[Aavso-photometry] New to photometry

[Wolfgang Renz]

Hello Iakovos

Which of the Atic cameras do you have ?
Which chip ?
http://www.atik-instruments.com/html/cameras.html

Did you already test your camera for linearity ?

Did you already post a graph of the 44 i Boo in your web page ?

Which exposure time did you use ?

With bright stars like e.g. beta Per the main source of noise with
CCD measurements is usually scintillation noise due to the small
aperture of camera lenses and the nevertheless pretty short ex-
posure times. See e.g.:
So its recommendable for exposures of less than ~ 30s to stop
down the aperture (at such small apertures below ~ 5 cm it should
not have an as large influence as with normal scopes), take and
stack as many images to reach that value, and/or defocus slightly
if the var and the comps have no close companions that interfere
with aperture photometry. On could even operate in a "PEP-like
mode" with severe defocusing up to half the chip hight (the other
half for background subtraction) and image the var and the comps
in sequence. When the next galactic SN outbursts, this will be the
mode CCDs have to operated in when they observe it to handle
the brightness of the SN at all. An other alternative would be to
add an additional grey filter to the setup. Something like a moon
filter (~ 10% transmission) is usually sufficient to lengthen the ex-
posure time for bright vars and not. When using one of these it
would be best to use a polished one (that can be used further
away from the focal point) with an AR-coating (to avoid reflec-
tions). But this is probably just recommendable with camera
lenses that cann't be stopped down with a f# ring and with sco-
pes to avoid the penalty of an increased scintillation noise.

If you have sharp optics or stopped down the camera lense severe-
ly the medium faint stars might occupy just 1, 2 or 4 pixel (when the
star falls at the center, edge or corner of the pixel). If the images are
so severely undersampled, one will get issues due to intra-pixel sen-
sitivity variations and non-sensitive areas between the pixel of ABG
chips. The rule of thumb to avoid this is to have a FWHM of at least
2.5-3 pixel. This can often just be reached by a slight defocusing if
one cann't do subpixel dithering to even out this issue.

Is the 58mm-135mm a zoom lens or do you have several camera
lenses in that range ?
Zoom lenses are for ppi (pretty picture imaging) usually not recom-
mendable as they usually have just one or two sweet FLs where the
image quality is better than with the other FLs. But as they have to
have more lenses as fixed FL camera lenses, their transmission
values are usually worse and they are more prone to reflections if
their AR-coating is not really good. Some zoom camera lenses are
also prone to FL shifts. Especially when closer to the zenith.
But for photometry and smaller CCD chips they can be acceptable.

If the star sizes vary significantly with the distance for the optical axis,
one way to avoid issues is to place the var and the comps at about
the same distance from the center. This often also helps to minimize
flat calibration issues (at least if one doesn't have an overlayed edge-
to-edge gradient).

Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany



----- Original Message ----- 
From: "Iakovos Marios Strikis"
To: "AAVSO CCD" <aavso-photometry at mira.aavso.org>
Sent: Sunday, May 17, 2009 4:19 PM
Subject: [Aavso-photometry] New to photometry

Hello all 
 
My name is Iakovos & I live in Athens Greece.... I am 25 years old & I just
got a CCD (Atic IC model mono.) with the Schuler V filter for photometry...
I have a small Eq. 3-2 go-to mount (not the best thing to have) and some
telephoto lens (58mm-135mm & 200mm) of my old Zenit film camera. I
want to start doing some photometry on bright & fast stars (with a period
of 4 to 10 hours & mag. 5 to 8) so when I get a better mount with a small
refractor to do some more difficult objects (but I will have some experien-
ce).... 
 
I would like to ask how is the procedure of taking images and have not
scattered data on the photometry plots (I did some first experiments on
the i-Boo {44 Boo} and I can recognize a curve but the data I have are
not so good)... My camera is not linear & also has an Anti-Blooming
system ... Also the temperature of the ccd is -20 celsius from the envi-
roment...
 
In the next few hours I will post a graph of the 44 Boo in my web page
(done with the MaxIm DL) so to see what I mean for the scatter of my
data...
 
I will be glad if you could really help me with my first photometry steps...
 
Clear Sky to all 
 
J.D.Strikis