1

TECHNIQUES FOR IMAGING VERY FAINT OBJECTS

NO MAGIC, JUST DO EVERYTHING RIGHT

(or at least try to!)

by Paul Boltwood

ITS November 8, 2002 Salem Oregon

This talk is on the conference CDR

Copyright (C) 1996-2002 Paul Boltwood

Slightly updated January 15, 2006

2

SKY & TELESCOPE DEEP FIELD CONTEST

• one of the reasons I was invited here was that I won this contest with an image that reached mag. 24.1

• see Bradley Schaefer’s articles S&T May 1998 and May 1999

• I should not have won because I had:

• suburban skies

• a CCD with 40% peak quantum efficiency

• 16" aperture

• done a crude reduction

• many amateurs surpass me in all of these departments

• as soon as I heard that I won, I re-reduced the image carefully and reached mag. 24.5 – but too late for S&T

• in this talk I will tell you how I did this

3

PHILOSOPHY BEHIND THE TECHNIQUES

• when I started in 1989, I wanted to do some publishable science and I ended up doing photometry of blazars, ~100,000 images

• having my data accepted by scientists was my motivating force and my strategy was:

“try to do everything right”

• most professionals in North America assume that an amateur is incompetent until proven otherwise

• when I design something, I try to learn the theory behind my equipment & techniques and be very thorough

4

WHY IS SO MUCH HOMEMADE?

• during this talk you will see that most of my equipment and software is homemade. Everything in this talk that does not have a brand name, I have designed, and either built or had built.

• I like designing and building this stuff, and I know how to do it

• I started before much of today’s equipment was available

• to keep costs down, I used my labour rather than money

• for the scientific work especially, I needed to know that everything works properly and in a known way. Equipment manufactured for amateurs is deficient in this regard:

• optical performance data is not available, esp. non-visual

• software source code is not available

• mechanical performance is usually inadequate

• database handling is not provided

• wrong approach taken for scientific work

5

WHY DO I HAVE AN OBSERVATORY?

• allows use of superior, non-portable, equipment

• avoids losing alignment and collimation

• has electrical power

• observing in the Frozen North near Ottawa, Canada makes an insulated heated equipment room especially desirable:

• makes long exposures feasible

• allows use of long winter nights even at -35°C

• protects computers and electronics

• stores books and accessories

• allows money earning activities while waiting for exposures

6

BOLTWOOD OBSERVATORY (1)

In The Far Backyard At Home

7

BOLTWOOD OBSERVATORY (2)

Roof Rolled Off To The South

8

BOLTWOOD OBSERVATORY (3)

Telescope Room, Old Homemade Camera

9

BOLTWOOD OBSERVATORY (4)

Telescope Room, AP7p Camera

• telescope room is cool in the sun because double skinned and ventilated, including between skins. Thermal design by Rob Dick

• biggest fault is no dome. Cannot observe if windy

• walls are black because original use was visual with a refractor. Roll-off roof ceiling is white to provide light while working on equipment

• convenient access more important than dark skies for my work so I located at home

10

MOUNT• Byers Series 2 (sort of), with

modifications:

• backlash removal

• stiffened DEC drive (15* better than Byers)

• stiffened mounting plate

• overloaded with 180 lb optical tube assembly

• tracking error in 2 minute exposure is imperceptible

• periodic error correction not used or needed

• no guiding either

• stepping motors limited to 15 times siderial rate

11

OPTICAL TUBE ASSEMBLY (1)

• focus flap motors, no longer used

• rotating secondary, 4 ports

• eyepiece focuser

• S.S. angle stiffeners epoxied on

• 11*80 finder

• square wooden tube:

• allows home carpentry

• supports extensive baffling

• corners allow tube currents to escape away from light path

• easy to mount items on

• corner spaces are useful

• allowed a demountable primary cell

12

OPTICAL TUBE ASSEMBLY (2)

Primary Cell – Designed And Built By Max Stuart

collimation adj. using a rod end

13

OPTICAL TUBE ASSEMBLY (3)

Primary Mirror Support

holes for rod ends

14

OPTICAL TUBE ASSEMBLY (4)

Spider Assembly And Baffling

15

BAFFLING

• no non-imaging light should get to the focal plane or intermediate optical surfaces

• optics often have light-scattering dust on them

• baffles at right angles to the light are the primary defence. Black paint is a secondary defence

• unbaffled focusers, extension tubes, cameras are major sources of flare

• you should be able to work near the moon as long as it is not actually shining on your primary

• you should be able to turn on the observatory lights with little impact while CCD observing

16

OPTICS

• 16" f/4.7 primary on Astro Sitall zero expansion substrate by Peter Ceravolo

• 2 times better than diffraction limit

• zero expansion substrate replaced a previous pyrex primary for a several times reduction in focus drift

• 3.1" Newtonian secondary by Galaxy, RMS .016 wave

• Televue 2" Big Barlow normally used

• Televue Paracorr used for wide field

• Televue 5X Powermate intended for narrow field but not fully tested

• UBVRI filters for photometry by Omega, RGB filters for astrophotography by Custom Scientific

17

OPTICS PROBLEMS

• amateur optics don't cover CCD spectral extremes

• refractive optics suffer from chromatic aberrations, and filters may pass unexpected UV or IR. Do not use photographic filters

• wish I had a Ritchey-Chretien Cassegrain matched to camera

• before buying your 4096*4096 CCD, check your optical aberrations in the corners, and alignment and focus capability. This stuff is hard enough at 512*512

• before using multicoated optics, check what happens at the spectral extremes

• design such that all optical surfaces are far from focus. Then dust will not affect your flat fields very much

• black anodizing is clear in the near IR, so paint it

• I use Krylon Ultra Flat Black spray

18

FOCUSER & CAMERA ASSEMBLY

• anti-backlash

• umbilical

• focuser frame

• focus sled

• optics tube

• collet

• AP7p camera

• filter wheel

• fan for 3°C cooler CCD

• tarp on ceiling

19

FOCUSER

• 5.5 inch range to allow for magnifying optics

• stepping motor and limit switches

• linear bearings

• the camera is always at same rotation on sky

20

EXPLODED CAMERA ASSEMBLY

magnifiers with buttress thread baffling

Collet

filter wheel

Apogee AP7p

extra filter holder

camera clamps

filter disk motor

21

FILTER WHEEL

• 10 positions for 1" filters

• microstepped stepping motor

• wheel is carefully balanced

• 2 optical switches for positioning wheel:

• one with 1 hole, other with 10 holes

• switches are checked to be sure that the correct filter is in place at each move

• AP7p not fastened in the normal way – clamps are used

• AP7p penetrates into face of housing

• filter is very close to housing

• permits the use of 1" filters – 2" is normal

• note the baffle to correct shiny ring in the camera

22

CCD CAMERA

• current camera Apogee AP7p with SiTe 502A chip

• has blue sensitivity and 80% peak quantum efficiency, more thermal noise and hot pixels, poor temperature control, 512*508 pixels

• old homemade camera with Thomson TH7883 chip in vacuum, liquid cooled, chip at -72°C, low thermal noise and very few hot pixels

• has little blue sensitivity, 40% peak quantum efficiency, excellent flatness, good temperature control, 382*574 pixels. Used for S&T Deep Field

23

CALIBRATION FLAT FIELD LIGHT SOURCE• purpose is to flood telescope entrance

aperture to simulate a sky background:

• puts diffuser across full aperture and in contact. Need exactly the same light path as for sky background

• diffuser needs to be evenly lit

• spectra to match night sky (not likely!)

• four 35W tungsten halogen lamps with blue-green and IR cut filters on each

• regulated power required

• inside covered with crumpled Al foil

• clips onto front of OTA, 40"*40"*11.5"

• 4% variation across diffuser - best I could do in an 11.5" thick package

24

BOLTWOOD OBSERVATORY (5)

Equipment Room

25

ELECTRONICS (1)• power and switch panel

• rack of microprocessor boards

• flat fielder power supply

• modular

• use Microchip PIC microcontrollers, one per function

• complex stepping motor control firmware in 4 of them

• all time critical functions are done in the microcontrollers

• almost electronics all are in equipment room to ease repairs and reduce thermal wear & tear

26

ELECTRONICS (2)

• control of telescope fans & heaters, button box sound, temp. readouts

• RA & DEC stepping motor drives, button box

• filter wheel & focuser motor drives

• serial backplane controller with parallel port interface to PC

27

ELECTRONICS (3)

Rack Backplane For Microcontroller Boards

28

COMPUTER AND SOFTWARE SUMMARY

• observatory computer is a Pentium II 333 MHz, 256 MB RAM, 60 GB hard drive

• 17" 1600*1200 display

• software:

• Windows 2000 because it allows simultaneous star mapping, data base, observing, reduction, and non-astronomical work

• telescope control

• image data base

• reduction

• MaxIm

• star mapping:

• Guide 8.0 with A2.0 catalog to mag 20 - 526,280,881 stars and a million galaxies

• DSS available at house over web

29

TELESCOPE CONTROL SOFTWARE• VB and C++ software

• position & exposure info. usually from “.aux” files or script

• uses MaxIm for camera driver, auto- focus, mount jogger, image display

• corrects for refraction and flexure for position and focus

• corrects for temperature for focus

• optionally runs scripts in my own language

• images are rotated to have north at the top immediately upon readout

• often not attended for 90 minutes

• normal exposure is 2 minutes with many frames merged in reduction

30

DATA BASE

• has index to 120,000 images, all FITS with extensive headers

• allows semi-automatic data reduction

• observations are grouped by “group files” to aid reduction

• observing and reduction software updates data base automatically

• user interface package:

• searches data base in several ways

• uses MaxIm to display images

• allows display and editing of FITS headers (singly or in bulk)

• allows display and editing of “group files”

• VB and C++ software

31

DATA BASE USER INTERFACE

32

CALIBRATION FRAMES

• I do 16 frames each of bias, deferred charge and flat fields which are then averaged to reduce noise

• deferred charge frame was used for my old homemade TH7883 camera, not yet for AP7p:

• was several times more important than bias frame

• added to image to compensate for trapped electrons

• 10+ hours of 1000 sec. dark frames for each CCD temperature:

• calibrated and summed to form thermal frame

• prorated by exposure time when used – assumes good camera

• quality is dependant upon camera temperature regulation

• normal image reduction procedures are used to create master calibration frames from raw calibration frames

• bad pixels in master calibration frames are marked and not used in image reduction

33

REDUCTION SOFTWARE (1)

• produces master calibration, astrophoto and photometric reductions

• runs scripts

• all pixel computations are 32 bit floating point, 16 bits inadequate

• reduced images have a large dynamic range due to merging

• C++ software

34

REDUCTION SOFTWARE (2)

• automatic, driven by:

• "group" file listing image files and FITS headers

• control file giving star, sky, and exclusion zone locations

• merges images interpolating between pixels to correct for translation, scale, and rotation

• cosmic rays removed from calibration and image frames during:

• overscan averaging

• raw frame calibration

• frame merging

• bad pixels (due to many sources) are marked and avoided – merge is corrected for these missing pixels

• variance frames are maintained (primarily for photometry)

35

REDUCTION PROCEDURE SUMMARY

• manually delete bad images from the group file

• reduction software then:

• calibrates each raw image pixel by pixel

• measures each calibrated image

• merges calibrated images based on those measurements into buffers. Each image is multiplied by a weight that is larger for better images

• completes the merge

• optionally flatten the image to correct errors in flat field calibration

• optional MaxIm deconvolution and other fiddling

• will use S&T Deep Field image as the example in what follows

36

CALIBRATE EACH RAW IMAGE

• scan lines have 32 overscan pixels beyond the real pixels

• average these along the line

• subtract overscan from each pixel to remove certain camera problems

• apply bias, deferred charge, thermal, and flat master calibration frames

• for each pixel:

cal = (raw-overscan- bias+defchg-(prorated_therm))/flat

• bad pixels are marked and not usedS&T Deep Field Raw Frame

37

MEASURE EACH CALIBRATED IMAGE

• using pattern matching, locate the "key" star

• measure:

• sky background

• centers for each of 2 "locating" stars

• shape of "locating" stars

• reject any calibrated image where "location" star elongation is too large

• estimate the variance of a faint object in this image:

est_var = sky_var * exp(key_mag - key_min_mag) * (fwhm*fwhm) is proportional to it

• weight for frame when merging is 1/est_var

• compute translation, scale, and rotation required for registration

• compute weighted sky estimate and add to sum

38

MERGE EACH CALIBRATED IMAGE

• merge this calibrated image into 12 merging buffers

• 12 needed to handle variance, weight, and cosmic ray removal later

• bad pixels are skipped

• subpixel merge into the image buffers using bilinear interpolation

• subtract sky from each pixel because it is so dominant

• weight each pixel by 1/(est_var)

• add weight to the sum_of_weights buffer for each pixel

• for each pixel location, remember max and next_to_max value seen in buffers for cosmic ray removal later

39

COMPLETE MERGE

• remove cosmic rays. For a pixel:

• use merged image value excluding max and next_to_max values

• statistically decide whether max is a cosmic ray, ditto next_to_max

• if cosmic ray, remove from sum and sum_of_weights buffers

• compute for frame: sky = weighted_sky_sum / sum_of_sky_weights

• compute for each pixel: pixel = (weighted_sum / sum_of_weights) + skyS&T Deep Field Merged

40

FLATTENING THE IMAGE

• why isn't it flat now?

• I did flat field every raw frame but there is an extreme sensitivity to flat field errors due to light pollution

• range of image is 1.444e6 to 1.459e6 photons/pixel, just 1%

• image is 99% light pollution

• sky flatness failure is 6000 photons/pixel, just .4%, but that is 40% of the total image range

• flattening method:

• separate sky from stars and other objects

• examine sky near each final image pixel to get a sky estimate for that pixel

• for each pixel subtract off local sky estimate, add on overall sky average

• only works with small objects, and fails with nebulosity

41

S&T DEEP FIELD FINISHED IMAGE

Mag. 24.5 Objects Have SNR Of 3 According To Bradley Schaefer

42

FAINT TARGET URBAN CCD PROBLEM

• flattening the sky background is the #1 problem

• any spectrally independent lack of flatness in the CCD chip does not matter as long as your master calibration frame is good (this does require care)

• what matters is any spectrally dependant lack of flatness

• spectral variation of each pixel’s sensitivity creates the problem because sky background, target, and the flat field light source all have a different spectral content

• back illuminated chips are especially bad due to interference effects

43

CCD FLATNESS COMPARISON

• Thomson CSF TH7883 and SiTe 502A were compared

• used master flat calibration frames for B, V, R, and I photometric filters where the average pixel value was 1.

• measured standard deviations of flat differences between filters:

TH7883 SiTe 502A

B-V 0.0144

V-R 0.0027 0.0052

R-I 0.0058 0.0168• the TH7883 should be able, ultimately, to go substantially deeper,

but with much longer exposures.

44

SPECIAL URBAN REQUIREMENTS

• high flat field quality

• proper baffling

• perhaps filtering for light pollution. I have not tried this except to use a photometric I filter to darken sky

• my skies are suburban at <19.3 mag./sq.arcsec (dark is 22) so my advice may not be entirely appropriate

• the ability with the CCD to subtract the sky makes the big urban difference in comparison with film or eyesight

• unfortunately, due to higher sky noise and flat fielding failure, an urban site still cannot match a dark site

45

ASTROPHOTO SUGGESTIONS

• when doing astrophotos, avoid the hackneyed objects, or at least do some different view of them

• pick suitable targets:

• that fit the chip well

• are overhead at the middle of the observing session

• do long exposures

• get in close

46

EXAMPLE OF CLOSE IN - CORE OF M31 (1)

• usual amateur picture covers 3 degrees and the center 15 arcmin is burned out

• this has .47 arcsec pixels, 3.0*4.5 arcmin

• 7" refractor, homemade CCD camera, no filter

• 54 min. exposure

• some interesting dark patches on the left

47

EXAMPLE OF CLOSE IN - CORE OF M31 (2)

• same image, different stretch

• center of the reduced image was not saturated and this shows that M31 has a very bright point-like center - not evident in most photos

48

EXAMPLE OF CLOSE IN - CORE OF M31 (3)

• same image

• unsharp masked in MaxIm (but the official way failed)

• mask made using low pass FFT filter with 2.5% cutoff and 100% weight

• image - mask + 10000 using pixel math

• 10000 added because MaxIm does not understand negative numbers

49

EXAMPLE OF CLOSE IN - CORE OF M31 (4)

• deconvolved using MaxIm

• in this image the background is not the sky – it is the star clouds of M31

50

EXAMPLE OF A DIFFERENT VIEW - NGC 206 IN M31

• I R V photometric filtered images rendered as R G B

• faintest star visible limited by confusion - more resolution is needed to do better, not more exposure

• pixel size 1.09 arcsec

• exposures: I 348 min.

R 562 min.

V 434 min.• 16" Newtonian, homemade

CCD camera

• weighted merge was tuned to enhance sharpness

51

ALIGNMENT FOR GERMAN EQUATORIALS (1)

• I use Project Pluto's Guide for map of the polar region

• mark refracted pole location

• do rough alignment some other way

• have RA drive on

• use low CCD magnification

• aim telescope at pole with telescope on one side of pier

• start 1 min. exposure

• after 30 sec. pull counterweight shaft around 180 degrees slowly in 30 sec.

52

ALIGNMENT FOR GERMAN EQUATORIALS (2)

Center of half circles is where the polar axis is pointing. Adjust mount elevation and azimuth until correct. Sorry – these pictures are not mates

Shim OTA on saddle and adjust DEC to center the half circles. Set DEC circle to 90°

53

ALIGNMENT FOR GERMAN EQUATORIALS (3)

Doing Precision Adjustment Of Elevation And Azimuth

54

OPTICAL ALIGNMENT TOOL• fits into vane holder in

place of secondary to first align the spider

• fits into eyepiece focusers 1.25" & 2"

• fits into collet on front of filter wheel

• HeNe for 1/2 sized spot

• aluminized center dot on primary with clear donut around it instead of a gummed reinforcement. Gum will streak when washing mirror with alcohol

Homemade HeNe Laser Collimator

55

TECHNIQUES FOR IMAGING VERY FAINT OBJECTS

Paul Boltwood, paul@boltwood.ca

1655 Stittsville Main St., Stittsville, Ont.,

Canada K2S 1N6

(613) 836-6462

More at ottawa.rasc.ca under the Astronomy button.

A more technical talk on the S&T Deep Field image given at Starfest 2000 is on the CDR and the web site. The techniques in it are somewhat different.

56