Astrophotography - Equipment

Written and Edited by David Pearson Some material extracted from Antonio Miro original Beginner’s Class

Astrophotography handout

Astrophotography

Astrophotography is taking photographs of objects in the sky

• Records the objects to see for history

• Records once in a lifetime celestial events

• Allows seeing dimmer objects that can not be seen with your eyes or

visually with your telescope

• See objects in color

Astrophotography can be performed with

• Only a tripod with SLR or DSLR (digital camera), or any camera with

tripod connector

• Telescope with SLR/DSLR, or just hold a point and shoot camera up

to the eyepiece of a telescope

• Webcam, video or specialized digital camera can be used with a

telescope

– Computer may be required to see the objects, and to combine or

process the images

Astrophotography

Types of pictures that can be taken

• Star Trails

• Asteroids, Meteors and comets

• Eclipses

• Transits - Planet transits across Sun

• Celestial objects (stars, planets, clusters, galaxies, nebula, etc.)

• Occultation's

• Sun spots

• Moon and craters

• Meteor Planet impacts

• Constellations

• Aurora Borealis

• Rainbows & Sun dogs

Necessary Functions to perform Astrophotography

• Camera Equipment Support

– Purpose – Support weight of all equipment, provide pointing capability to find and

track and/or guide sky objects

– Consists of tripod/pier, manual or motorized mount, Camera and/or telescope, and

imaging accessories.

• Tracking

– Purpose –To rotate the telescope and/or camera at the speed necessary to move at the

same rate as the Earth’s rotates and keep selected object in camera field-of-view

– The mount performs this function. Can be manual (user controlled) or motorized.

• Focusing

– Purpose – Focus the telescope and/or camera for best object resolution/view

– Consists of a focuser that connects an eyepiece/camera to the telescope. Can be

manual or motorized

• Polar Alignment

– Purpose – Align mount coordinate frame to Sky coordinate frame to allow good

tracking performance

– Depending on polar alignment method, different hardware and/or software is used

Necessary Functions to perform Astrophotography

• Guiding

– Purpose – To actively keep chosen sky object in the field-of-view of the

camera or telescope.

– The mount along with camera/telescope and other accessories are used to

perform this function (not all mounts can perform this function)

• Camera

– Purpose – Capture selected sky object image by leaving camera shuttle open

for extended amount of time or to capture video

– Camera and/or computer with software controls this function

• Scripting

– Purpose – Automate processes to perform all astrophotography functions

– Computer and software performs this function

• Image Processing

– Purpose – To capture, calibrate and process image(s) to bring out detail in the

final image.

– Computer software performs this function

Tracking

• The main purpose of tracking is to keep the selected object in the

camera field-of-view. To do this precisely, the mount or the user

must move at the combined apparent sky rotational speed of 15

deg/hr (15 arc-sec/sec) and the much smaller effect of the earth’s

motion around the sun of 0.0412deg/hr. Some mounts ignore the

smaller term or a portion of it.

• There are two basic mount types that perform tracking differently;

Altitude-azimuth and equatorial

• An altitude-azimuth (alt-az) mount has two perpendicular

axes, one axis that rotates 360 degrees around the horizon

from north, to east, to south, to west and back to north, and

one axis that is perpendicular to the other that rotates from

horizontal (0deg) to vertical (90deg) and back to horizontal.

• Types: Tripod, Tripod with manual alt-az adjustment,

and manual or motorized alt-az mount

• The alt-az mount must mimic the apparent sky motion using

two axes and over time an unwanted rotation occurs, called

field rotation (see field rotation discussion).

• The alt-az mount needs to be aligned with the sky to perform

tracking. This is performed by performing a star alignment

(see polar alignment section)

Note: picture of

manufacture product is

for reference only and

does not recommend a

specific product

Tracking

• A equatorial mount has two perpendicular axes, the right

ascension (RA) axis is pointed at the Earth’s rotation axis and is

driven at apparent sky rate to allow object tracking. The

declination (DEC) axis provides a second axis that when

combined together with the RA axis allows pointing at any

object in the sky.

•An equatorial mount only needs one axis (called right

ascension (RA)) to move to match the apparent sky rate.

The one axis motion can be performed either by the user or

can be motorized.

• Most equatorial mounts come with an azimuth and altitude

set of adjustments to precisely allow pointing the right

ascension (RA) axis at the Earth’s rotation axis (polar

alignment).

•To assist in polar alignment, some mounts come with a polar

alignment scope in the mount

Types:

• DIY Barn Door Tracker

• Dedicated camera mount

• Telescope equatorial mount

Note: picture of

manufacture product is

for reference only and

does not recommend a

specific product

Tracking DIY Barn Door Tracker

• A very simple way to track is to use two pieces of wood, a hinge, and a ¼

- 20 bolt. A quarter turn of the bolt every 15 seconds matches Earth rate.

It is both cheap and fun to build, and you can have very nice results.

Adding a rotating head between the barn door tracker and your camera

will help orienting the camera to get the field of view you want. You may

even add a one rev/min DC motor to make the tracking more automatic.

• Do need to point the hinge at the North Star (Polaris)

• Can allow exposures up to about 2 to 2 ½ minutes

http://cloudbait.com/projects/barndoor.html http://exmodula2.com/

Tracking Dedicated Equatorial Camera Mount

Vixen Polarie

iOptron Sky Tracker

AstroTrac TT320X AG

AutoGuiding Tracking Mount

Attach a DSLR to a camera equatorial mount

which is attached to tripod

Just a small sample of available products

Do need to point the

camera mount at the

North Star (Polaris)

Note: picture of manufacture products is for reference only and does not

recommend a specific product

Focusing

• The purpose of focusing is to obtain the best view and resolution of the object being

capture by camera. In practice, this can be very difficult.

• The focuser connects an eyepiece/camera to the telescope, and can be manual, or

motorized with manual or computer control.

• If a DSLR or SLR with a lens is being used than the camera must be focused. Focusing

is more critical as the focal length increases, because as the field-of-view decreases the

image gets larger and any focus errors will be more apparent leading to shapes of objects

that are not desired. Also, note that standard camera lens were designed to be used

during the daytime, as a result the optics may not be as good as a telescope. Focusing

may be difficult because star will not be pinpoints, but sometime strange shapes, so

manual and auto focus may be difficult. Depending on camera type, one of the

following focusing method can be used.

• Four focusing methods (worse to best); 1) do a manual focus at time of exposure, 2)

determine the infinity focus setting during the daytime and mark the position on the

lens with tape or mark, 3) If camera has live view and magnify-the-image capability,

use it to focus the best you can, 4) Same as option 3), except use auto focus mode then

switch back to manual (beware this doesn’t always work depending upon camera. If it

doesn’t work use option 3).

• Manually focusing a telescope with a non-DSLR/SLR camera is also difficult. Many

techniques can be used to assist the process.

Focusing – Aperture Mask

• The simplest and cheapest is a simple aperture mask over the front of the

scope that has two holes cut 180 degrees apart. The size of these holes varies

depending on the aperture of the scope you are using (about 2" diameter for an

8 inch aperture and maybe 3" for a 10"). Unless you have a computerized

scope with "Go To" capability, you must use the finder scope to make an

accurate mental or hard copy map of the position of the scope while it is on the

object to be photographed. You now move the scope to a bright star nearby

and look through the camera viewfinder for the star. Most likely, you will see

two stars with the same brightness.

• Move the scope focus in or out and you will notice the stars will begin to

converge or diverge. What you want is to converge them into a single stellar

image as accurately as you can and after doing so, you are in focus. Now move

the scope back to the photographic object and using the locator map you made

previously, position the scope EXACTLY as it was before. You can now make

the photograph.

Bahtinov Mask Hartmann Mask

or Scheiner Disk

Note: picture of manufacture product is for reference only and does not recommend a specific product

Focusing – Focault method

Focault method - A razor edge to cut a star beam.

• Use a "knife edge focuser" to get a precise focus. This employs the fact that a well

focused star makes a small pinpoint of light on the focal plane of the camera. The

knife edge focuser in simple terms is a very thin edge that when placed at the

precise prime focus of the scope will cause a well focused star to quickly vanish

from view when the scope is moved in a direction that will cause the knife edge to

occult the star. An out of focus star will not quickly vanish but will appear to

slowly dim out as it is occulted.

http://www.sciencecenter.net/hutech/mitsub/focuser.htm

Mitsuboshi knife-edge focusers

Note: picture of manufacture product is for reference

only and does not recommend a specific product

Focusing – Off-axis Guider

• An off-axis guider allows simultaneous visual viewing of the camera image.

Since the eyepiece has the same focal distance as the camera, once you focus

using your eye in viewing the image, the camera is also in focus.

Mitsuboshi Off-Axis Guiders

http://www.sciencecenter.net/hutech/mitsub/oag.htm

Note: picture of manufacture product is for reference

only and does not recommend a specific product

Focusing - Camera

• Another method of focusing is to take a short exposure of a star field using the

camera and then adjust the focus until the smallest stars are achieved. Note

that the telescope/lens diameter and optical quality, telescope collimation and

atmospheric conditions will limit the minimum star size that is achieved.

• A more expensive method, but easier is to use is a computer software program

(FocusMax or similar) to analyze the star field image and adjust the focus until

the smallest star diameter is achieved. In addition to the software, a motorize

focuser (FeatherTouch, MoonLite, or similar) and controller must be

connected to the computer to use this method.

Sample Focuser Manufactures

2”, 2.5”, 3” and

3.5” format

Note: picture of manufacture products is for reference only and does not

recommend a specific product

Polar Alignment

In general, Polar Alignment is required to maximize the viewing time for a given

telescope field-of-view. However, for astrophotography a good polar alignment

minimizes image smearing and reduces one cause of elongated stars during image

capture. The process of obtaining a good polar alignment is different for different type

of camera/telescope mounts.

• For equatorial mounts, the RA axis is made parallel to the Earth’s rotational axis. The mount

only has to move the RA axis to move at the Earth’s rotational speed.

• For alt-az mounts, a multiple star alignment must be performed for the mount to determine its

telescope coordinates relative to the sky coordinates. The mount must move its two axis

simultaneous to move at the Earth’s rotational speed.

Both mount types can be used for astrophotography, but the best choice is an equatorial

telescope mount

Methods for Polar Alignment (all methods are for equatorial mounts, except for 7.)

1. Manual method using eye and mount or telescope reference aid

2. Crosshair or double crosshair reticule (lighted reticule works better)

3. Polar align reticule

4. On a equatorial telescope mount, use a Polar alignment finder-scope that is mounted

inside mount.

5. Star Drift method

6. Polar alignment software using camera

7. Star Alignment

Polar Alignment

1. Manual method using eye and mount or telescope reference aid

• With this method, the user sights along a reference straightedge to point the

telescope RA rotation axis of the mount at Polaris. The reference can be the

long edge of the telescope, mount’s RA axis or two screw heads on opposite

ends of the telescope. To adjust use the tripod legs, tripod head, or

latitude/azimuth adjustment bolts/knobs on the mount. Note if the telescope

is used as the reference straightedge, the telescope must be parallel to the RA

axis (DEC = 90) and in the highest position over the RA axis.

• If mount has latitude scale, adjust for your latitude. Double check using

above method. Recommend that the telescope mount is leveled first.

2. Crosshair or double crosshair reticule (lighted reticule works better)

• Can be used in finderscope or telescope eyepiece(focuser) holder. If using

finderscope, the finderscope must be aligned with the telescope.

• Insert reticule into finderscope or focuser.

• Put the telescope parallel to the RA axis (DEC = 90) and in the highest

position over the RA axis.

• Adjust the tripod legs, tripod head, or latitude/azimuth adjustment

bolts/knobs on the mount until Polaris is center in the crosshair.

Polar Alignment

3. Polar align reticule

• Can be used in finderscope or telescope eyepiece(focuser) holder. If using

finderscope, the finderscope must be aligned with the telescope.

• Insert polar align reticule into finderscope or focuser.

• Put the telescope parallel to the RA axis (DEC = 90) and in the highest

position over the RA axis.

• Adjust the tripod legs, tripod head, or latitude/azimuth adjustment

bolts/knobs on the mount until Polaris is centered in the polar align reticule

quadrant shown for current day and time (see picture).

Polar Alignment

4. On a equatorial telescope mount, use a Polar alignment finder-scope that is

mounted inside mount.

• Same method as 3), see previous page

5. Star Drift method

• The star drift method can be used to align your telescope to any accuracy.

First, level your tripod and orientate it to Polaris (not necessary, it just helps).

If you have a Polar alignment circle, it can also help to use them.

• Look at a star near the Equator, South. Track the star in RA only, and look if

the star goes up or down in your eyepiece (supposing your are looking

straight at south with your head vertical). Rotate the azimuth of your

telescope to adjust it until the star is not moving anymore.

• Then, move to a star at East (West works also). Do the same, but adjust the

altitude this time (the angle between your telescope RA axis and the horizon).

By switching several times from South to East (West), you should be able to

adjust your polar alignment quite quickly. Of course, the first time you will

spend a lot of time; take notes of what you are doing, and it will be much

quicker the next time you do it.

• Continue star drift process until the star does not move for the time you want

to take an exposure

Polar Alignment

6. Polar alignment software using camera

• There are many software programs to help in polar alignment using a web

cam or camera. Some mimic the star drift method by displaying the star drift

over time. Others require precise star alignments and the software computes

the polar error by comparing the star coordinate to what the telescope mount

measured. Also some continuous displays the error as the polar axis is being

modified by the user to provide real time updates and to know when

complete. There are to many programs available to name them all. Here is

an incomplete list; Alignmaster, EQAlign, PoleAlignMax, PemPro, WCS,

Polar finder, StarTarg2.0.

7. Star Alignment (Required for alt-az telescope mounts)

• Star alignments are performed by choosing a known bright star, placing it in

the center of the telescope eyepiece, finder or camera, and then tell the

computer which star it is. This is performed one, two or three times. When

complete, the computer determines the coordinate transformation between

the mount/computer and the sky. This is not technically a polar alignment

method, however it basically does the same thing, determines how to move

the telescope to rotate at Earth rotational rate.

Image Smear without Guiding

e

s

Pf

It

872.787 where

t = Exposure Length, min

Is - Image Smear, microns

f = Focal Length, mm

Pe = Polar error, arc-min

http://articles.adsabs.harvard.edu

“Polar axis alignment requirements for astronomical photography”, Hook, Richard N., 1989 volume 99 page 19-22, British

Astronomical Association provided by NASA Astrophysics data system

Nomograph

Image Smear without Guiding (continued)

Nomographs

Guiding

• Guiding is the act of continuously correcting track errors in real-time to eliminate image

smearing and minimize elongated stars in capturing images. Track errors can consist of

polar alignment errors, Earth rotational rate errors, refraction induced errors, atmospheric

induced errors, wind induced errors, and mount/telescope induced errors. Although the

purpose of guiding is to remove errors, it can also add errors by just the act of guiding,

such as sensitivity to telescope vibration, and the interaction with guiding exposure rate,

atmosphere fluctuations, and mount periodical tracking errors. There are two methods,

manual and automated.

• Manual Guiding

• Consists of using a crosshair reticule as an eyepiece in either the finderscope,

telescope or off-axis guider that is attached between telescope and camera. The

objective is to maintain the guide object in the center of the crosshair by using

mount hand controller.

Note: picture of manufacture product is for reference only and does not

recommend a specific product

Guiding

• Auto Guiding

• Auto guiding is performed by the computer using guiding software and a guiding

camera attached piggy back to the telescope or in-line with the imaging camera and

telescope. The use of a guiding camera attached piggy back is best when used with

a short focal length telescope. For long focal length telescopes, the in-line guide

camera is best with the use of an off-axis guider, inline-axis guider, or a imaging

camera that has a build-in guiding port or sensor.

• There are numerous Guiding software available, such as Maxim DL, PHD,

CCDSoft and MetaGuide.

Note: picture of manufacture product is for reference only and

does not recommend a specific product

Guiding Port on-axis guider off-axis guider

Guiding - Auto

• Auto guiding has a sensitivity to guiding exposure rate, atmosphere fluctuations, and

mount periodical tracking errors. The atmosphere can cause frequency fluctuations of

the guiding star image in the order of less than 1 hz. As long as the guide image

exposures are several times longer than 1 sec (3 or 4 sec is better than 2 sec), the ability

of the guider to track the guiding star image is very good. The longer exposure averages

the star fluctuations over a longer time period and therefore the mount does not try to

correct the much higher frequency fluctuations. However, with sever periodic error in

the mount, usually due to low quality gears or hard grease balls in the gears, guide

exposures of 2-4 sec may not be possible. This can cause the stars in the image to be

elongated. To prevent this, the mount should be of good quality, which usually means a

higher price. Another option, may be to use multiple star guiding that is available in

Maxim DL (not known if other software offer the same capability). Or if using a

separate guide camera, adding a focal reducer to brighten up the guide stars has shown to

increase guide rates from 1 to 2 seconds.

Sample High End Telescope Mounts

Note: picture of manufacture product is for reference only and

does not recommend a specific product

Camera

• Depending on the setup the imaging camera can be a webcam, DSLR/SLR, or

astrophotography CCD. Although a smartphone, or point and shoot camera can also be

used by taking a picture through the eyepiece. The difference between the DSLR/SLR

and the astrophotography CCD, is the CCD camera can be cooled below the ambient

outside temperature which reduces the noise and allows dimmer objects to be exposed

with the same exposure duration.

• A DSLR/SLR has interchangeable lens that changes the focal length which changes the

field-of-view and magnification. Usually with this configuration the camera is piggy

back on a mount or telescope. Using low focal length lens or fisheye lens provides large

field-of-view to capture constellations, aurora borealis, meteor showers, polar star trails,

dark sky with lighted foreground or wide view of Star, planet, moon conjunctions.

• Using an astrophotography CCD or DSLR/SLR without a lens on a telescopes fixes the

field-of-view and magnification. Therefore depending on the image or object to be

capture, the object size needs to be matched to the telescope field-of-view or telescope

focal length (see how to compute field-of-view with camera and telescope topic)

Note: picture of manufacture product is for reference only and does not recommend a specific product

Camera

• The webcam can be used for astrophotography, but only in unique situations as the

equipment and image processing is different than other cameras. Since the webcam

captures images at 10-60Hz or higher, a computer is needed to record the images

and the image must be bright since each exposure is 0.1 to 0.017 seconds in

duration. With the exposures so short, the images are susceptible to atmospheric

fluctuations and therefore a lot of images must be taken to capture the best images

to combine. The video images are combined by tools like “RegiStak” or “PIPP” to

produce a single image.

• Astrophotography CCD cameras come in two types; monochrome or one shot

color.

• Monochrome cameras require separate exposures of a sky object through Red,

Green and Blue (RGB) filters to obtain a color image. Usually a fourth

exposure, called luminance, is also obtained using a clear filter. These four

exposures when combined by processing produce a astrophotography picture.

To simplify the exposures using multi-filters, a filter wheel can be added to the

camera.

Note: picture of manufacture product is for reference only and does not recommend

a specific product

Camera

• One shot color cameras are similar to a DSLR in that only one exposure is needed to

create a color image. However, to achieve this requires four side-by-side pixels to be

combined, each with a different color filter sitting over the pixel (called a Bayer matrix).

This technique to obtain color images comes at some loss of resolution in exchange for

the ability to capture color in one exposure.

• Astrophotography one-shot color camera require longer exposures to obtain good color

images than required by a monochrome camera shooting with four filters. The advantage

with one-shot color cameras is the total image time is substantially less than using a

monochrome camera and having to image through four filters.

• Remember to capture the images in “RAW” format when using DSLR or CCD cameras.

The “RAW” format is best if image processing is to performed.

RGGB BGGR GBRG GRBG

Scripting

• Scripting is the process of automating the steps in operating your telescope and camera in

taking astrophotographs. There are several computer programs available that automate

the steps of focusing, taking and saving pictures, guiding, roof control, camera on/off,

slewing to objects and determining where the camera is pointed. Scripting programs

include, but are not limited to CCDCommander and CCDAutoPilot. There are also

methods to capture DSLR photographs through an intervalometer, tablets, smart phones

and computers.

Image Processing

• Imaging processing includes capturing images, image calibration and image

processing. Generally, image capture is not included as image processing,

however, it is here as the image capture process is directly related to image

calibration.

•Capturing images consists of taking one or multiple exposures (subs) of a

fixed length of a single sky object. Images should be saved as “RAW”.

Most DSLR cameras have this capability. For astrophotography cameras

save as a “FITS” file.

• Image Calibration consists of taking one or multiple dark, bias and flat

frames to calibrate the camera/filter/telescope to improve image quality. See

following pages for definitions.

• Image processing consists of aligning sub images, combine or stack subs ,

and photo processing the combined image to obtain an image that can be

seen in detail with appropriate brightness, contrast, range of grayscale, and

(if applicable) color balance. Image processing computer programs, such as

photoshop or PixInsight can be used. Also software experts such as Richard

Berry, Bruce Johnston, Michael Newberry, Douglas George, and Christian

Buil (and others) have all written full-range programs which may be capable

of reading and manipulating the raw images from your camera.