Hottie Pixels and Processed Images

I've been lazy about image processing. To get nice astro images, it takes more than aligning and summing multiple images. Why?

CCD images have artifacts.

Here are the problems:

1. Hot pixels - These are bright dots that appear in the same spot on each image.
2. Vignetting - This is uneven illumination of the field.
3. Dust spots - These end up looking like unfocused blobby blobs.
4. Noise - Read noise is what gives a grainy appearance to an image.

Above: M57 (Ring Nebula). 15 2-second exposures with Opticstar CCD on 14-inch w/ focal reducer. Fully processed.

What to do? Let's start with the noise. Electronic noise can be subtracted out with a "bias frame". A true bias frame is an image of zero exposure time where the CCD is read out without having been exposed to any light (scope is covered). This allows you to isolate the effect of read noise. The shortest exposure time I can get with camera control software I have (Nebulosity) is 10 ms (0.01 seconds), so this is what I use for my bias frames. It's best to take many bias frames and average them to make a master bias frame.

Above: M31 (Andromeda Galaxy). 20 2-second exposures with Opticstar CCD on 14-inch w/ focal reducer. Fully processed. This galaxy is 2.5 million light years distant!!

To get rid of the hot pixels, you use a "dark frame". This is simply an image with no illumination (again, the scope is covered) taken under the same circumstances as your real astro image. Darks should be taken the same night the real image is taken. It's considered good practice to take about the same number of exposures and same exposure time that you have of the real image and average them into one happy dark frame.

Above: M15. 20 2-second exposures with Opticstar CCD on 14-inch w/ focal reducer. Fully processed.

To minimize vignetting, you use a "flat frame". Flat frames are images with an even illumination. You can use the sky at twilight or a white screen. The recommendation is to take several flats and combine them. The software I use (Nebulosity) scales the intensity of the flats, so the exposure time isn't so important. I use the same exposure time for the flats as I do for the real images.

The flat frame should be pre-processed, that is you should subtract the dark frame and the bias frame from the flat.

The Nebulosity software has a processing algorithm. All I have to do is combine the dark frames and bias frames, and pre-process and combine the flat frames.

Above: My flat frame. Combination of 30 2-second exposures. Telescope was pointed at a white posterboard.

In summary:

To minimize read noise, subtract the bias frame from your real image.

To remove hot pixels, subtract the dark frame from the image frame.

To minimize vignetting, divide your real image by the flat frame.

Or, in equation form:

Good = (Raw - Dark - Bias) / (Flat - Dark - Bias)

The processing equation above is applied to each of the real images. Next the images are aligned and combined. Once again, Nebulosity has an algorithm for aligning and combining. For each image you select two stars, and the software will translate and rotate each image before they are combined.

Saturday night I took dark frames, flat frames, and bias frames. I also took more exposures of each image. I think the images are looking better!

The Queen, the Dragon, and the Flying Horse

Finally, a gorgeous and clear Saturday night, and my first chance to test out the new focal reducer. Long story short, it's awesome.

My life will never be the same.

It is simply fabulous to be able to fit M13 (Hercules Cluster) on the chip. It's like having skinny jeans for a telescope. Maybe that's a bad analogy. Anyhow, I took a few images of the bright globular, but honestly, they were sort of meh. I've gotten better results with the Canon Rebel DSLR attached with a T-mount. But still, happy dance because M13 fits!

Here's M13 w/ the focal reducer (it's not too bad):

M13 w/ Opticstar DS145C on 14-inch, 6 2-second exposures stacked

The scopes were well behaved (no alignment issues) and I therefore had lots of time to spend actually looking at objects. Here's the rundown of what I saw by constellation:

The Queen (Cassiopeia)

• NGC 457 (a.k.a the Owl Cluster, the ET Cluster, or Caldwell 13) is an open star cluster about 7900 light years distant and approximately 21 million years old. Discovered by William Herschel in 1787.

  

  NGC 457 w/ Opticstar DS145C on 14-inch, 6 2-second exposures stacked

• M103 (a.k.a. NGC 581) is an open cluster between 8000 to 9500 light years distant and about 25 million years old. Discovered in 1781 by Pierre Méchain.

  

  M103 w/ Opticstar DS145C on 14-inch, 6 2-second exposures stacked

• M52 (a.k.a. NGC 7654) is an open cluster with an uncertain distance between 3000 and 7000 light years and an estimated ate of 35 million years. Discovered by Charles Messier in 1774.

  

  M52 w/ Opticstar DS145C on 14-inch, 6 2-second exposures stacked

The Dragon (Draco)

• NGC 6543 (a.k.a. The Cat's Eye Nebula) is a planetary nebula. I was super excited to see it, but it didn't photograph well. It's small and faint. Basically it looks like a fuzzy little star and of course nothing at all like the Hubble images.

The Flying Horse (Pegasus)

• M15 (a.k.a. NGC 7078) is a globular cluster estimated to be about 12 billion years old. (One of the oldest globulars!) It was discovered by Jean-Dominique Maraldi in 1746.

  

  M15 w/ Opticstar DS145C on 14-inch, 7 4-second exposures stacked

Overall, I think the astrophotography is getting a little easier, and a maybe just a little better with each attempt. The major lesson learned from these images is that I really need to take more, and perhaps longer exposures.

I need more photons.

Focal Reducer

I've played around with our Opticstar CCD cameras enough now that I feel limited by their relatively small field of view (FOV). For the 14-inch telescopes the FOV w/ CCD is approximately 8 x 6 arcminutes. Saturn and the Ring Nebula (and other planetary nebulae) fit just fine, but I'd also like to image larger objects, like globular clusters.

The easy solution is a focal reducer. We recently purchased what seems to be the only focal reducer in the universe that will work with our CGE Pro EdgeHD telescopes. It's a beast of an accessory at 3.25 pounds, so today we balanced one of the scopes with the focal reducer and camera.

The reducer will reduce (go figure) the the focal length by 0.7. By my calculations, the new field of view (for the CCD camera) should be about 12 x 9 arcminutes. It's not a huge gain, but it will allow me to fit globulars like M10 and M12 (constellation Ophiuchus) in the field. The Hercules Cluster, M13, may be too big, but I'll likely try to image the core anyway.

Focal reducer at base of 14-inch w/ Opticstar CCD camera

BTW, I calculated the FOV in arcminutes using this:

FOV = (S x 3438) / f

where S is the size of the CCD chip in mm, and f is the focal length of the telescope in mm. Exciting, I know.

I don't plan on changing out the focal reducer anytime soon since balancing the scope is a pain in the keister. What will the FOV be with a regular eyepiece? A rough calculation gives me 32 arcminutes for our 26 mm eyepiece and 44 arcminutes for our 40 mm eyepiece. Big, but not quite big enough to fit the 60 arcminute Brocchi's Cluster. We may have an eyepiece (a Panoptic?) with a larger apparent FOV, though. I'll have to check.