Astrophotography --- eyepiece projection or Barlow lens?

moren...@mailinator.com

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Oct 30, 2011, 4:23:14 PM10/30/11

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Hi group,

So, I recently (early September) took a few moon shots, some of
them prime focus, some of them using eyepiece projection (my
scope is a Newtonian reflector, 1000mm focal length, so I wanted
a little more magnification, so I used my 12mm Vixen Lanthanum
eyepiece for it).

The eyepiece pictures show hints (actually, quite evident) of
chromatic aberration, a blue-ish hint on the edges --- prime focus
ones, taken with one or two days difference, show absolutely no
trace of chromatic aberration. This tells me that either the
eyepiece
or something I'm doing wrong in the eyepiece projection setup is
causing the distortion. (the Moon was quite high above the
horizon on both occasions, so atmospheric-related chromatic
aberration seems to be clearly out as an option).

The guy at my local astronomy store, who seemed to me quite
knowledgeable, tells me that there simply is no way to avoid it,
that it is a "natural" defect of that technique; he recommended
that I use a Barlow lens instead.

Questions:

(1) Does this guy's suggestion have real merit? (using Barlow
lenses to get higher magnification with little --- or no? ---
chromatic
aberration?)

(2) If yes, would I have to get some special design, or somewhat
top or very high quality Barlow lenses for this purpose? What
should I look for in its characteristics/specifications?

Thanks for any comments,

Carlos
--

Chris L Peterson

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Oct 30, 2011, 8:53:23 PM10/30/11

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On Sun, 30 Oct 2011 13:23:14 -0700 (PDT), moren...@mailinator.com
wrote:

>The eyepiece pictures show hints (actually, quite evident) of
>chromatic aberration, a blue-ish hint on the edges --- prime focus
>ones, taken with one or two days difference, show absolutely no
>trace of chromatic aberration.

>...

>The guy at my local astronomy store, who seemed to me quite
>knowledgeable, tells me that there simply is no way to avoid it,
>that it is a "natural" defect of that technique; he recommended
>that I use a Barlow lens instead.
>
>Questions:
>
>(1) Does this guy's suggestion have real merit? (using Barlow
>lenses to get higher magnification with little --- or no? ---
>chromatic aberration?)

Absolutely. Eyepieces are not designed as imaging elements for
extending focal length. Eyepiece projection can be useful when you
have a very narrow field- for instance, when imaging a planet. But for
the most part, it will not produce very good results. A barlow is
definitely the way to go when you want to extend your telescope's
focal length for imaging.

>(2) If yes, would I have to get some special design, or somewhat
>top or very high quality Barlow lenses for this purpose? What
>should I look for in its characteristics/specifications?

I have found very little difference in optical quality between various
brands of barlows, assuming they come from reputable suppliers. For
imaging, I've had very good success with Orion 2X and 3X barlows,
which are very economical.

Chris.B

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Oct 31, 2011, 2:18:32 AM10/31/11

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Hi Carlos

An interesting problem. Are you using a digital SLR with a removable
lens? May we some of your images?

I have used eyepiece projection with various compact digital cameras
on my achromatic refractors. I usually very black lunar shadows but
the limb is quite often greenish, blue or violet. The effect is of a
gentle watercolour tint on a monochromatic (B&W) lunar image.
Enlargement does show a violet tint to shadows on some of my 150mm/6"
F:8 refractor images. This is despite using a Fringe Killer filter.

I find a 15mm (or longer) eyepiece works best. Vignetting becomes
ridiculous with higher power eyepieces. A Barlow helps by allowing
much large diameter optical elements in lower power eyepieces.

http://fullerscopes.blogspot.com/2011/05/may-moon.html

http://fullerscopes.blogspot.com/2010/04/rekindled.html

Martin Brown

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Oct 31, 2011, 4:23:14 AM10/31/11

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On 30/10/2011 20:23, moren...@mailinator.com wrote:
> Hi group,
>
> So, I recently (early September) took a few moon shots, some of
> them prime focus, some of them using eyepiece projection (my
> scope is a Newtonian reflector, 1000mm focal length, so I wanted
> a little more magnification, so I used my 12mm Vixen Lanthanum
> eyepiece for it).
>
> The eyepiece pictures show hints (actually, quite evident) of
> chromatic aberration, a blue-ish hint on the edges --- prime focus
> ones, taken with one or two days difference, show absolutely no
> trace of chromatic aberration. This tells me that either the
> eyepiece
> or something I'm doing wrong in the eyepiece projection setup is
> causing the distortion. (the Moon was quite high above the
> horizon on both occasions, so atmospheric-related chromatic
> aberration seems to be clearly out as an option).

Essentially the eyepiece is highly optimised for looking through with
effective image at infinity and is less able when used to project an
image at a finite distance behind the lens.

>
> The guy at my local astronomy store, who seemed to me quite
> knowledgeable, tells me that there simply is no way to avoid it,
> that it is a "natural" defect of that technique; he recommended
> that I use a Barlow lens instead.
>
> Questions:
>
> (1) Does this guy's suggestion have real merit? (using Barlow
> lenses to get higher magnification with little --- or no? ---
> chromatic
> aberration?)

Yes. Although any chromatic aberration already present will be more
obvious at a higher magnification. You can also push a Barlow or
Teleconverter combo by adding extra extension tubes provided you do not
run out of back focus. SCTs will tolerate a fair amount of this:

http://www.nezumi.demon.co.uk/astro/zoom/zoom.htm

High contrast target weather vane showing the various combos I had.

>
> (2) If yes, would I have to get some special design, or somewhat
> top or very high quality Barlow lenses for this purpose? What
> should I look for in its characteristics/specifications?
>
> Thanks for any comments,

You can do pretty well with photographic teleconverters if you have an
SLR they work very well with long focal length lenses.

http://www.digiscoped.com/teleconverters.html

You can get 1.4x, 1.7x, 2x and 3x nominal factors.

--
Regards,
Martin Brown

Chris L Peterson

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Oct 31, 2011, 10:18:42 AM10/31/11

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On Sun, 30 Oct 2011 23:18:32 -0700 (PDT), "Chris.B"
<chr...@nypost.dk> wrote:

>I have used eyepiece projection with various compact digital cameras

>on my achromatic refractors...

Don't confuse eyepiece projection, which is a focal technique like
focal plane imaging (that is, there are no lenses on the camera) with
simple afocal imaging, which is imaging through an eyepiece with a
camera that has its own lens. These are different methods, described
by different analyses, and provide different results.

Chris.B

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Oct 31, 2011, 1:17:20 PM10/31/11

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On Oct 31, 3:18 pm, Chris L Peterson <c...@alumni.caltech.edu> wrote:
>
> Don't confuse eyepiece projection, which is a focal technique like
> focal plane imaging (that is, there are no lenses on the camera) with
> simple afocal imaging, which is imaging through an eyepiece with a
> camera that has its own lens. These are different methods, described
> by different analyses, and provide different results.

Quite right. :-)

I was wondering if our friend was actually using an SLR.

Afocal photography is an easy and valid way for almost anyone to
capture some snaps of the moon or sun. (the latter only with a
suitable full-aperture filter)

It might not occur to everyone to have a try with whatever equipment
they have at hand. I have more success with a digital compact, in one
night, than I ever did in decades of using a film SLR at the focal
plane.