concerned about 'viewing' with the 40mm, i only want to use it to get a larger area of the sky in the field of view, thus making finding dso's easier. once in fov il switch to 20 or 15 mm

[pvaz]

I see, for that purpose any will do. I forgot to mention that a wider then 6mm exit pupil sometimes show the shadow of the secondary mirror in the center of the view. If that happens then it won't be of much use even as a finder EP. A 32mm plossl with 52º FoV would show the same wide field as a regular 40mm 43º plossl and you'd be keeping it on the safe side.

Personally I find DSOs are easier to detect and observe with the widest FoV possible and a smaller exit pupil. The faint fuzzies need a good dark area around them to pop up.

If you are planing to stay at 1,25" format the 24mm Panoptic (or the much cheaper equivalent explore scientific) will do the job as both a finder EP and provide high quality views.

TFOV for 40mm = (43º * 40mm) / 1000mm = 1,7º

TFOV for 24mm = (68º * 24mm) / 1000mm = 1,6º

Telescope House Explore Scientific 25mm 70° eyepiece

EDIT: Seams like the ES is a 2" so unless you have a 2" focuser that's not an option.

[great_bear]

The real equation for true field requires knowing the eyepiece field stop diameter

As discussed on a long thread on CN a while back, even that doesn't work too well either. The only way to accurately measure the apparent field of view is to shine a torch backwards through the thing, and physically measure the size of the circle it projects onto a wall in relation to the distance you are from that wall.

(or a similar solution, such as looking at a wall through the eyepiece, with the other eye open to "size" the viewing circle against reference points on that wall - this is essentially the same method as described above, but without involving a torch )

[rightguard]

first clear skies for AAAAGGGEESS tonight so il test it out

[great_bear]

the 24mm Panoptic (or the much cheaper equivalent explore scientific) will do the job

Telescope House Explore Scientific 25mm 70° eyepiece

The Explore Scientific in that link is a 2" eyepiece.

Actually, I'm not really convinced that any of those 70-degree eyepieces are new. They look like the Meade QX range rehoused, as I mention in this thread here <click>

[rightguard]

just tried the 40mm on a terrestial tree (is there any other kind )

noticably larger fov with no black spot from secondary, so looks like all is well

[pvaz]

Oh, only noticed you already have the EP. I read your post fast and thought you ware thinking about buying one. Sorry about that.

[rightguard]

yea think it was 12 quid from astroboot, nice n cheap

[Capricorn]

I bought a 40mm eyepiece for the same reason. Didn't care about the quality of view, just wanted the widest I could get my hands on for locating things through a Mak.

If the image wasn't sharp I didn't care just as long as it was in view.

Hope it does what you want, but at £12 you cannot really go wrong.

Think a 40mm will have a stop that limits it actual field of view, the chromed tube at the bottom acts as a field stop. So it all works out much the same as a 32mm, but it is a teeny bit more.

[pvaz]

I bought a 40mm eyepiece for the same reason. Didn't care about the quality of view, just wanted the widest I could get my hands on for locating things through a Mak.

If you don't have a decent widefield EP I'm sorry to say you're missing out on some of the most interesting DSOs. Objects like the Orion nebula, North America Neb, Trifid Neb, Veil neb, double cluster (...) really need that FoV so that you can frame them. I find the 2.1º I get with my Nagler 31mm barely enough for some of them.