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What filter? DSLR/photographic? WebObs

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KBJ's picture
What filter? DSLR/photographic? WebObs

Sorry, bit thick and can't seem to find any info. I haven't uploaded any obs lately due to a long run of bad weather/work commitments and I notice the WebObs form seems to have changed. First, what's the difference between Photographic and DSLR? I use a DSLR and last time I uploaded, Photographic was the only option. Second, it tells me I must fill out the Filter box. I shoot unfiltered, my DSLR is unmodded, and the only two options are "Unfiltered with V zeropoint" and "Unfiltered with Red zeropoint". Considering I don't have a clue what a "zeropoint" is anyway, you can understand I'm a little lost here.

Any advice will be gratefully received. Thanks!

Cheers -

Rob Kaufman, KBJ
Bright, Vic, Australia

KTC's picture
...what's the difference

...what's the difference between Photographic and DSLR?

Any search engine will lead you to information like this if you use search text "photometry photographic magnitudes":

Specifically:  "Another type of magnitude which is sometimes encountered is the photographic magnitude, . Photographic magnitudes were determined from the brightness of star images recorded on photographic plates and thus are determined by the wavelength sensitivity of the photographic plate. Early photographic plates were relatively more sensitive to blue than to red light and the effective wavelength of photographic magnitudes is about 4200Å. Note that photographic magnitudes refer to early plates exposed without a filter."

Now you can see that your DSLR is a quite different detector than photographic emulsion.

" tells me I must fill out the Filter box. I shoot unfiltered...."

From your own web site:

"My method is to take unfiltered DSLR images, extract the green channel, and make visual estimates from the comparison stars marked on relevant AAVSO charts."

No, you don't shoot unfiltered, you take green-filtered images...yes?

Or have you altered your observation methods?

But we have a significant problem:

"My method is to take unfiltered DSLR images, extract the green channel, and make visual estimates from the comparison stars marked on relevant AAVSO charts."

Does this mean that you do not use photometric software to analyze your DSLR images?

If not, why not?

The good news is that your rig and techniques can be easily improved.

Get some photometry software, and feel free to send me some of your raw and calibrated images.

Talk to you soon!

KBJ's picture
Thanks KTC, your reply is

Thanks KTC, your reply is appreciated.  I'm still none the wiser on the filter box.  I have an inkling that the red filtering may refer to modded DSLRs which would leave only the "Unfiltered with V zeropoint" option.  But I could be very wrong and I just wanted it confirmed one way or the other.

As far as Photographic and DSLR goes I do understand the basics of film emulsion and digital sensors but a Google search won't necessarily tell me which option to select.  All my previous DSLR obs were uploaded as photographic, there being no other option.  Really (in my ignorance) I just wanted to know whether the DSLR option was somehow tied to the DSLR photometry program or whether I've graduated from "Photographic" to "DSLR".

As far as my website info goes, it's maybe a little out of date.  I mostly do nova searches now as well as monitoring a suite of known and candidate recurrent novae whenever I can.  I submit the latter as 'fainter thans' - their useful currency is very limited but they provide a good record of how often these objects (or non-objects!) are being observed.  And I just use the RGB image for that so they are unfiltered.  These days I only submit magnitude estimates very rarely where there might be a big gap - eg very early observations of new novae.  And I use visual estimates of the green channel for that.  I'm rapt that AAVSO has introduced their DSLR photometry program but like most of this 'high-end' amateur astronomy it's too expensive of time, money and brain power for me.  The appeal is to the affluent tech-savvy (or tech-interested more to the point).  And good on them!  I have very simple basic gear and like my astronomy simple.  Hence gradually working out a way I can usefully contribute without equipment/software upgrades or engaging in the complexity.  A bit like visual observing using a DSLR.  Each to their own!  And I wouldn't have a clue what a 'calibrated image' is LOL...

Thanks again -


KTC's picture
Ring me up when you get serious

Ring me up when you get serious.


roe's picture
The "Unfiltered with V

The "Unfiltered with V zeropoint" has nothing to do with modded DLSRs.  Star magnitudes are an expression of the RATIO of two brightnesses (or, more properly, fluxes).  When you image a star field with a variable and one or more comp stars in it you compare the signals of the variable to the comp star (usually software does that for you).  You can then say, hey, the variable is X brighter or fainter than the comp star.  If you know how bright the comp star is from one of the excellent sequences provided by AAVSO, you can then add or subtract X from that number to get an idea of how bright the variable is.

The challenge is, the comp stars were measured with very specific filters and the brightnesses depend on which filter.  Which brightness value (ie, which filter band) will you choose to use when you add or subtract X?  It's up to you.  But, if you use, say, V report your measurement as using a V zero point.  If you use the R value for the comp star, report an R zero point, etc.

With a DSLR image, it would probably be best if you extract the green channel from the RAW image to make your measurements.  Then use the V value for the comp star and report a V zero point because the standard V filter is closest to green.  To really get with the program, you would want to determine transformation coeffiecients to adjust your reported values to more nearly compare to everyone else.

KTC's picture
DSLR transform coefficients

"To really get with the program, you would want to determine transformation coeffiecients to adjust your reported values to more nearly compare to everyone else."

Jim, if you help him do this...I'll buy you a beer! ;-)

HQA's picture
DSLR photometry

Hi Rob,

What you are doing with your DSLR - displaying the image and visually estimating the brightness relative to the sequence stars in your field of view - is very similar to how people estimated brightness on photographic plates.  The measurement error is probably also very similar - in the 0.10-0.20 magnitude range.  It is a little hard to get a good error estimate because it probably depends on how you have your monitor's grey scale set up, and how your display program handles scaling.  You are probably using jpeg images rather than raw images as well, with more limited dynamic range and the issue of compression. Still, it is a simple method of making an estimate and doesn't require any special software or image processing.  As you are observing in Australia, even crude estimates have value for many of the southern stars.

The question then is how to report your measurement.  It is not photographic, since you aren't using photographic plates nor the wavelength sensitivity of such emulsions.  It is not digital, since you are not using an image processing program to obtain an aperture magnitude estimate.  We don't currently have a method of properly handling such a report.  What I would do is select "DSLR", select "Tricolor-Green" and then in the notes, indicate "visual estimate from computer screen".


KBJ's picture
Thanks Arne.  That's exactly

Thanks Arne.  That's exactly the situation.  I'm only uploading fainter-thans from RGB jpegs, on a small selection of CVs.

Cheers -


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