Image Processing of ST2000XM Images with Small Focal Length

Transcription

1 Image Prcessing f ST2000XM Images with Small Fcal Length Part1 CCDSTACK Wrkflw (Release 1.2) Bernhard Hubl, CCDSTACK_ST2000XM_eng.dc / 8

2 1. CALIBRATION WITH CCDSTACK 1.1. Requirements fr darks The dark current f SBIG cameras cannt be neglected. S, a dark subtractin is very imprtant. The dark current f my ST2000XM camera mutates very slwly. This is the reasn, why I use dark frames with an age f up t 2 mnths. I sht darks nly in astrnmical unusable nights. The bserving time in clear nights is t precius! The darks are created with exactly the same adjustments as used fr the light frames: Same binning Same chip temperature Same expsure time The number f dark frames shuld be equal r higher than the number f light frames Master darks The first step f the calibratin prcess is the creatin f the master darks using CCDSTACK: Prcess -> Calibrate -> Make Master dark. Cmbine Settings: Clip min/max Mean With adjustment 2/ Requirements fr the flats T reach very deep images, a high quality flat is abslutely necessary. If yu let yur camera at yur ptics withut ding any rtatins r ther mvements, then it is nt necessary t sht each night a new flat. I have used flats, which were lder than ne mnth, withut any prblems. A careful inspectin f the psitin f the dust particles is very imprtant, if yu use ld flats. Basically, I create a separate master flat fr each used filter. All flats are sht in 1x1 binning, even if the clurs f the bject are captured in 2x2 binning. The average ADU value shuld reach 35 t 50% f the saturatin value. This means fr the ST2000XM: Saturatin = > average between 22,000 and 31,000. The expsure time shuld nt be t shrt (pssible disturbance thrugh shutter) and nt t lng (fr fast creatin f flats). Expsure times between 0.4 and 5 s are useful fr my equipment. Because f these shrt expsure times it is nt necessary t sht flat darks with exactly the same expsure time as the expsure time f the flat, but a master bias subtractin r a master dark with a standard expsure time f 2 s is adequate. 20 t 30 flat frames are captured fr each filter. 50 dark frames with an expsure time f 2 s are used fr the master flat dark. I use tw methds fr the creatin f the flats. Bth methds lead t similar results: Sky flat: The evening sky is captured ppsite t the sun in a height f 45 t 60. The telescpe is mved during capturing f the flat frames t get a mvement f the stars frm ne expsure t the next expsure. The filter sequence is: Halpha / R / G / B / Clear. The expsure time has t be changed cntinuusly t reach apprximately 50% f the saturatin level. Mnitr flat: A white clth is clamped ver the dew cap f the refractr. The telescpe pints directly twards a mnitr, which shws a pure white image. The distance CCDSTACK_ST2000XM_eng.dc / 8

3 between the mnitr and the frnt lens f the refractr is apprximately 30 cm. This setup leads t similar flats as with the sky flat methd Master flats The first step is t create the master flat dark. This can be dne in the same way as in chapter 1.2 described. The master flat can be created with CCDSTACK in the fllwing way: Prcess->Calibrate->Make Master Flat The request dark/bias subtract each flat frame shuld be answered with Yes and then yu shuld chse the master flat dark, which had been created in the previus step. Cmbine Settings: Sigma reject Mean / nrmalize = aut / sigma multiplier = 2 / iteratins limit = Calibrating f the light frames File -> Remve All Images File -> Open -> Open all light frames thrugh ne filter Prcess -> Calibrate -> Calibrate Calibratin Settings: Dark subtractin: Chse apprpriate master dark and set adaptin = nne Flat Field: Chse apprpriate master flat and d nt activate subtract bias Apply t All Adjust Display: activate DDP / activate Apply t All / Click Autscale. File -> Save data -> All CCDSTACK_ST2000XM_eng.dc / 8

4 2. REGISTRATION WITH CCDSTACK The further prcedure depends n the desired reslutin f the finished image: riginal reslutin duble reslutin with drizzle methd 2.1. Registratin with riginal reslutin Luminance registratin At first, the luminance frames are registered. The reference image shuld be an image, which shws very gd stars and which is in the middle f the dither area. I prefer a step size f 1 pixel fr luminance dithering (square with 5x5 pixel -> 25 psitins). If yu are sure that there is n hint f rtatin between the single frames (which is very seldm the case), then yu can chse FFT using the ptin Shift Only and yu will get a very gd registratin. If rtatin is necessary, then FFT can nly be used fr prealignment. An exact registratin can nly be reached with a fllwing Star Snap. I prefer t chse 5 stars fr Star Snap : ne star in each f the fur image crners and the last chsen star in the image centre (=rtatin centre). The selected stars shuld be bright, but nt saturated. After clicking Align All yu shuld make a carefully inspectin f the registratin via blinking f the images. If single images are nt ptimally aligned, then it ften helps, if yu d a manual prealignment f the prblematic image r if yu chse ther stars. If yu are happy with the registratin, then yu have t resample the images. This can be dne with Apply->Csine fr strngly undersampled images like mine r with Apply->Bicubic Bspline fr better sampled images. The registered single frames shuld be saved in a separate directry (e.g. align_lum) RGB registratin The registratin f the RGB images is very similar t the registratin f the luminance frames. At first, the same luminance image, which has been used as reference fr luminance registratin, shuld be pened. Then all RGB images are pened. If the RGB images are binned 2x2, then these images are autmatically resized t the size f the luminance frame. Afterwards, yu can use the same prcedure as fr the luminance registratin. If the RGB images are 2x2 binned, then it is smetimes better t chse brighter reference stars, which can als run int saturatin a little bit. The registered RGB images shuld be saved in a separate directry (e.g. align_rgb) 2.2. Drizzle Drizzle wrks best, if yu use a nt whle-numbered step size fr dithering (e.g. 0.5 pixel) and if yu capture many single frames fr an ptimal S/N rati. The clurs shuld be captured with 1x1 binning. At first, all luminance frames are resized by a factr f 2. This can be dne ptimally with: Edit->Transfrm->Resize->Specify Scale Magnify = 2 Interplatin = Nearest Neighbr CCDSTACK_ST2000XM_eng.dc / 8

5 Afterwards, yu can d the registratin f the luminance and the clur frames in the same way as described in chapter 2.1 (riginal reslutin). There is ne difference: Yu shuld use the interplatin methd Nearest Neighbr instead f Csine fr drizzled images. CCDSTACK_ST2000XM_eng.dc / 8

6 3. COMBINING WITH CCDSTACK 3.1. Nrmalize The data reject functins are very pwerful tls f CCDSTACK. These functins need very well nrmalized images fr ptimal results. The nrmalizatin can be dne with: Stack -> Nrmalize -> Aut Typically, it is nt necessary t select an area. After the nrmalizatin an infrmatin windw, which shws the scale factrs f each image, appears. All factrs shuld lie in the range between 0.7 and 1.3. Otherwise, the nrmalizatin prcess did nt wrk ptimally r it is a hint, that the single frames were captured under strngly changing cnditins. (Maybe bad images shuld be remved.) If yu are sure that all images were captured under excellent transparency and that there was nly a variatin f the sky backgrund (e.g. mn rise), then there is als the pssibility t weight all images with the same factr and t cmpensate the varying sky backgrund by subtracting an ffset: Stack->Nrmalize->Cntrl->Offset Select a big area f the image with sky backgrund The selectin f the reference image fr luminance nrmalizatin is uncritical, but the reference images fr RGB shuld be chsen carefully: The RGB reference frames need very gd transparency and the highest pssible height f the bject fr minimum influence by extinctin. If yu use a filter wheel, then it is a gd idea t switch cntinuusly between the filters R, G and B. Afterwards, yu shuld chse three cnsecutive R, G and B images, which were captured at high elevatin and during excellent transparency. This prcedure is the basis fr an easy t achieve clur balance. Finally, the quality f the nrmalizatin can be checked easily: Adjust display -> Activate Apply t all Blink the single frames -> All Images shuld shw a similar brightness Data rejectin and cmbining Data rejectin means that single pixels in single images are excluded frm the cmbining prcess. This cncept allws the eliminatin f satellites, tracks f air planes, csmics and ht/dark pixels, which ccur nly in single images. Prvided that the nrmalizatin wrked well, then the data rejectin in CCDSTACK is very easy. The help f CCDSTACK suggests Pissn Sigma Reject, but I prefer STD Sigma Reject, when at least 6 frames are available. The fllwing adjustments wrk ften very well: Stack -> Data Reject -> Prcedures STD Sigma Reject Tp image %: D nt activate, but input the factr manually. Typical factr fr 6 frames: 1.8 Typical factr fr 20 frames: 2.2 N iteratin limit Activate Clear befre apply Apply t all CCDSTACK_ST2000XM_eng.dc / 8

7 The rejected pixels are shwn in red clur. S, the prcess can be easily checked and the factr can be mdified if necessary. When the rejectin is kay, then the cmbining f the single images is dne via mean: Stack->Cmbine-> Mean CCDSTACK_ST2000XM_eng.dc / 8

8 4. RGB AND LUMINANCE EXPORT 4.1. RGB factrs The pdf dcument describes the determinatin f the RGB factrs. A crrectin f extinctin is very imprtant fr a lw bject height. Fr calculatin f the extinctin yu shuld chse the bject height f the same frame, which had been used fr nrmalizatin. If yu knw the RGB factrs, then the multiplicatin can easily be dne with CCDSTACK: Prcess->PixelMath->Multiply by 4.2. RGB creatin Clr->Create 4.3. DDP Luminance: Nne Filter factr: = rati f single expsure times (e.g: If yur single expsure times are R=600s/G=300s/B=300s, then yu have t input R=2/G=1/B=1). Set Backgrund: Select neutral backgrund area Apply DDP shuld be used frt the RGB image and fr the luminance image. A very gd starting pint is the standard CCDSTACK DDP: Activate DDP in the Adjust Display Windw and click n Autscale. The backgrund shuld be decreased a little bit. If yu are happy with yur settings, then the image can be saved as 16 bit scaled tif, which can be easily imprted in Phtshp. CCDSTACK_ST2000XM_eng.dc / 8