Capturing Large Objects with CCD Camera
by Jan Wisniewski
Extended objects are not commonly imaged with CCD cameras because CCD chips are relatively small (unless very expensive) and will cover only small objects at optimal resolution (~2 arcsec./pixel).
To get around this "CCD size problem", shorter focal length can be used to achieve larger field of view while sacrificing resolution:
Cookbook camera on 8 inch SCT f6.3 yields 16 x 12 arcmin. field, 2.7 arcmin./pixel
Cookbook camera on 8 inch SCT f3.0 gives ~ 36 x 27 arcmin. field, 5 arcsec./pixel
50 mm f2.0 lens attached to Cookbook camera shows ~ 7.5 x 5 degrees field, 1.2 arcmin./pixel (it is like seeing really deep with the resolution of unaided eye - (click here for an example)
Mosaic allows an assembly of even larger images using multiple tiles
Step-by step protocol:
I use only grayscale images to create moasic of large objects. It boils down the amount of time one wants to invest in acquiring all the necessary exposures (and the number of clear night in a season, of course). Smaller mosaics can be assembled in color.
Images of first field are taken (usually 3 - 5 exposures with 8 inch SCT f3.0, 1 min. each to avoid guiding), then telescope is shifted in RA to adjucent field (with some overlap). Then the cycle is repeated until a complete single row is acquired.
Then the scope is shifted in declination (again leaving some overlap) to start the next row. If all the desired rows are not finished in a single night, then on the next clear night the telescope is carefuly repositioned (using last image) to continue imaging.
Once acquisition is finished, I correct all images using dark and flat frames, tham stack each tile separately from corresponding exposures. During stacking I also adjust brightness with gamma-log scaling. It is important to calibrate each image properly to avoid problems with gradients in sky background! Then all tiles are converted into tiff format.
In the example below, 48 tiles (total of 240 images) were used to completely cover Veil Nebula. Individual tiles were copied into a single docuabout 80 meg of RAM were required). I use PhotoPaint 8 to assemble my mosaics, however other photoediting software will work, too.
( Note: To assemble small mosaic, one can use "stitch" function in PhotoPaint 8. It seamlessly joins up to two rows of tiles. For reasons discussed at the end of this article, it would probably not work with more than 2 or 3 images in each row )
Tiles were then aligned and overlapped. If there is enough overlap between tiles, their edges can be "feathered" to create less noticable junctions. Keep them still in multilayer format! Save intermediate files frequntly under consecutive version #s - it is a lot easier to go back one step than to start all over again in case of computer mishap etc.
The brightness of each tile is then corrected to create as smooth overall background as possible. It helps to temporarily combine all the layers and strongly increase brightness and contrast to check for small variations in the background - however, never overwrite multilayer image as you would loose all the assembly work done so far!
Once background looks reasonable, all the layers are combined, image is cropped and its brightnes and contrast are adjusted. Save it as a tiff file. Resulting raw mosaic (reduced in size) is shown below.
In the end, image needs to be edited manually with paint tools to remove "spikes" emanating from bright stars. Click here to see finished Veil Nebula mosaic.
The final mosaic assembled as above is to some extend distorted. Each tile is affected by optical distortions of you imaging system. In the above case I used "skewing and translation" to aliogn individual tiles but "rotation and translation" works even better. Due of the large angular size of the resulting mosaic, it will always be somehow distorted on the screen (even if fitted to conform to any of the standard cartographic projections.
Note: Assembly of small mosaics with Photoshop was described article by Robert Gendler & Joel Gelber published in Sky & Telescope (June 1999) http://www.geocities.com/CapeCanaveral/Hall/6432/Article3.html
See more examples of large CCD mosaics
Back to Index Page
Number of visitors:
© Jan Wisniewski