Richard, I have a h-Alpha here somewhere, I will have to look at it to verify
the size. I have both siszes in my filter box. You are welcome to use it if I
have what you need.
Contact me off list
Beevo@xxxxxxx
Beevo
---- Richard Harshaw <rharshaw2@xxxxxxx> wrote:
=============
Hi, All!
I need some help, if any of you can provide it.
I have a new Skyris 618C camera for speckle interferometry, and before it
can be used for speckle work, I need to calibrate the pixel scale precisely.
Using double stars of known separation and position angle is risky since
speckle is about 20-100 times as sensitive as micrometers and CCDs, and
while a reading in close agreement with a "known" pair would suffice for a
micrometer or CCD, it may be far off for the ultra-discretion of a speckle
system.
So I had a grating made to fit over the corrector plate of my C-11 and was
told to use an H-alpha filter to generate a "string of pearls" image in the
camera and then use special data reduction software to measure the plate and
compute the separation between the central image and either 1st-order image.
Since I did not have an H-alpha filter, I used an O-III instead, since it
has a narrow bandpass too (albeit in the green part of the spectrum, not the
red).
But my Lumicon O-III has a bandpass of about 100 angstroms centered on 5,670
and this wide a band caused migraines for the reduction software. Since I
did not have a discrete enough band passing through the grating, my camera
recorded oval images instead of disks. These oval images gave the reduction
software fits as it tried to find the centers of each point spread function
and apparently was off by about 20%. I know this because when I measured a
known pair using the given pixel scale from my grating measurements, the
separation was about 20% off.
Since then, I have resorted to timing drifts of stars across the camera
chip, and got better results, but still not good enough to begin serious
work.
I have plans to use a green laser and a convex mirror in the next few weeks
and try generating my data with that, but in the meantime, I'd like to try
the H-Alpha filter idea, as another guy on our speckle team did that and got
excellent results, since the H-Alpha filters are much narrower band-pass
filters than the O-III's.
However, I am seeing good quality 1.25" mounted H-Alpha filters go for
around $400 (as per AstroDon and other good sources). That, frankly, is not
an option at this moment, after having shelled out over $1,200 for the
camera, motorized focus control, Johnson-Cousins R filter and other optical
goodies.
Soooooo-does anyone have a 1.25" mounted H-Alpha filter I could borrow for a
few nights? (Obviously, if I damage it, break it or lose it, I will replace
it, but that is not likely.)
If you do, please contact me off list (at rharshaw2 <at> cox <dot> net and
let me know. We can make arranges to do the handoff both ways, and I would
be very grateful to you for your loan. In fact, I will write a paper on
this process for a scientific journal and will give you mention as a
contributor to the effort.
Thanks in advance!
Richard Harshaw
Cave Creek, AZ
Brilliant Sky Observatory
