Then and Now: Astrophotography on the Simple Side?

I spent some time this morning with PixInsight on a stack of M-42 images. This is the result. PixInsight is an impressive, though oddly challenging, piece of software. The interface still eludes me at times. The results are splendid, however.

This image was taken through a Nikon D-810a at f/4, 200mm, tracked on an iOptron mount in gusty winds. This piece is the result of three major processes:

  • All images were aligned using stellar centroids.
  • The images were then stacked… this is an image integration of 100 seconds worth of exposures.
  • PixInsight was then used to do a Dynamic Background Extraction to essentially perform a flat field thus removing the lens’ vignetting. I still can’t get over this process: no flat fields required… though I bet real flats would result in a better overall image.

The camera does its own internal bias and dark subtraction. The image was then brought into PhotoShop for adjustment to levels and cropping.

M42 color stack using Nikon D-810a

Now… compare that colorful image with the monochrome one: that was taken way back in 1986 on Tri-X Pan film pushed to about 1000 ASA by boiling it in nitrogen. The image is a 20 minute exposure through a Celestron C-8 at f/10, manually guided with an illuminated reticle eyepiece. I developed this in my bathroom using duct tape and towels to block all external light from entering.

M42 film

What a difference! New technology brings better sensitivity and a whole new world of imaging…. but we knew this. I’ve been playing with CCDs since the early 1990s. No surprises. The real surprise? Cost! All this tech adds up in cost. I am not really sure that it saved me a whole lot of time to make the new image with the new tech… perhaps if both images were color? Then, yes, the new tech has saved me time. Simple? M’eh. It’s about the same level of technical detail. It ends up being about one’s knowledge base: software or film developing? You choose. Certainly some of my best images were taken with film. Which do you prefer? It’s totally up to you. Like vinyl records, film is making a comeback, but hasn’t made its way to the realm of astrophotography again. I am pretty sure that CCDs and CMOS sensors are here to stay for astro-art imaging.

  • PixInsight sounds interesting: check out their site here.
  • iOptron? Check out their site here.

Taking Flat Fields

Introduction:

When taking CCD images, and particular, when trying to use those images for scientific purposes, it is important to reduce the amount of unwanted signal and unwanted noise from each image. Optical path “noise” (some of which is actually signal), is such a problem that many astronomers really never come to grips with it. Their data suffer, and the end result is poorer science. This treatise will spell out the simplicity of taking good flat fields to reduce optical path noise and CCD sensitivity issues and will also walk you through a couple of methods to get flats done.

Optical Path Noise:

Telescopes, CCD chips and filters all block light as well as transmit light. They also harbor dust, finger prints, and other unwanted shadow producing things in the light path. The result of such optical path obscuration is an unevenly illuminated CCD chip. This is a real nightmare for anyone doing photometry, in which a standard star of known brightness might measure a bit faint one night because it was being imaged on top of a dust speck on the filter glass! Optical path vignetting and other physical path obstructions will also cast large, non-discernable shadows onto your CCD, causing poor even illumination.

CCD Sensitivity:

In the spatial realm both on the multi-pixel and single-pixel level, a CCD chip will display uneven sensitivity to incoming light. This can depend on the thickness of the substrate and uneven cooling among many other issues. This creates issues very much like those mentioned already in the optical path noise section above.

The Solution:

Take flat field images and divide them out of your images. A flat field is an image taken of an evenly illuminated object like the dusk sky, or a special illuminated white card hanging on the wall of the observatory. These images are taken through the telescope:

· at the same temperature as your nightly work,

· through the same filter/s as your nightly work,

· at the same focal point and at the same rotational angle being used all night,

· and with integration times to allow the flat to reach an average of between 20 to 50% full well capacity of your CCD chip. Flat images should never bloom, but should also not be less than a second in integration time.

For precision work, 20 to 30 or more flats through each filter should be taken each night you are collecting science data. Each flat of a given filter should then be averaged together to create a master flat which is then divided out of your light frame on a pixel by pixel basis. These details are usually all handled automatically by your software. I will assume you are using MaxIm DL software revision 5+ for the following examples.

In Practice – Taking Sky Flats:

Taking flats is easy. Here is a step-by-step method to take sky flats which has worked well for me for years. You need no special equipment other than that you already own to take CCD images.

1. Wait until the sun is setting, but still just above the western horizon.

2. Turn on your observatory: EVERYTHING. The mount, the fans, the CCD, the PC, lights normally on, etc.

3. Cool down your CCD to the night time working temperature. Wait 10 minutes for it to settle to the working temperature.

4. If you are using filters, you should take flats in order of densest filter to most transmissive. I work in the order of Ha, B, V, R, then lastly I. Set your filter wheel to the first filter.

5. Set the focal point of the system. Minor adjustments through the night are ok in order to allow for temperature changes of your optical tube assembly. Do not make changes more than a mm or so. You’ll have to take new flats if you do make larger changes.

6. Set the CCD camera’s angle to the system. Leave it here all night.

7. Point your telescope at the blue sky towards the western side of the meridian. Avoid areas of sky where there are bright stars (which will not be visible yet, as the sun is still up).

8. Take a 1 second integration.

9. Once it downloads, use MaxIm DL’s Information Window in Area Mode to inspect the average pixel count of the image. If it is too bright, some pixels will be saturated, and you will have to wait until the sun sets some more. If you have an image that reads about 20-50% of the full well count, then proceed immediately to the take a series of flats.

a. Generally the Sun is at a point in the west where its light might just be still touching the top of the treetops on the eastern horizon. Stars are not visible to the eye, nor generally to the camera yet.

b. My full well count with an SBIG camera is 65535, so I aim to get flats with an area average of 20000.

c. You can use MaxIm DL’s image series command to take a set of flats with any given filter. Repeat all the steps above as needed until you have flats for what you need.

10. You can use these flats for as long as you wish, but for precision work, flats are taken every night and sometimes in the morning after your imaging is complete. If you are not after precision work, then taking flats once a week is enough. Some would say that’s sacrilege!

A helpful hints:

If you want to start taking flats earlier, just to give yourself some time, cut out sheets of frosted mylar (used in silkscreening) to cover the objective of the telescope. Use 5 to 10 sheets of this milky white plastic material to basically dim the incoming sky brightness to the optics.

You can take flats while aiming at evenly illuminated clouds. This is ok!

I have gotten away with as few as 6 averaged flats. For truly accurate work, I have gotten up to 40 averaged flats.

Here is a flat. Look how ugly it can be! The donuts are dust. The edge darkening in the corners is caused by vignetting.

flat.jpg

A typical flat field frame. This one is of the evening sunset sky taken through an H-alpha filter. Note the dust donuts and uneven illumination of the chip. This is what we use to correct our images for these issues.