distortion

[womblemaster]

Was browsing through an optics website and i read this:

Bushnell 1.25" Format 2x Barlow Lens 78-0105 This high quality 2x Barlow Lens effectively doubles the magnification of any eyepiece in a way that eliminates the distortion and other negative effects of using extremely short focal length eyepieces. A 2x Barlow lens doubles the magnifying power of your eyepiece by doubling its effective focal length. For example, an 20mm eyepiece used with a 2x Barlow lens would have the magnifying power of a 10mm eyepiece.

AM very new to using astro lens, have used spotting scopes for 30years.

AM exploring the possibility of using astro gear for viewing birds at approx 200m.

SO far i have had bad results, but theni bought cheap gear to start the experiment.

Maybe i would be better to go with a 30mm lens with a barlow, for daytime viewing? rather than a 15mm or less lens.

ANyone know what this "distortion" is......is that the same as colour aberation?

Perhaps for daytime viewing its essential to go with ED lens? OR does lanthium glass have the same effect.....quality birding scopes use a mineral called flourite, which is something similar i imagine....?

[Merlin66]

In the bad ol' days the simple eyepiece designs like the Ramsden and Kellner could not cope with the incoming cone of light from a "fast" system and were optimised for f8 and above. They suffered from field curvature, coma, spherical and chromatic abberations..

The newer Plossl and wide angle designs are much better and are usually OK down to about f5. Below f5 you really need something special - Nagler type etc.

So, with the barlow increasing the focal ratio say from f5 to f10 more " basic" eyepieces can be used.

Hope this helps.

[womblemaster]

good info thanks)

SO what makes one barlow better than another.....obviously there are alot on the market, and its confusing to pick one.

Again should i go for ED glass or do certain makes/models stand out ? eg meade teleconverter i see advertised alot.

I have a celestron zoom, would that be improved by using a barlow.....ive not heard anyone mention using zoom and barlow together.

[The Warthog]

You can use a zoom lens with a Barlow, and you can stack Barlows to get multiples of the power. I.e. a 2x Barlow stacked with a 3x Barlow will give you 6x. To identify birds at 200m I would guess you need about 100x. I have identified large birds (cormorants, turkey vultures, etc.) at almost a km in my 65mm 90x Mak spotting scope. So, an astronomical scope that is capable of 400x or more may be surplus to your needs. As you are not looking at point sources of light (unless those birds are really far away ) I don't think the kind of distortion and false colour that plague astronomers will really be an issue.

That being said, a well made scope of 3 to 4" diameter in either a catadioptric or a short refractor, with medium priced wide-angle eyepieces would work a treat for observing birds. If you are observing from a hide, and portability isn't so much of an issue, you could take advantage of the capabilities of a longer refractor. If you go for a larger scope, consider whether lugging it out to your site will get to be a pain.

A 15mm Plossl is a nice comfortable eyepiece to use, BTW. If I were you, I would go for an ep with a good eye relief and a field of view of about 70º.

If you want to really impress your friends, though, get an orthoscopic ep. With its restricted FOV, it will show less of the bird's surroundings and make them think your scope is more powerful than it really is.

[womblemaster]

"If you go for a larger scope, consider whether lugging it out to your site will get to be a pain."

is exactly why birders ignore astro scopes. they arent portable. or weather proof.

anything over 2400g is mostly ignored.

most portable scopes have obj lens of 80-100mm so its rare to get magnifications over 70x. OK in brilliant sunlight 80x is feasible, but not in uk. where light is often dull, due to overcast condiitions.

"I have identified large birds (cormorants, turkey vultures, etc.)"

try identifying a bonapartes gull at 170+m. Tahts the kinda unreal colour definition i am trying to find)

http://www.birdweb.org/birdweb/bird_details.aspx?id=195

[John]

"If you go for a larger scope, consider whether lugging it out to your site will get to be a pain."

is exactly why birders ignore astro scopes. they arent portable. or weather proof.

anything over 2400g is mostly ignored.

most portable scopes have obj lens of 80-100mm so its rare to get magnifications over 70x. OK in brilliant sunlight 80x is feasible, but not in uk. where light is often dull, due to overcast condiitions.

Bonaparte's Gull - BirdWeb

I've occasionally used a 5" inch schmidt-cassegrain (SCT) scope for birding and it worked very well - those only weigh a few lbs but capture a lot of light and have a long focal length in a compact tube so high powers are achievable relatviely easily, even in fairly dull conditions. I've even seen an 8" SCT being used this way although that was a semi-permanent set up at a reserve.

With the long focal lengths of SCT's you don't really need a barlow lens but it's advisable to use eye pieces with long eye relief and a wide field of view as has been said.

John