Astro Focusing problems

Dear All
I have bought a skywatcher 120mm x 600mm refractor to use mainly as a guide
scope. However I have been trying to image through it as a test. It
certainly produces a nice purple fringe around bright objects visually but I
expected this from an acromat.
I can image and produce sharp stars when I use a red or green filter but I
can't seem to get the image through a blue filter to focus. I realise that
the focus will be in a different position through the shorter wavelength
filter but I thought that the blue filter which is relatively close to the
violet should come to a pinpoint focus also.
The accompanying pics are little subframes of a bright star with a cross
over the front to produce diffraction spikes. These help with focus like a
hartman mask.
The red and green images produce nice diffraction spikes but the blue (the
first image) won't do it. All the filters also stop IR (allegedly) so I
think this should not be the problem. THe filters are astronomic LRGB
filters.
Any suggestions?
Is this inherent with all achromats or do I have a dud?

On Wed, 06 Jun 2007 02:53:48 GMT, "Terry B"
wrote:

I have bought a skywatcher 120mm x 600mm refractor to use mainly as a guide
scope. However I have been trying to image through it as a test. It
certainly produces a nice purple fringe around bright objects visually but I
expected this from an acromat.
I can image and produce sharp stars when I use a red or green filter but I
can't seem to get the image through a blue filter to focus. I realise that
the focus will be in a different position through the shorter wavelength
filter but I thought that the blue filter which is relatively close to the
violet should come to a pinpoint focus also.
The accompanying pics are little subframes of a bright star with a cross
over the front to produce diffraction spikes. These help with focus like a
hartman mask.
The red and green images produce nice diffraction spikes but the blue (the
first image) won't do it. All the filters also stop IR (allegedly) so I
think this should not be the problem. THe filters are astronomic LRGB
filters.

I suspect the result through the blue filter is a combination of
chromatic aberration and spherochromatism. Assuming normal glasses,
the focal length is very short for a 120 mm objective and the result
is less control of aberrations.

Bud
--
The night is just the shadow of the Earth.

Terry B wrote:

Dear All
I have bought a skywatcher 120mm x 600mm refractor to use mainly as a guide
scope. However I have been trying to image through it as a test. It
certainly produces a nice purple fringe around bright objects visually but I
expected this from an acromat.
I can image and produce sharp stars when I use a red or green filter but I
can't seem to get the image through a blue filter to focus. I realise that
the focus will be in a different position through the shorter wavelength
filter but I thought that the blue filter which is relatively close to the
violet should come to a pinpoint focus also.
The accompanying pics are little subframes of a bright star with a cross
over the front to produce diffraction spikes. These help with focus like a
hartman mask.
The red and green images produce nice diffraction spikes but the blue (the
first image) won't do it. All the filters also stop IR (allegedly) so I
think this should not be the problem. THe filters are astronomic LRGB
filters.
Any suggestions?
Is this inherent with all achromats or do I have a dud?

Yes this is inherent to all low cost achromats especially at f5.

William's diagnosis is accurate for this f5 Skywatcher lens. The color
displacement especially in the violet is very typical. It would be interesting
to see your red and green shots for comparison.

If you happen to have a 300-600 line per mm diffraction grating perhaps in slide
form and it produces a bright 1st order spectrum, hold the grating over a
25-30mm eyepiece focused on Vega. You
should see a fairly well defined line from red into blue but very
pronounced vertical splaying of the line into the violet. This is a
literal depiction of what your achromat is doing, chromatically.
If you have a dob around try the same test and you will see the
difference.

Now imagine that star line converted to a round airy disk - the
very same chromatism seen in horizontal form with the grating is
now being distrubuted out spherically. But the use of the grating
allows you to see in graphic terms what your achromat is doing
chromatically vs. say a newtonian mirror.

Notice also the three rounded triangles of your star images vs a
tight circular airy disk. These rounded points in the airy disk are the
spherical component William is speaking of. Very likely your lens has an outer
zone of undercorrection. Rack in a star until you get a ringed diffraction
pattern. Very likely your pattern shows a bright thick outer ring which is the
result of undercorrection at the edges of
the lens. Sometimes if you lessen the tension on the retaining ring
at the front of the lens you can improve slightly the affects of
undercorrection. You certainly cannot do any harm.You can always put back the
same retaining ring tension (and image quality) you had before.

Everything you show is classic and normal for the lens you have.

Best regards -
Jerry