Greetings fellow stargazers
Apologies in advance for the rather long winded intro, but wanted to give some background before I dive in with questions.You can skip the italicised text to get straight to the point if you wish.
I recently signed up to AAVSO after watching some of the informative ONLINE webinars held over last few months and was inspired to try my hand with using my modified DSLR and see if I could produce some reasonable results.
Clearly using a CCD coupled with appropriate Johnson-Cousins, Bessell filters would be ideal~(leaving aside discussioin around manuf/availibility currently of some of these) , but before I plunge head and pocket first into things I want to see where my Canon 700D, SMC Pentax 135mm F3.5 prime lens and Astrotrac portable tracker with wedge can get me first.
I have a Celestron CGEM mount, Skywatcher and Celestron 80mm ED APOs, William Optics 100ED APO and 11" SCT as well numerous filters (Astronomik 12nm narrow band, Astronomik CLS-CCD light pollution filter and a 1.25" Astronomik lrgb set for filter wheel) some of which may come into play at a later stage if my initial foray proves rewarding and successfull!
Some background: I am a long time DSLR user(Canon mainly) and amateur astronomer ( primarily imaging ) and also have a small online business doing IR/UV filter removals/clear glass replacements mainly aimed at astro/photographers who want to increase the Ha response of their cameras.My other big big passion is Cosmology, for which I started studying towards an Hons after a few decades of leaving University.
After a fair bit of research,reading quite a number of quite technically in depth articles and documents on this matter, from AAVSO and elsewhere I am still battling to find a concise and fairly straight forward way to ensure my science images are sufficiently/ properly calibrated in order to analyze them photometrically.
I have played around a bit with APT,Muniwin and briefly VStar but at this stage would appreciate hearing what explicit process/software others are successfully using to calibrate their frames ( green would be a good start I guess).
Also are there check/comparison star fields for S Hemisphere ( I am based there). I know of Landolt, but that is for decl. > -20 and I'm at -34 deg S so not much helkp for my southerly latitude.
I also have Adobe PS CS6 V13 for image maniuplation, Deep Sky Stacker for calibration and stacking that I use for my astrophotography images.
Thanks in advance, look forward to your input!
I’m using stock Nikon equipment, specifically a D610 with a 180mm f2.8 Nikkor, and a D300 on a Celestron C14 with a Hyperstar lens assembly. All factory filters are still installed. I normally use the interval timer routines to make 10 30-second exposures of each target. All images are stacked to produce 3 images- a 30 second average stack, a 150 second average stack, and a 300 second sum stack.
I use AIP for Windows for all calibration, processing and measurement. All calibration frames are made using by extracting the luminance channel from each raw image and saving as FITS. I created master files for each camera using 20 bias frames, 20 dark frames and 20 white card flats. AIP4WIN DSLR conversion settings will automatically calibrate the images as they are converted from raw files.
I follow the guidance in the AAVSO Manual for DSLR Photometry available from the website.
I hope this answers your question.
Best wishes,
John Pickett
>Also are there check/comparison star fields for S Hemisphere ( I am based there). I know of Landolt, but that is for decl. > -20 and I'm at -34 deg S so not much help for my southerly latitude.
For general calibration and obtaining color-terms, the equatorial standards remain useful everywhere except near Earth's poles. There are additional high-precision primary standards in the south. Specifically, there are the SAAO E-region standards at -45 Dec, mostly relatively bright stars suitable for short focal lengths/small apertures like telephoto lenses:
https://ui.adsabs.harvard.edu/abs/1989SAAOC..13....1M/abstract
In addition there are the southern Landolt fields, which go much fainter:
https://ui.adsabs.harvard.edu/abs/2007AJ....133.2502L/abstract
...and some southern pairs of stars in a very useful, but oft-overlooked paper by Landolt:
https://ui.adsabs.harvard.edu/abs/1983AJ.....88..853L/abstract
FWIW, I have all these stars plus much else compiled (with accrate coordinates and source bibcode for every entry) in a growing list of UBVRI photometry:
ftp://ftp.lowell.edu/pub/bas/starcats/ubvri.dat
(about 100,000 lines in a 10Mb flat-text file).
You may also wish to explore the huge amount of high-quality data given by Peter Stetson in 100+ fields, mostly around globular clusters, mostly in the south:
http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/community/STETSON/index…
\Brian
"Also are there check/comparison star fields for S Hemisphere ( I am based there). I know of Landolt, but that is for decl. > -20 and I'm at -34 deg S so not much helkp for my southerly latitude."
I'm at -27.5 deg and use standard stars from the E Regions (Menzies J.W. et. al. 1989, S. Afr. Astron. Obs. Circ. 13, 1) at DEC -44 DEG hence good for your latitude. They are relatively bright, from 5th to 10th mag in V and thus more useful for shorter fical length imaging systems.
Roy
... shorter focal length ...
https://www.aavso.org/apps/vsd/stdfields
Click the blue name of the field to get the VSP chart. You can sort the table by clicking the arrow point next to column title.
Phil
Thank you both Roy/Phil.
Will definitely take a look at both your references you've supplied in due course.
Hi,
So you have nice gear already, small aperture for bright stars and faint stars with the C11.
>>>I have played around a bit with APT,Muniwin and briefly VStar but at this stage would appreciate hearing what explicit process/software others are successfully using to calibrate their frames ( green would be a good start I guess).
Every Dark & Flat must be done for the same filter. For TG, the Darks and Flats must be also converted to TG, ...the same for TR and TB.
DARKS - the dark current is increasing significantly every 5 - 6 °C, at a rough estimate. (for details see the specs of your camera..)
I have a dark library for every ISO and exposure time. Than I am looking for the temperature of the DSLR CMOS sensor (Canon can read the temperature of the sensor)
If one night stays at the same temperature, everything is good. You take the Darks with the same temperature.
If one night goes down from 20°C to 13°C, now the experimenting is on! You can take darks with same iso & exp. time,
a) from the whole temperature range (if one is lazy)
b) make two masterdarks, one for each half of the temperature range..
c) anything else you want to experiment with.. (but at some point it is too time consuming..)
FLATS:
Are really important! Every time of defocusing or rotating the Canon, a new masterflat is important!
I use the white T-Shirt method. (But if you do photographs, you'll already have your method..)
The variable should be in the center. The compare & check stars should be near the variable. Do not use compare stars at the very end of the image. If you look at the C-K diagram, those stars have not a straight line..
LINEARITY
I use ISO 100 - 400 and 800 max! The linearity or the full well capacity, is decreasing the higher the gain (iso) ISO 1600 in my opinion is not good anymore...
For 14 bit,(16 381 ADU) you can do exposures until the star has ~70% or ~12 000 to 14 000 ADU max, to avoid saturation. The Linearity comes to a problem if the variable is done ok (e.g. 12 000 ADU) but the e.g. brighter compare star has already exceeded over 14 000 ADU and is saturated.. -> one gets wrong results then...
CS, Bernhard