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Uncool Imaging

The Meade DSI, ZWO, Orion Starshoots, modified web cameras and modern DSLR camrea demonstrates that CCD and CMOS sensor technology has improved over the past decade to the point that, for moderate exposure times, dark noise can be managed without cooling the sensor. Conventional astronomical CCD cameras employ power hungry peltier arrays, hermetically sealed dry chambers, large noisey power supplies and bulky heat sinks to cool the chips. Additionally, fast USB/Firewire standards have emerged that allow the CCD to be read directly into the PC without extensive processing or memory on the camera. These factors allow the DSI to produce images of a quality that until now, required cameras costing nearly 10 times as much.

I have written several articles on the topic of imaging with uncooled sensors. Below is a link to paper that previously appeared in the OPTAS newsletter on the topic.

Uncool Imaging: Video Cameras and Uncooled CCD's

As the paper discusses, the key to getting the most of a uncooled imager is to choose the longest exposure time that does not saturate the detector with thermal electrons (dark noise). Often, it is possible to make due with shorter exposures. Good quality images have good signal to noise (S/N) ratios. To improve images, that means reduce the noise or get more signal (larger aperature).

Those familiar with cooled CCD imaging will need to learn a few new tricks with uncooled imagers. The idea is to stack MANY images to reduce the noise, rather than chill the sensor. It is a time vs money trade, as are many thing in life. The DSI software is designed to automate much of this process so that for moderately bright deep sky objects, the total time to a finished color image is similar to taking 3 filtered exposures (R,G & B) with a cooled camera. For fainter objects, longer total times will be required.

One of the ancillary benefits of making a finished image up of many shorter exposure times is that, with a moderately good telescope drive system and alignment, it is not usually necessary to guide images. This is another big time saver. With long exposure cooled imagers, it often it can take a significant portion of the total time to find a guide star and train the drive system.

For more Uncool imaging tips see the notes at the end to this page.

Slides from a recent lecture titled: Introdction To DSI Imaging can be downloaded here as a Adobe PDF file.

As time permits, more practical tips will appear on this page.

Gallery

Crab Nebula (M1) - 12" Lx200gps @f3.3 34 minutes (68x30 sec.) 11/14/2004

M74 - 12" Lx200gps @f3.3 20 minutes (40x30 sec.) 11/14/2004

NGC 1977 (The Running Man) - 12" Lx200gps @f3.3 22.5 minutes (55x30 sec.) 11/4/2004

Eastern Veil Nebula (Caldwell 33/NGC 6992)
12" Lx200gps @f3.3 20 minutes (5x240 sec.) 7/20/2004

M2 - 12" Lx200gps @f3.3 9.5 minutes (19x30sec.) 11/2/2004

M33 - 70mm Achromat @f3.5 21 minutes (42x30sec.)10/31/2004

Double Cluster (NGC884+NGC869) - 70mm Achromat @f3.5 12 minutes (12x60sec.)10/31/2004

M31 3x3 Mosaic - 70mm Achromat @f5 150sec/per panel (10x15sec.), Assembled with Panorama Maker
Click here for full resolution image.

M16 - 8" Lx200gps @f3.3 10 minutes (20x30sec.) 6/23/2004

IC5146 Cocoon Nebula - 12" Lx200gps @f3.3, 24 minutes (48x30 sec.) 9/1/2004

Helix Nebula - Caldwell 63 (NGC 7293) - 12" Lx200gps@f3.3 12 minutes (24x30sec.) 10/13/2004

Orion Nebula - Messier 42 - 4" SCT @f5 12 Minutes (48x15sec.) 10/28/2004

Details: This image used shorter exposures to prevent the Trapezium from buring completely out. Also, the chip antiblooming can be overwhelmed by an object this bright. Finally, since the image is undersampled (~4.5"/pixel), adding many images that are spatially dithered, prevents the on chip color matrix from producing artifacts.

 DSI Imaging Tips

The DSI imager capture can be approached at two levels: 

1) Beginner level which automates all processing

- Dark Subtraction
- Alignment
- Image Selection
- Combining
- Resampling to Square Pixels
- Color Demosaicing

 This is a powerful tool, but necessarily makes many assumptions.

 2) There is a "Save Option" to "Save all raw images". When used in this mode, all raw frames (dark subrtacted if this was enabled) are saved to separate intermediate files for later off-line processing. You still get the auto processed image, but have all the pieces that made it up. This is for the advanced users who wish to flat field, use third party image processing (or AutostarIP), chase hot pixels, apply frequency domain filters etc...

 The Keys To Success:

 A) Regardless of which method you use, serious imagers should be sure they are saving in one of the FITS formats. Anything else is going to save the "display presentation only!!!!!" and severly limits what enhancements can be done.

 B) There are two color demosaicing methods supported in the software: Color Sharp and Color Soft. The color soft method gives better color saturation at the expense of spatial resolution. I personally prefer the Color Sharp method and set it. I can use other programs to adjust the saturation later. 

C) There are two readout modes for the camera, a high speed mode for planets and terrestrial uses, and a slow "low noise" method for deep sky imaging. When the exposure time is 0.5 seconds or less, you get the fast readout method. If the exposure time is greater than 0.5 seconds, you get the lowest noise readout method.

Be careful to get the exposure you desire. When "Live" is checked and "Preview" is not checked, you get exposure time next to "Live". When you check "Preview" you get the exposure time indicated next to "Preview" whether "Live" is checked or not.

 D) The brightness and contrast controls in the capture program are very rudimentary. Autostar IP has more flexible stretching, color adjustment and filtering. For quick images, like that M42, I let the automated robot do most of the work. Save as FITS3P and then load it into Autostar IP and stretch and color balance there. Total processing time is only a minute or two per image.

 When I want the very utmost, I save all the intermediate files and use AutostarIP/MaximLD and Photoshop to squeeze out the last bit.

Be careful. You must:

0- If you had dark subtract enabled, you should first add the dark used back into all native frames. You can then apply a different dark. Best darks are made by bracketing the exposure. Take 6 more more dark frames in "deep sky" mode with out autoalignment saving all raw frames. Repeat after the imaging frames. These two sets averaged together will approximate best the temperature range over which the images were made. After backing out the auto-darks, you can apply your high precision darks.
1- Dark subtract in native format
2- Flat field in native format
3- demosaicing the frames into R,G,B,L frames
4- Process all frames identically
5- Select, Align
6- Filter, Stretch, Patch noodle whatever
7- Combine R's, G's, B's, L's
8- Color Merge
9- Balance/Adjust
10- Resample to square pixels (flux conserving or anti-aliasing)
11- Print it!

 E) The IR blocking filter is removable by design. This allows greater sensitivity when using the imager as a mono imager or guider. It is said that you can also do deep sky color without it, but color balance can be funny, mostly on reflection nebula. Handle it carefully, if you screw up the coatings, you will have to order a replacement from Edmunds optics.

 

 


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