Re: Dumb question

["Richard Miles" <rmiles@baa.u-net.com>]

Tom,

Certainly no offence taken on my part - I was pleased to get such a lengthy
response.
I'll try to expand on my earlier points.

But before I do I should just say that we may be 'comparing apples and
pears' as they say.
I am mainly referring to photometry where several images taken at differing
airmasses are used in reducing data to the Johnson V magnitude system
primarily.
On the other hand if your data reduction methodology relies on reference
stars included in a single image plus a knowledge of your transformation
coefficients, then provided the image quality is quite uniform across the
field you should be O.K. using 'cut-down' apertures.  I shall have a browse
of the TASS technical papers and see what methodology is followed.


>
> I think this is all a question of error.  99% of the light flux for what
> error?  For what absolute photometric error?  I am still struggling to
> decide what I should be trying to do.  Meanwhile, I am keeping the raw
> data, and I hope enough calibration stuff so that I can later reprocess
the
> data to get the lowest possible error.
>
> My real goal is to get something useful done in my lifetime.  I define
> lifetime as ten years (if I am lucky).
>
> So I think in terms of what I might get done in ten years.  This is not
> really much different from a professional who is lucky if he can keep
> funding for a project like this for ten years.  ;^)

Don't get me wrong  - I am full of admiration for the achievements of TASS
and regard it as the pinnacle of achievement for amateurs doing photometry
at present.

>
> I am in a suburban location with lots of light pollution.  I don't think
it
> is possible to do the best absolute photometry here.  So I plan to do what
> I can and to try to characterize the error.  I think I will be quite
> content to get absolute photometry of 0.05 to 0.1 mag.   When I read
> experts comments on the quality of the currently available catalogs of the
> size I expect to accumulate (1 billion measurements of 5 million stars) I
> think that this might be useful.  Experts might comment.

Yes - you've hit the nail on the head - we are desperately short of an
accurate homogeneous system of V(RI)c photometry across the entire sky with
an adequate number density of catalog stars.  Indeed this is what
fundamentally limits the achievable accuracy for people like you and me who
generally have to contend with non-photometric skies and light pollution for
most nights.  I will be happy to get to better than +/-0.03 mag for 90% of
the targets.

>
> My plan is then to do what is possible here.  I have processed some data
> sets with different apertures.  I find that Michael's choice of 4 pixel
> radius aperture photometry to give near optimum results with my data.
>
> I would be very interested in arguments as to why I should do something
> else.  I stand ready to support anyone who wants to do work on this with
> suitable data sets.

I can have a look at the TASS methodology and see if there's some other
factor that could be taken into account and which may help but I suspect
that your flat fielding might also be the more important factor.

A possibly original suggestion on my part is that one keeps optical surfaces
(in particular the filters and CCD window) as clean as possible and then try
comparing precision and accuracy of the photometry WITH and WITHOUT
flat-fielding.  One useful test is to image M67 and model the variation in
response (un-flat-fielded) across the chip as a polar diagram, i.e. a smooth
function of radius from, and angle around the optical axis.  Don't assume
the optical axis coincides with the center of the chip however - it usually
doesn't!  This type of approach, may I suggest, is an alternative to
flat-fielding in the conventional sense.  It of course largely misses the
localised 'donuts' and other dust motes but since you are drift-scanning
then you are tracking across the chip and such donuts (which usually have
max. absorptions of <5 or 10%) are averaged out.  Of course you mustn't
disturb the optical setup between calibration runs on M67 or any similar
field.

>
> >The
> >quest for accuracy in deriving magnitudes to a standard system is what
> >absolute photometry is all about.
>
> While I too wish to strive for the best possible accuracy, I also wish to
> strive to get something done.  There is some compromise to be reached
> between generating a lot of data of poor quality, and a small amount of
> data of high quality.  I must say that my bias is to try to produce a
large
> catalog of pretty good relative photometric accuracy with a knowledge of
> the absolute error.

Fully agree, Tom.  The most important thing is to have a good knowledge of
the absolute error then the data is good for posterity.

>
> >I intend to advise anyone wishing to use the
> >spreadsheet to reduce CCD images to make sure that they choose a
photometric
> >aperture that includes 98-99 percent of the light flux from nominal point
> >sources not only for the entire area covered by the CCD image but also
from
> >image to image used in the reduction.
>
> Why 98-99%?  Why not 99.99%? This must imply some anticipated absolute
> error.  In fact, as the aperture increases the collected light increases,
> and the noise increases.  At some point the signal goes into the noise and
> no further benefit of increased aperture is obtained.  Seems to me that
> such a statement should include "the aperture is selected to reduce the
> error to a minimum with the observed sky brightness".   Possibly this
> anticipates only using data from the best photometric nights.  I don't
have
> many of those.  I always have a lot of light pollution.  So the choice
> depends on what one is trying to do.  I think this points out the
> difference between making the best measurements on a single star and
> measuring all the stars in the northern sky between mag 8 and mag 14 in
> many epochs.   That is what I am trying to do.

Yes - you are right in that as you go fainter sky brightness soon kicks in
as an important factor and for badly light-polluted or moonlit nights then
there is a magnitude limit fanter than which your comment that "the aperture
is selected to reduce the error to a minimum with the observed sky
brightness" is valid.  Assuming though that one is not sky brightness
limited, then we are limited by the degree of spatial and temporal
uniformity in atmospheric transparency.  Fine, there's no point in battling
on when the sky is 'falling apart'.  But for reasonably photometric nights
then the limit is that catalog accuracy and the separation distance to
suitable catalog stars.  Since the catalogs with reasonable number density
such as Hipparcos are no better than +/-0.01 mag (against the standard
system) there's no point in striving for 99.99% of the light to be included
in the aperture.  Indeed the actual figure depends on the absolute
photometric accuracy of the catalog you are using (as well as the image
quality).  The principle here is that the uncertainty introduced by omitting
up to x% of light from the aperture should be around half the uncertainty in
the catalog itself (my opinion).  So if we put some numbers in as an
example:  If the catalog uncertainty is +/-0.02 mag then the aperture should
include at least 99% of the total flux, and on average for reasonably good
quality optics the integration software will register 99.5 +/- 0.5% or so of
the actual light, such that the error introduced in this case is only about
1/4 of the uncertainty in the photometric catalog itself - that's a good
ratio especially since atmospheric transparency will also be competing to
degrade photometric accuracy!.

So there you have it for now.
I need to do my homework as to what is already explained in the TASS
technical papers and do hope that I am adding something new to the
discussion rather than teaching my grandmother to suck eggs as we say here
in the UK.
>
> Tom Droege
>
>
>
>