Astrophotography

 

HIGH RESOULUTION ASTROPHOTOGRAPHY

 

FIVE ESSENTIAL REQUIREMENTS

1) EXCELLENT SEEING

2) EXCELLENT OPTICS

3) PERFECT FOCUS

4) ACCURATE GUIDING

5) PROPER FILM AND f/RATIOS

  


Seeing

The steadiness of the air, is undoubtedly the most important single factor in astronomical photography. Atmosphere clarity becomes of paramount importance when long exposure photography of faint objects is done. Apart from such obvious drawbacks is a naturally cloudy climate, the worst enemy of atmospheric transparency is the proximity of large cities, which not only pollutes the atmosphere but also illuminate the pollution. Artificial pollution, is always

worst in winter, and during the late afternoons at all times of the year.

AS A RULE OF THUMB, IF FINE DETAIL APPROACHING THE LIMIT OF RESOLUTION OF THE TELESCOPE CANNOT BE SEEN VISUALLY, IT WILL NOT BE PHOTOGRAPHED EITHER.


Excellent Optics

Obviously, the telescope should be of the highest optical quality a defect in the mirror or lens will spread light that blurs fine detail, reducing image contrast.


Perfect Focus

Particularly when eyepiece projection is used, focusing must be critical. The ground glass screen should be replaced with one designed of astro photography.

TWO TYPES

1) CROSS-HAIR PATTERN SCREEN

2) SEMI-FROSTED SCREEN

 These focusing screens are far superior to the ground glass screens that come standard on most cameras.


Testing the Accuracy of a Telescope Drive

You can test the accuracy of your telescope drive by placing an object of known angular size (i.e. a planet or double star) on the crosshair of an eyepiece and then adjust the drive rate of the telescope so that the object position is relatively stable. Then watch the distance the star moves in the eyepiece and estimate the magnitude of drift. You must first know the field of view of your telescope to use this test. A star on or near the celestial equator moves westward at the rate of fifteen degrees per hour. By timing the passage of the star across the field of view, you can determine the angular field of your telescope.

EXAMPLE

If the star takes 4 minutes and 30 seconds or 4.5 minutes to drift across your eyepiece you would multiply the time in minutes by 15 to get the field in minutes of arc.

4.5 minutes * 15 = 67.5"

or

67.5" * 60 = 4050'

This test will give you a close calculation of your telescope drive's gear-worm error.

If you wish to perform this same test photographically, you will get a much better idea of the quality of the drive. You can actually measure the magnitude of the errors accurately and track down the source of the errors to a particular gear. To perform this test you should purposely offset the telescope polar axis about 5 degrees in azimuth from the true pole. Then find a field of faint stars near the equator and set the clock drive just a bit slow and begin an exposure. The exposure should be long enough to allow the worm to make 2 or 3 revolutions

(20 to 30 minutes is usually an adequate exposure time). You should not attempt to guide the telescope during this exposure. Be sure that you record the exact length of the exposure time for calculations.

When the film is processed, you will find a series of parallel wiggly lines. The regular wavy pattern is the periodic error while the tiny bumps that are superimposed on those wavy lines are the erratic errors. If the total exposure time was E, and the actual length of a star trail is A, and the length of the periodic portion of that trail is P (i.e. the distance from peak to peak on the star trail), then the true period of the periodic error is given by ( P/A*E ). The period of this error will almost always be found to equal the period of the worm gear.

The magnitude of the error can be found by measuring the height of the sinuous trail and then calculating the image scale of the telescope using the formula:

S = 135/f(in)

Do not be surprised to find that the periodic and erratic errors found in a standard inexpensive drive system to exceed several minutes of arc.


Precise Polar Alignment

This alignment method is desirable only if you intend to make long exposure, guided astrophotographs. The advantages are, no image drift in declination and no star trailing caused by field rotation. It eliminates the need to make corrections in declination during exposures and allows the operator to concentrate on Right Ascension corrections.

After a quick alignment of the telescope, insert an illuminated reticle eyepiece into your telescope (a barlow lens will help speed up this procedure considerably) and point the telescope at a fairly bright star near where the meridian and the celestial equator intersect, preferably within +/- 1/2 hour R.A. of the meridian and +/- 5 degrees of the celestial equator). Position

the reticle so when the telescope is moved in R.A. the star will move parallel to a crosshair. Monitor the declination drift (ignore any drift in R.A.).

a) If the star drifts south, the polar axis points too far east

b) If the star drifts north, the polar axis points too far west

Move the telescope's polar axis in the appropriate direction until the north or south drift stops. Accuracy of this adjustment will be increased if you use the highest possible magnification and allow the telescope to track for a period of time.

Now repoint the telescope at a fairly bright star near the eastern horizon and near the celestial equator. The star should be at least 20 degrees above the horizon and +/- 5 degrees from the celestial equator).

a) If the star drifts south, the polar axis points too low

b) If the star drifts north, the polar axis points too high

Again monitor only declination drifts. After you have made the necessary adjustments to stop the declination drift, you will have achieved a highly accurate polar alignment.


Proper Film and f/Ratio Combinations

The film must be capable of resolving all the detail a telescope can show. It should therefore have a theoretical resolution about three times that of the telescope. (linear resolution at the film plane, which is a function of the equivalent f/ratio of the total optical system )

In celestial photography resolving power is a product of the Focal length of the system and not by the aperture. Resolution of fine detail on the negative depends on the product of the telescope's focal length (F) and the emulsions resolving power (R). Convert the focal length in inches to meters and use films high contrast resolution in lines/mm for(R).

EXAMPLE

Focal length of Celestron 8:      80 inches
2415's resolution in lines/mm:   320

Meters = 80 / 39.37

FR = Meters * 320

The FR value for the Celestron 8 is 7O4.

A FR value of 700 is great for average seeing conditions. With lower values, details in the negative suffer more from grain and not seeing. Values >700 would be needed to take advantage of superb seeing conditions.


HYPERSENSITIZATION OF RED AND INFRARED EMULSIONS

TREATMENT TIME (min)
TREATED
FOG INCREASE
TREATED
GAIN FACTOR
Water   6 0.04 10X
Water, vacuum,
Hydrogen Soak 
6 0.05 15X
Ammonia, 6% 6 0.25 30X

Silver Nitrate
     
2 * 10-4 6 0.05 27X
4 * 10-4 M 6 0.20 48X
8 * l0-4 M 4 0.10 48X
1.2 * 10-3 M 4 0.05 55X
M = Molarity      

 

HYPERSENSITIZATION OF FILM USING AMMONIA LIQUID

 

AMMONIA FORMULA

1) Ammonia (0.91 Sp. Gr.)............. 2cc

2) Alcohol .................................. 275cc

3) Distilled Water ....................... 725cc

 

 *** THIS PROCESS MUST BE DONE IN TOTAL DARKNESS ***

Soak the film to be hypersensitized using the ammonia formula above for 2 minutes. Do not rinse the film in water after soaking. Dry as quickly as possible after removing surface liquid. Be careful not to scratch the emulsion when wiping. Do not touch emulsion side of film with fingers.

SPEED GAINS USING THIS PROCESS

lX to 4X on Normal Films

5X to 25X on Infrared Films

 

 

HYPERSENSITIZATION OF TP 2415 BY BATHING IN SILVER NITRATE

Hypersensitization of Kodak Technical Pan 2415 film can be accomplished by bathing for 3 minutes in silver nitrate solution. The sensitization results in a speed gain of 15X to 25X.

Treatment

*** THE FOLLOWING MUST BE DONE IN TOTAL DARKNESS ***

1) Load unexposed 2415 film onto a dry developing reel.

2) Immerse the reel in An 8.3*10-4 molar solution of AgNO3 and distilled water.
   Agitated gently for 3 minutes at 20 C.

3) Pour off the AgNO3 solution and discard.

4) Rinse the film in distilled water at 20 C with vigorous agitation.

5) Pour off water and use a second rinse of 95% isopropyl alcohol for 2 minutes.
   This alcohol rinse removes most of the residual water from the film and
   Hardens the emulsion.

6) Pour off alcohol and save for re-use. Tap the reel containing the film
   sharply on a towel to dislodge the remaining droplets of alcohol.

7) Hang the film vertically in a stream of cool air from a fan for 5 to 10
   minutes to complete the drying process.

8) The film can then be loaded into a 35mm cassette or place in a film holder,
   if sheet film is used.

NOTE: AgNO3 treated film must be exposed immediately. The film cannot be stored, a few hours
of storage in a deep freeze results in a completely fogged negative of ND = 5.0. The film must be processed immediately, like 2415, AgNO3 film is highly touch sensitive so handle it with care. 

Results

1) Speed gains of 15X to 25X

2) Contrast index, grain structure, and developing times are unchanged by the
   hypersensitization process.


3) The background chemical fog is about ND = .014 above the normal ND = 0.1
   density of the ESTAR-AH base.

***IMPORTANT NOTE ***

UNIFORM FILM DRYING REMAINS A PROBLEM FOR THIS TECHNIQUE.