About John A. Blackwell

Director of Phillips Exeter Academy Grainger Observatory, research astronomer, astronomy & education consultant.

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.

A Meeting of the Moon and Venus

Mark your calendars for June 16th 2018: the Moon and Venus will slowly get to within 2.3 degrees of each other making for a lovely sight. All you have to do is head out in the early evening just after sunset and look to the lower western horizon.  You might also catch some bright later-Winter stars as well.  Here’s the view (click to make larger):

Venus and the Moon June 16th 2018

Venus and the Moon June 16th 2018 looking west after sunset.

While you are enjoying the view, take a close look at the Moon. You might just see something special, some Earth-Shine.  When the unilluminated portion of the Moon is visible, this is due to sunlight reflected off the Earth, bouncing back to light up the Moon, and making it appear a faint eerie blue color. Binoculars will make this really prominent. Enjoy!

Spring: the time for planets. They’re Back!

Now mid-March 2018 and there are planets in the sky!  Here are some of the notable moments.

If you look to the west right after sunset you will catch bright Venus and fleeting (and fainter) Mercury. On March 18th, just after sunset, you might also be able to catch the very young, sliver moon, low on the western horizon.

March 18th 2018 looking west right after sunset.

March 18th 2018 looking west right after sunset.

Are you and early riser? Then you will be able to catch the other bright planets, Mars, Jupiter and Saturn.

Jupiter, Mars, Saturn and the Moon visible at about 5:30am looking southeast on March 12th 2018.

Jupiter, Mars, Saturn and the Moon visible at about 5:30am looking southeast on March 12th 2018.

 

HDR of Eclipse Totality

With some more time back at the regular computer, I have had the ability to merge some of the eclipse images together to make a solid HDR image of totality. This is what I call quite the Photoshop workout. This is a merge of 12 images ranging from innermost corona out to the outermost corona that did not wipe out the moon’s dark disk.  Some 20 hours went into this. The star to the lower left is Regulus in Leo.

2017-08-21 Eclipse

HDR of the 21 August 2017 solar eclipse.

With just a little more editing, the image can come out even better…..

2017-08-21 Eclipse Totality HDR refined.

2017-08-21 Eclipse Totality HDR refined.

Totality from Casper, Wyoming

The eclipse was a perfect success! Clear skies prevailed throughout totality, with forest fire smoke and clouds coming in after it was all done. The temperature dropped from 88 to 68F, stunning! The corona was larger than those I had seen before. We had prominences and more. MANY images to deal with and not fast internet, so please be patient while I get these things edited and out there. It might take a while… a week or more. In the meantime, here are a couple of unedited shots.

Casper was most generous: we had an excellent space from which to observe: green grassy fields on to the hill to the east of town. The parking lot lights and grass watering sprinkler systems had been disabled for the day. The medical center even provided a lunch at the end!

Mid Corona 2017

Totality: This is the mid-corona using a longer exposure to show the fainter regions.

 

Inner corona 2017

Totality: The inner corona. Note the fain, pink solar prominences on the right limb.

 

Partial stage 2017

Partial stage as the eclipse begins. There are a few sunspot groups to see there.

 

Diamond ring 2017

The diamond ring as the eclipse reaches totality.

A Casper Saturday: People Arrive!

The town of Casper, WY is now hopping along. A lot more people are here today. We started out with a visit to the Geology Museum on the southwest side of things. They have an excellent display on geologic time along with representative minerals and fossils from each period/era. They also have their very own T.rex skeleton, a nice surprise. It is being slowly picked out of the encasing rock….

So, there was a lot of good paleontology to enjoy there for sure!

Back into town, the place is swinging. Lots of people wandering about with geeky T-shirts… yes, these are eclipse watchers for sure. The town has closed off the central area for shops to show off their goods, for people to mill about, to have a quick bight to eat/drink, and relax in the summer sun, a hot summer sun, pushing to 90F. The skies today: crystal clear. Absolutely lovely.

Arrived: Casper Wyoming

All, I am safely here at Casper, Wyoming, right on the line for the eclipse. We DID have rooms in the local motel (YAY!) and have been rambling through town a little to get oriented. The town is divided in a couple of neat ways: the Platt River runs through as does a major train route frequented by the BNSF RR and others. The town has a basic grid layout with a couple of highways to help with faster circumnavigation. It is clear, sunny and hot, and the weather is supposed to stay that way through the eclipse. Restaurants and businesses are all out in full force with eclipse gear: shirts, hats, pins, logos, stickers, drinks. One can not make way through town without seeing some reference or another. Yesterday as we drove into town, there was little in the way of traffic, so I surmise that there will be a LOT of traffic today and tomorrow. In short: ALL is GO for eclipse 2017 here.