SQM-L
Today, my Sky Quality Meter arrived (SQM-L). The Sky Quality is a measure of light pollution. The SQM measures the Sky background Brightness in Mags/Arcsec2. Values are between 16 (heavy light pollution, moon light) to about 22 (very dark).
The SBB is strongly related to the limiting magnitude (the brightness of the faintest visible star) and to the maximum exposure time of astrophoto's.
See this page for a graphical relation betwee SBB and Limiting Magnitude: http://members.ziggo.nl/jhm.vangastel/Astronomy/visibility/PredictionTool.htm.
From now on I will write down my SQM-L value (SBB) with all my astrophoto's on this site.
La Palma
I was in La Palma in the past two weeks. I took my travel astro equipment with me by plane. Total weight for this equipment is about 10-12kg, including laptop, star atlas, EOS 350D with 2 lenses and Powergrip, Min-EQ mount with contra-weight and RA motor and handcontroller, heavy(!) 12V battery for the RA motor and several cables for computer control of the lot.
I managed to take about 100-150 8Mb RAW frames on 4 nights and did the computer processing in following days. The accuracy of the Min-EQ mount allows exposure times of 1-3 min when using the 135mm lens. Max exposure time depends on Declination of the object and of the accuracy of the polar alignment that I needed to do every evening.
I have a total of seven satisfactory end results. I reported detailed info in the results section:
- overview of Orion with the Barnard's Loop
- a detailed Orion view with the Belt and the Sword
- The Witchhead Nebula in Orion near Rigel
- Rosette Nebula in Monoceros
- California nebula
- The M41 star cluster near Sirius
- two star clusters in Puppis.
Apart from astrofotography I used my 10x50 binoculair to view several deepsky objects that cannot be seen from latitudes above northern Africa. The best object was the globular cluster Omega Centauri. It was really special to view the brightest global cluster of both hemispheres. Next time in La Palma I will definitely take pictures of ω Centauri
Custom white balance
My DSLR is a Canon 350D that has its standard IR filter replaced by a neutral filter. This way the camera is more sensitive for Red and IR wavelengths. For instance, this camera is 5x more sensitive than a standard camera for the 656 nm wavelength. This is the so-called Hα-light that is abundant in reflection nebulae.
The side-effect of this camera modification is that daylight images have an overall red glow. This can be easily corrected afterwards in software like Canon's Digital Photo Professional (DPP) but it is a burden to correct all daylight images this way. The camera is no easy snapshot camera anymore.
There is an easy solution: I can change the white balance setting in my camera. This way, the RGB colours of an example image are analyzed in the camera and the correction factors to obtain a nice colour balanced image are added to the RAW data. When viewing the RAW images in DPP or other RAW viewer these correction factors are applied automatically. This is called Custom White Balance (CWB)
I wanted to know if this CWB correction affects my astro images when processing the RAW Canon data in Nebulosity V2. I took two pictures of a white laptop screen, one with automatic white balance and one with custom white balance. When viewing the AWB frame in a RAW viewer, the picture looks reddish. This is caused by the higher red sensitivity. When viewing the CWB in a RAW viewer, the picture looks grey, as it should be. The CWB info is applied by the RAW viewer.
Now the processing in Nebulosity. First I need to demosaic the raw data. Then I can save the Red, Green and Blue signals in separate files. Then I can determine the average (mean) signal strength of the three signals. This way I found out that the Red signal is about 25% higher than Green and Blue. Most important: Nebulosity does not show any difference between AWB and CWB images. The CWB info is ignored by Nebulosity.
I found out about another feature of Nebulosity: In "Edit→preferences" you can setup what DSLR camera is used. I tested the colour balance in Nebulosity when setting "Standard EOS 350D" and "Modded EOS 350D" using my modded 350D.
It follows that setting "Modded EOS 350D" corrects for the higher Red signal of the modded camera, but it does an overcorrection. Blue is 25% higher than Red and Green is 15% higher than Red. The picture looks blue.
In both cases (not corrected or overcorrected Red) the skewed white balance can be corrected in Nebulosity using "Image →adjust colour background offset". This equals the average signals for R, G, and B.
Conclusion:
- I need to try both Camera preference settings in Nebulosity using real astro frames to find out if there is any difference in results after using "Image →adjust colour background offset".
- Nebulosity ignores CWB info set up in the camera. I can use CWB settings in my camera in order to obtain proper white balance for daylight photography without changing my Astrophoto results.
Moon with DSLR
DSLR piggyback
DSLR Piggyback met 75-200m telelens. Volgkijker ETX105 met webcam op Vixen GP. Canon via aluminium strip aan Meade bevestigd, dus geen balancering in DEC. Handmatig volgen met MTsca.
Poolas zonder richten geplaatst. Webcambeeld van volgster bekeken als preview in K3CCDTools. Handmatig volgen op SLOW stand. Grofweg in centrum dradenkruis gehouden. Pauzes tussen correcties maximaal 20 sec. Vooral in DEC, ca 10% daarvan in RA. Achteraf is er op de foto geen volgfout te zien, ook geen beeldrotatie. Volgfout <20". Trilling is minimaal. Ik denk dat met meer aandacht en concentratie de volgfout nog wel 4x kleiner kan, dus volgfouten kunnen ook met F=600mm volledig vermeden worden. Fotograferen met Astrotech en volgen met ETX behoort dus tot de mogelijkheden.
Scherpstellen met DSLRFocus. Ik heb nog niet goed door hoe ik de numerieke indicatie van de scherpte kan gebruiken. Missschien gebruikte ik een te heldere ster. Nu scherpgesteld door via DSLRFocus een aantal foto's te maken met 4 sec belichtingstijd en op het oog de scherpte beoordeeld. Ik heb de indruk dat het resultaat nu goed was. Na in- of uitzoomen blijft het grofweg scherp maar voor mooie resultaten is opnieuw scherpstellen noodzakelijk.
Belichten: met handbedieningskastje. Ik heb nog geen parallele poort kabel voor lange belichtingstijden via DSLRFocus.
Beste foto is met 3 minuten belichting op 140mm met f/5.6. Qua achtergrondverlichting kan de belichting nog wel wat langer. Zwakste ster: m13.5.
Afstoppen is noodzakelijk. Ook bij afstoppen tot f/5.6 is aan de rand nog coma te zien. Gevolgd op Vega, die goed was te zien op de webcam, maar zwakkere sterren kon ik niet vinden met even zoeken. Het vinden van een goede volgster kan dus nog problematisch worden.
Todo:
- handleiding voor gebruik webcam achter ETX (scherpstellen, met/zonder barlow, handelingen in K3CCDTools)
- uitzoeken hoe je in K3CCDtools de zichtbaarheid van sterren kan verbeteren zodat ook zwakkere volgsterren mogelijk zijn. Opnemen in handleiding/checklist.
- ervaring opdoen met scherpstellen in DSLRFocus
- handleiding DSLRFocus schrijven
- parallele poort kabel voor DSLRFocus. Geen seriele kabel gebruiken want ik heb de poort nodig voor MTSca.
- fijnregeling voor focus zoomlens maken.Heeft geen prioriteit. Scherpstellen gaat nauwkeurig genoeg zonder experimenteren met langere belichtingstijden, ISO waardes en hogere diafragma waardes
- experimenteren met andere lenzen, bv. 55mm.
- Leukere onderwerpen kiezen. Bv. Pleiades, noord Amerika nevel, orionnevel, andromeda. Veel mogelijkheden natuurlijk, maar veel hangt af van het vinden van een volgster in de buurt.
- barlowlens op ETX gebruiken (F=3300mm) en schatting maken van grootte van trillingen die optreden.