March 2009 - Utah-Astronomy - mailman.xmission.com

Bias & Flats
by Jerry Foote 18 Mar '09

18 Mar '09

Hi Jim, I bias frame is an image of the noise that is contained in the readout register of the chip as well as the noise contained in the readout electronics plus any electronic offset built into the readout electronics. Ideally it should be taken with zero exposure time but on a practical side your fast exposure is so close to zero that it doesn't make any difference. The reason for a zero exposure time is to make sure that there isn't any thermal contribution to the bias frame. Every time an image is taken the contribution to the image from bias is included. This includes darks and flats as well as the actual image. In addition to the artifacts you mentioned in flats there is the individual photo site sensitivity of each pixel. Think of it as each pixel having a slightly different ASA speed rating. This comes about from uneven polishing of the silicon chip before fabrication of the imager and slight variations in the gate structures for each pixel. Remember that a flat image when ready to apply to an actual image has been normalized: the individual pixel values have been divided by the average pixel values in the central portion of the frame. The actual image is then divided on a pixel by pixel basis by this normalized flat frame. When making flat frames, be sure to take dark frames of the same exposure and temperature to subtract out from the flat frame. This also subtracts out the bias from the flat image. One additional thing about flats is that anytime the optical train is changed new flats must be taken. This includes focusing, filters, cleaning the optics or re-collimation. One additional item is that all frames taken with cameras, DSLR, CMOS or CCD contain noise and the only way to reduce that noise is to use take multiple darks and flats and then averaging, or median combining them. This reduces the noise content by a factor of the square root of the number of the frames. Thus if ten frames are combined the noise is reduced by over 3 times. If you want to get into this in depth I would recommend Richard Berry and James Burnell's book "The Handbook of Astronomical Image processing". As it also includes some very powerful software it is real value. Check out Willman-Bell. Jerry Foote ScopeCraft, Inc. 4175 E. Red Cliffs Dr. Kanab, UT 84741 435-899-1255 jfoote(a)scopecraft.com

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Bolide seen early morning wednesday
by Howard Jackman 18 Mar '09

18 Mar '09

So I was pulling into my neigborhood this morning at 1:22am and I saw one of the best meteors I've ever seen, it was very bright - brighter than any iridium flash I've seen it was moving nearly verticalwith a slight right to left slant. From my position in West Jordan it was just off to the south/southeast Besides how bright it was the most impressive part was how slow and deeply blue in color it was I was on the verge of expecting it to actually make it all the way to the ground since it was so low to begin with, maybe 1/3 of the way up from the horizon and it took so long to finish burning. Very impressive! I had to share it with my sleepy wife ;-) Anyone else get to see this? Howard

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Test
by Jim Gibson 18 Mar '09

18 Mar '09

Did my case study on bias show up? I didn't see it.

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Case Study in Bias and Flats
by Jim Gibson 18 Mar '09

18 Mar '09

This may be a repost. If so, sorry. I have been reading about support images; bias, darks, and flats. I am no expert and I am probable at the level of just knowing enough to be dangerous. Since my old brain’s signal to noise ration is about like my cameras’ I figure I need to go over this stuff 9 to 16 times to get a clear picture. You are my Ginny pigs. We always have Patrick to straighten us out. Bias Bias frames record the signal from the low-level charge that naturally occur on CCD or CMOS sensors and are added to every image taken . I shot a picture with the lens cap on, at room temperature; at the fastest speed my camera would go 1/4000 of a second. If I was doing this for real and planed on using it I would have to do this at the same temperature I would be imaging at. Looking at the image: http://www.slas.us/gallery2/main.php?g2_itemId=1306 It looks like a perfect black field. But taking the image into PhotoShop we see on the threshold display a little bit of dribble on the left side that represents the Bias. http://www.slas.us/gallery2/main.php?g2_itemId=1309&g2_imageViewsIndex=1 Then moving the threshold slider over under the dribble we begin to see the bias: http://www.slas.us/gallery2/main.php?g2_itemId=1312&g2_imageViewsIndex=1 Then sliding the threshold slider all the way to the left we really see the bias noise: http://www.slas.us/gallery2/main.php?g2_itemId=1315&g2_imageViewsIndex=1 With the eyedropper active I was seeing values from 10 to 80 that are added to every image I take including dark frames. Flats A flat-field frame is a picture of the optical system's in the condition you are about to shoot your image including peculiarities, such as vignetting, uneven illumination, and dust shadows. I got a hair brained idea for making a flat-field thingy instead of a light box. The light nbox I believe would be superior but one of its drawback is bulk. Someone probably thought of this before, but for me it just popped into my head. My idea cuts down on the bulk and is versatile enough for other things including Flat-fields mask, Hartman or Bahtinov masks, and crosses for diffraction spikes for stars. Oh, and the price is right. The basic materials are shown here: http://www.slas.us/gallery2/main.php?g2_itemId=1318 Yep, embroidery hoops with 2 ply of t-shirt material for flats, some vinyl for Hartmand or Bahtinov masks or what ever. Here is the flat-fild hoop for my 100mm ED http://www.slas.us/gallery2/main.php?g2_itemId=1321 Here is the flat-field hoop for my 12.5” dob: http://www.slas.us/gallery2/main.php?g2_itemId=1324 Here is the flat-field hoop in action: http://www.slas.us/gallery2/main.php?g2_itemId=1327 and this is what the flat looks like: http://www.slas.us/gallery2/main.php?g2_itemId=1330&g2_imageViewsIndex=1 I will probably take these images off this galary after a week or so. A word about bias. Although we have shown what bias looks like and we can see that it will be included in our images, what I read is that for DSLR imaging in subtracting regular dark frames you are also subtracting bias to some extent. It may be different for LRGB imagers; ask Tyler of Patrick for that. For scientific grade stuff you would probably be concerned about bias. For general deep sky imaging and especially if you are trying to learn the entire process of deep sky imaging, I wouldn’t worry about bias until you get down the road a ways. Know that it is there, and don’t completely ignore it. But, feel comfortable with you polar alignment, focusing, collecting 16 darks & flats or making a dark library and processing images. Then if you images need some improvement, look at bias as source of improvement. Jim

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Messier Hunting
by Troy Davidson 18 Mar '09

18 Mar '09

I wanted to say thank you to Kevin and David Hays for their presentation last night on viewing Messier objects. I have also found the Wikipedia entry for the Messier objects to be helpful when looking for them. Here is the link to the page: http://en.wikipedia.org/wiki/List_of_Messier_objects The page includes all objects, NGC numbers where applicable, common name, a picture of the object (The most valuable thing listed, in my opinion.), what the object is, distance, location, and magnitude. I just wanted to pass this info along to help fellow observers hunting for these objects. -- ++++++++++++++++++ Troy Davidson

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A case study in Bias and Flats
by Jim Gibson 18 Mar '09

18 Mar '09

I have been reading about support images; bias, darks, and flats. I am no expert and I am probable at the level of just knowing enough to be dangerous. Since my old brain’s signal to noise ration is about like my cameras’ I figure I need to go over this stuff 9 to 16 times to get a clear picture. You are my Ginny pigs. We always have Patrick to straighten us out. Bias Bias frames record the signal from the low-level charge that naturally occur on CCD or CMOS sensors and are added to every image taken . I shot a picture with the lens cap on, at room temperature; at the fastest speed my camera would go 1/4000 of a second. If I was doing this for real and planed on using it I would have to do this at the same temperature I would be imaging at. Looking at the image: http://www.slas.us/gallery2/main.php?g2_itemId=1306 It looks like a perfect black field. But taking the image into PhotoShop we see on the threshold display a little bit of dribble on the left side that represents the Bias. http://www.slas.us/gallery2/main.php?g2_itemId=1309&g2_imageViewsIndex=1 Then moving the threshold slider over under the dribble we begin to see the bias: http://www.slas.us/gallery2/main.php?g2_itemId=1312&g2_imageViewsIndex=1 Then sliding the threshold slider all the way to the left we really see the bias noise: http://www.slas.us/gallery2/main.php?g2_itemId=1315&g2_imageViewsIndex=1 With the eyedropper active I was seeing values from 10 to 80 that are added to every image I take including dark frames. Flats A flat-field frame is a picture of the optical system's in the condition you are about to shoot your image including peculiarities, such as vignetting, uneven illumination, and dust shadows. I got a hair brained idea for making a flat-field thingy instead of a light box. The light nbox I believe would be superior but one of its drawback is bulk. Someone probably thought of this before, but for me it just popped into my head. My idea cuts down on the bulk and is versatile enough for other things including Flat-fields mask, Hartman or Bahtinov masks, and crosses for diffraction spikes for stars. Oh, and the price is right. The basic materials are shown here: http://www.slas.us/gallery2/main.php?g2_itemId=1318 Yep, embroidery hoops with 2 ply of t-shirt material for flats, some vinyl for Hartmand or Bahtinov masks or what ever. Here is the flat-fild hoop for my 100mm ED http://www.slas.us/gallery2/main.php?g2_itemId=1321 Here is the flat-field hoop for my 12.5” dob: http://www.slas.us/gallery2/main.php?g2_itemId=1324 Here is the flat-field hoop in action: http://www.slas.us/gallery2/main.php?g2_itemId=1327 and this is what the flat looks like: http://www.slas.us/gallery2/main.php?g2_itemId=1330&g2_imageViewsIndex=1 I will probably take these images off this galary after a week or so. A word about bias. Although we have shown what bias looks like and we can see that it will be included in our images, what I read is that for DSLR imaging in subtracting regular dark frames you are also subtracting bias to some extent. It may be different for LRGB imagers; ask Tyler of Patrick for that. For scientific grade stuff you would probably be concerned about bias. For general deep sky imaging and especially if you are trying to learn the entire process of deep sky imaging, I wouldn’t worry about bias until you get down the road a ways. Know that it is there, and don’t completely ignore it. But, feel comfortable with you polar alignment, focusing, collecting 16 darks & flats or making a dark library and processing images. Then if you images need some improvement, look at bias as source of improvement. Jim

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Re: [Utah-astronomy] Shining a light on Wayne Sumner
by Wayne Sumner 16 Mar '09

16 Mar '09

Why Chuck, you old flatterer, you. Garsh.... Aw shucks.... Coming from you I count it as a compliment, but I can't hold a candle to all the great work done by so many of you in SLAS and OAS in bringing astronomy to the community. We are all so fortunate to live in the golden age of astronomy. I like to think we are pursueing the theme of Carl Sagan's book, "The Demon Haunted World: Science as a Candle in the Dark". So many adults are ill-informed about science and how we know what we know. I find astronomy useful as a tool to motivate students to want to learn more about math and science because of their innate interest in things astronomical. The fact is, we are all teachers every time we interface with the public about our passion: astronomy. I am grateful to be a part of this wonderful group. Wayne P.S. I'm still hoping the grand opening of the Northridge High School observatory will be in mid-April. Wayne A. Sumner Math/Physics/Astronomy/Engineering Boy's Tennis Coach Northridge High School Davis School District (801) 402-8610 >>> Chuck Hards <chuck.hards(a)gmail.com> 03/15/09 2:53 PM >>> Friday I got the nickel tour of Wayne Sumner's very high-tech, modern classroom/lab/shop facility at Northridge High, including an impressive collection of telescopes used for teaching astronomy. Wayne teaches math, physics, astronomy, engineering, and tennis at Northridge, and if every high-school teacher had his enthusiasm and drive, we would see far more students going into these fields in college and professionally. Wayne is an asset to the local astronomical community and deserves recognition for his efforts. His enthusiasm is infectious; as he puts it, "I *GET* to teach astronomy!" accompanied by a huge grin. I wish my own daughter's school had such faculty and facilities. Wayne, thanks again for the tour, and I hope I didn't embarrass you by calling everyone's attention to the stellar job you are doing as an educator and friend of astronomy. You journalists on the list, here's a worthy blog topic for you- give Wayne a call. _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy(a)mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://gallery.utahastronomy.com Visit the Wiki: http://www.utahastronomy.com

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Shuttle sightings
by Patrick Wiggins 15 Mar '09

15 Mar '09

So-so (low in the sky) pass of the shuttle earlier this evening but looks like an excellent one tomorrow and couple of good ones later in the week. Info here: http://spaceflight.nasa.gov/realdata/sightings/cities/region.cgi?country=Un… patrick

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Stargazers Asked to Help Document Light Pollution
by Ann House 15 Mar '09

15 Mar '09

http://www.sltrib.com/news/ci_11909076 -A

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UFO alert - again
by Kim 15 Mar '09

15 Mar '09

Thanks for all your comments. Chuck, if you were referring to the mylar balloons one finds in grocery stores, then what I saw doesn’t fit. The shape changed as if a large partially deflated balloon were tumbling and being deformed by the wind. It was also white, not silver. It actually reminded me of the kind of weather balloon that Stephen Peterson says he observed in 1956, but I saw no payload. I wish I had had time to set up a telescope, but I only saw it for a minute or two. Once from my home on west capitol hill in SLC I saw two indistinct and shadowy shapes in the sky above the downtown area. They were quite large and difficult to see, neither being illuminated nor reflecting much light. I was outside doing some urban astronomy with my 10-inch Newt, so I took a look through the scope. Only then could I see that they were very large bundles of helium balloons, apparently released from somewhere downtown. I looked carefully, but there were no lawn chairs attached. Kim No virus found in this outgoing message. Checked by AVG. Version: 7.5.557 / Virus Database: 270.11.10 - Release Date: 3/10/2009 12:00 AM

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