capturing ancient light

Capturing Ancient Light

Astrophotography

Basic tools and techniques

(and a little theory)Prepared for the OAS

byAl Tuttle

Disclaimer

This presentation is an amateur work, prepared for demonstrating some of the principles of astrophotography to our local astronomy club members. As such, do not rely on any information contained herein as fact solely based on this document.

This presentation is for informal training purposes only. No monetary compensation may be associated with obtaining, transmitting, or using this presentation in any way whatsoever.

I’m an Amateur, NOT an expert

I’ve been doing this for a year…. Getting up to speed has been like drinking from a fire hose.

Let me say: Many topics addressed here are deep and technical and I’ve

read long, contentious dialogs debating the pros and cons of how to best accomplish some of the tasks.

I’m sure many things here are over simplified, or just plain wrong.

However you don’t need to know it all to take satisfying images.

Give it a try. Start small, work with what you have, and learn as you go.

Why do it?After all there’s Hubble

It’s fun! Lots of new things to learn. You can “see” things you just can’t otherwise. The pleasure of being able to say “I did that”. Can be done under light polluted skies.

Besides…… Gives you something to do when you can’t sleep.

(Actually, you can often sleep while you do it)

Helps rid yourself of any of disposable income.

Wide field images (Milky Way, Constellations, Star trails, etc) Typical equipment is a 35mm film or DSLR camera & lens

mounted on a stationary tripod, or equatorial mount.

Lunar & Planetary Pictures For Lunar images, almost any camera will do. Film or DSLR

camera for lunar images and webcams are the most common. For the planets a CCD webcam coupled to a long focal length

telescope can do an excellent job.

Deep Space Objects (DSOs) CCD or CMOS camera capable of long exposure, a decent

telescope on an equatorial mount.

The basic types

Astrophotography: Overview

The basic types (cont)

Images may be taken Afocal or at Prime Focus

TypicalAfocal Setup

Also calledEyepiece Projection


Typical Prime Focus Setups

The basic types (cont) Images may be taken with a One Shot Color Camera

Typical OSC Cameras&

Connection to Scope (Note no filter wheel)


The basic types (cont) Or with a Monochrome camera and color filters

Typical Mono Camera and Filter Setup

SBIG Integrated Wheel


Lunar and Planetary Imaging

Almost any telescope, but a long focal length is helpful for planetary work. Barlows, Powermates also used.

DSLR cameras often used for Lunar images Remarkable images of planets can now be taken with an

inexpensive webcam. Short video clips are taken at 10 to 30 frames/sec to collect

hundreds or thousands of images. The images are sorted for quality, registered, and stacked to

create a much higher quality image than a single frame. Free software is available to stack and process images

(Registax, Kieth’s Image stacker, and others.)

Lunar & Planetary Imagining

Due to seeing conditions, it’s very difficult to capture a high quality, single frame image.

At left is typical movie clip.

Selecting, registering, and stacking yields photos like this. (actually much better than this)



Lunar/Planetary imaging is fast and fun Due to the short exposure times, precise tracking and

expensive mounts are not necessary. Many video clips can be acquired in just a couple hours

including setup. Processing is fairly rapid, typically just a few hours work to sort

and stack the best frames.

BUT! Quality of the image is very dependant on seeing conditions. Focusing can be very challenging Webcam sensors are very small. Getting the image on the

chip can be difficult.

The Ups and Downs

Deep Sky Photography

Dim targets mean longer exposures. DSO imaging is usually done with dedicated CCD imagers or

DSLR cameras. Frame exposure times of 1 to 30 minutes are typical. Total

times can be many hours. Long exposures can place severe demands on the telescope

mount to avoid tracking errors. An equatorial mount is best. It can be done with an Alt-Azm

mount, but field rotation will be an issue. Processing is substantially more involved.

However, great images can still be captured and processed with relatively inexpensive gear.


Basic Elements: (some things you’ll want to understand) How your mount works and how well it tracks.

Your telescope and camera specs (pixel and sensor size, sensitivity, focal length, etc)

Image capture techniques and pertinent issues. (Focus tools, Sky background/exposure times, etc.)

Techniques to improve the quality of the raw data.

How to assemble the image.


Tracking: Know your mount

Knowing how your mount performs is essential. Is it strong and on a stable tripod? Rule of thumb is for photography, the weight on the

mount should be half the “rated” capacity. Very close polar alignment is important. The closer it is,

the fewer corrections will have to be made. Balance of the scopes and weights is also very important. All motorized mounts have Periodic and Non-Periodic

Errors. Knowing how much of each, and how to mitigate the errors is

important. Some mounts have PE correction (PEC) built in.

Deep Sky Photography: The Mount

The Equatorial Mount

The Go-To pointing system uses both RA & DEC.

When Tracking, the mount rotates on the RA Axis.

When Guiding, corrections are made to both RA & DEC.


Polar Alignment: The better the RA axes is aligned with the NCP, the better the mount will track, and fewer corrections will have to be made.

Deep Sky Photography: Polar Alignment

Tracking: Balance the Load

Balance is very important to: Keep the load on the gears to a low, safe level. Minimize the effects of backlash (play) in the gears. Minimize the non-periodic errors due to rotational friction. The Balance is adjusted by moving the counterweights

and/or OTA so that the RA axis is just a touch East heavy. Balance in DEC is less important, but most set it a little tail heavy.


Tracking: Guiding

You can manually guide. Not many folks do any more. Auto-Guiding fixes a lot of problems.

Many would say it is required for long exposure imaging. Corrections are made at the sub arc-second level. Can easily correct for substantial PE and some Non-PE. Some dedicated imagers like the SBIG ST and STL series of

cameras can be ordered with an integrated 2nd “guide camera”, so no second scope or camera is needed.

The equipment used for auto-guiding does not have to be expensive. Inexpensive scopes and cameras can be used. There is very capable free software available (PHD, Guide Dog,

and others).


Guiding setups

Deep Sky Photography: Guiding

Piggyback and Side-by-side setups are common…. But watch out for flexure in either case.

Cameras…

Dedicated CCD cameras and DSLRs are the most commonly used for DSO imaging. Sometimes modified DSLRs are used that have the I/R

filter removed or replaced to enhance long wavelength response.

Cost goes from around $150 to many thousands. Filters are also needed for “monochrome” CCD

cameras…. But then…

Deep Sky Photography: Imagers

Actually….

All CCD/CMOS cameras are Monochrome. To get natural color, the light is filtered through Red,

Green, and Blue filters. The collected data is then added together to form the RGB

composite (i.e. color) image.

“Color” cameras have the RGB filters dyed onto the CCD or CMOS sensor in a certain pattern.

A little better resolution is obtained with a monochrome camera, with separate filters.


Cameras… The Sensor

For OSC sensors, a picture might help This represents a typical

color CCD/CMOS sensor. Note the pixels arranged in

a grid. The “filters” are dyed on

the CCD in what’s called a Bayer Array.

Monochrome cameras don’t have the dye. Separate filters are used in front of the camera.


Sensor in an OSC camera

Cameras… Sensor Specs It can be confusing; Megapixels, Quantum Efficiency,

ADUs, ABG, NABG, ….. . Think of the sensor as a bunch of tiny buckets in a grid.

The number of buckets is the number of pixels…. Megapixels. As a photon lands in a bucket, it adds to the charge the bucket

contains. How easily it adds to the charge => QE. When a bucket is full it’s called Saturated.

If full, a bucket may overflow into adjacent buckets. It’s called Blooming.

Some cameras control (gate) how that happens… they are called anti-blooming or ABG. If it doesn’t control it’s a NABG.

The charge for each pixel is converted to a pixel value (ADU) when an exposure is read out.


Sensors really should be matched to the telescope & target: Field of view (FOV): Determined by the size of the

sensor and the FL of the scope. The native image size is determined by the sensor size. Focal reducers can give some flexibility at the cost of resolution

(and watch for color shift and vignette). Large sensors are becoming available at reasonable cost.

Pixel size “should” be matched to the focal length. A large pixel size sensor (e.g., 20 uM) on a short FL scope would

be undersampling, giving chunky/square stars. Values like .75 to 2.5 arc-secs/pixel is typically recommended

depending on seeing conditions.

(Check out The CCD Calculator, from the New Astronomy Press)

Cameras… Sensor Size Issues


CCD & CMOS sensors contribute noise to the image. The noise is the result of defects (like hot pixels) and the

electrical currents always present in the sensor (called dark current and bias noise).

The amount of noise is sensitive to temperature. The colder the chip is, the lower the noise. Rule of thumb, every 5 deg C, the noise is cut in half (down to about -15C.)

The more expensive dedicated imagers have a device (TEC) to cool the chip, and may have electronics to closely regulate the temperature.

Regulated temperature makes correcting for the noise much easier (makes subtraction of Dark Frames more accurate & consistent).

Some cooled cameras don’t regulate, but get the sensor so cold that Dark frames can be neglected.

Cameras… Sensor Noise


Image Capture & Processing….

First a couple notes on image file formats: Most common file type used is the FITS format for CCD

imagers, and RAW for DSLR cameras. These are lossless formats, preserving all the data captured.

FITS: Flexible Image Transport System. FITS is designed specifically for scientific data and hence includes provisions including human readable data along with image information.

Raw: So named as these files contains minimally processed data from the image sensor of a digital camera.

Other formats can also be used (TIFF, JPG, BMP) depending on what camera/software used. Note it’s best to use a 16-bit or better format to preserve as much data as possible.

Deep Sky Photography: Processing

Make an Image… The basics

Regardless of “color” or “monochrome” the process is fundamentally the same: Collect images of each color desired (Typ: Lum, Red, Green, Blue). Calibrate and stack the images for each channel. Register/align the images. Combine them into an RGB/LRGB composite (color) picture.

Deep Sky Photography: Process

Basic RGB Process….

Monochrome cameras Take, calibrate, and stack the R, G, B (L optional) images into

“master” R, G, B, & (L) frames. Most folks stretch, and clean up the final masters prior to color

combining, but it can be done after. Register (align) the master frames. Color combine them by adding the masters together (there are

several programs that will do this task for you). Adjust the color levels and saturation. Do final processing in your favorite photo editing software.

Photoshop CS seems to be the favorite, but many folks use PS Elements, Paint Shop X2, or the Gimp (however these are 8-bit so only 256 shades of gray vice 65000).


Process in Favorite Editor

TakeImages

Combine masters for color image

Deep Sky Photography: Process Overview

Calibrate,Stack

Images

(For OSC cameras, just one set of images is taken and processed.)

Basic RGB Processing….

OSC cameras The data from the Bayer matrix of an OSC camera must be “de-

bayered” to create the RGB(L) channels, then recombined into a color image.

Programs like Deep Sky Stacker will automate the process of calibrating, aligning, stacking, de-bayering, and creating the color image. OR

You can do it yourself. After calibrating each frame: Use a program to de-bayer and extract the L, R, G, & B images as

individual files. Process (stretch, smooth, et. a.l.), Align and Stack each of the channels. Then RGB(L) combine, just as would be done for a monochrome camera. This is a little more work, but gives added control and the ability to stretch

and clean up the L, R,G, & B masters prior to color combining. This typically results in a bit smoother image.


Signal vs Noise (S/N Ratio)

This concept is probably the single most important aspect of taking and processing your images.

Simply defined: Signal is the image info you meant to get, noise is the other stuff: Such as dust on the optics, dark current, and hot pixels on the

CCD. Some noise is random (dark current, stray light), some is not (hot pixels).

Several factors affect the total noise in the final image. Some noise is under your direct control when imaging

(exposure time, filters, optics, …. ) Some noise is inherent in the equipment, and mitigating

techniques are used during processing (e.g., Stacking).

Deep Sky Photography: Down with Noise

Signal vs Noise: Taking Images There are things that can be done to minimize noise as

you take pictures: Shoot under the darkest skies possible. If in a light polluted area, use I/R and/or Light Pollution filters. Exposure length (often called integration time), affects the

signal to noise ratio. There are some pretty fancy formulas to figure out the optimum times, but in general:

Under dark skies: Shoot the longest exposures you can (but try to get at least 5 subs).

Under not so dark skies: Shorten the exposure time, and shoot many, many more subs.

With DSLR cameras, the ISO selected makes a difference.

Deep Sky Photography: Down with Noise

Signal vs Noise: Image Calibration First… some terminology:

Light Frame: The individual pictures you take of your target. Dark Frame: An image taken with no light hitting the sensor. Flat Frame: A half saturated white light picture used to correct

for optical path problems Flat Dark: Same as a Dark frame but at the exposure time of

the Flat (it is applied to the Flat). Bias Frame: A minimum time exposure dark frame taken to

account for CCD internal baseline charge. (can be safely ignored unless doing advanced imaging & cal).

ADU or PV: Anolog Digital Units or Pixel Values. The “count” of how many photons hit a given pixel.

Deep Sky Photography: Image Calibration

Image Calibration: Images (Light frames) are “calibrated” to remove noise

from the Sensor and Optics chain. Dark Frames

Taken with the scope/camera covered and the sensor at the same temp as the Light frames to be taken.

The Dark frames exposure length should be the same as the Light frames. Several frames (5 to 8) should be taken and stacked/averaged to make a “master dark”.

The Master Dark frame is subtracted from the Light Frames. This removes the noise added by hot pixels, dark current, and

bias noise in the CCD itself.

Deep Sky Photography: Image Calibration - Darks

Flat frames Used to correct for imperfections (dust, vignette, etc) in the

optical train. Taken at focus, under a uniform, white, dim source, at an

exposure time long enough to about half saturate the CCD pixels.

Note this is process is very fast. Typical exposure lengths are 0.3 to 2 seconds each.

Like the darks, many frames should be taken, stacked and averaged. After the Flat is stacked. It is normalized (each pixel value

divided by the average PV of the frame). The Flat is applied by dividing it into the Light frames.

Flat Darks Flat Darks are taken at the same exposure time as the Flat

frames. Used just like regular dark frames against the individual Flat

frames to corrects the Flat frames for CCD noise.

Deep Sky Photography: Image Calibration - Flats

Deep Sky Photography: Raw Light

Deep Sky Photography: Dark Subtracted

Deep Sky Photography: Flat Applied

Image Stacking After calibrating, images are stacked (averaged)in order to

reduce other sources of random noise… increasing the S/N ratio. Stacking lots of frames will significantly improve the image.

However it’s not a linear relationship: Stacking 4 frames will cut the noise in half. Stacking 16 frames will cut it in half again.

The frames are aligned by picking a number of stars, and using those to index the placement of each image.

Stack: Various methods of averaging the data Average, Median, K-sigma… Used for various situations like

number of images, amount of noise, and others. For example Median stacking will remove satellite trails,

Averaging won’t.

Deep Sky Photography: Image Stacking

Deep Sky Photography: Histogram Stretch

Deep Sky Photography: Register for RGB Combine

Deep Sky Photography: Histogram

Deep Sky Photography: Time to Process

Deep Sky Photography: Other Tools

Resources

Websites

www.cloudynights.com (visit the forums here… great people willing to help)

www.starizona.com (good site for equipment and info)

www.newastro.com (good resource for book, info, software)

groups.yahoo.com (great set of forums)

www.astromart.com (good deals can be found)

www.sbig.com (good products, and good info)

www.starklabs.com (solid, practical, inexpensive software)

Books

Handbook of Astronomical Image Processing, William Bell & James Burnell

The New CCD Astronomy (Ron Wodaski, The New Astronomy Press)

Deep Sky Photography: Other Tools

Resources - Software

Capture & Processing

AIP4Win (Comes with Book)

PixInsight (LE version is free, but dated)

CCDSoft (Software Bisque)

CCDOps (SBIG Software)

Envisage (Meade Autostar Suite)

Nebulosity (Low Price)

Deep Sky Stacker (Free)

Maxim DL

AstroArt

Registax (Free)

Guidedog (Free)

PHD Guiding (Free)

Photo Processing

PhotoShop CS3, CS2, Elements

Paint Shop Pro X2

The Gimp (Open Source)

Planetarium and Tools

Stellarium

The Sky

Voyager4

CCD Calculator (The New Astronomy Press)

AstroPlanner

WCS - Drift Alignment Software

My Gear

Deep Sky Photography: Equipment

1st Night Out M31 - Mono DSI Pro

Your first try is bound to be better than mine.

2nd Attempt - M13 - Mono DSI Pro

I felt real good about this one…. considering

Another Early Attempt M81 - Mono DSI Pro

Starting to feel this is actually possible

Orion Nebula M42 - Mono w/fltrs DSI Pro

Lunar Eclipse, Webcam on 80mm scope

Silverdale Jr High, Feb 2008

Cocoon Nebula IC5146 (DSI2 Pro w/fltrs)

(Taken at the Hurricane Ridge Star party this year.)

M33 Pinwheel Galaxy (OSC ST-2Kxcm)

NGC 2237 Rosette Nebula

Eastern Veil (OSC ST-2Kxcm)

Whew!! FinallyGot Questions?

Prepared forThe Olympic Astronomical Society

by

Al Tuttlewww.alberts-astro.com

[email protected]

capturing ancient light

Documents

thousands of images

satisfying images

planetary work

monochrome camera

dslr camera lens

process images registax

planetary imagingalmost

overviewthe basic types