So you want to jump into prime focus astrohotography eh? Well here is a simple technique to let you know if you are focussing your camera be it a SLR or a CMOS imager, be it a Meade DSI, a toucam or even a fancy SBIG camera

The technique I will describe is the Hartmann mask. A very simple device built out of nothing but cardboard

A hartmann mask is is simple sheet of cardboard with multiple cut holes on the face. This card is mounted on the objective of your scope be it a Newtonian, or catadioptric variety.

TO use the mask, mount it on to the objective and point it either to a star or a very distant lamp post…. Connect the

When the scope is not in focus you see diffraction patterns arising from each of the holes. As you get the target object in focus, the spots move in to each other yielding a single point image of the star.

When that happens, congratulations, your camera is in focus

This page is meant for amateurs who want to take astrophotographs with a small telescope and a camera. Here is a rough field guide to determining the kind of exposure you will need to capture the target object

The first thing you need to know is the concept of a stop ratio or the f – number. Yes these are those funny looking numbers you might have seen on the lens of your camera… they look something like
f/1.4, 2, 2.4, 4, 5.6 etc

Officially speaking the f – number is the ratio of the focal length to the diameter of the optical system. SO, before we do anything, this is an important piece of information you need to know before you go out in the field.

To determine the f number (f/#) of your system, you need to know what kind of optical system is going to be used. Astronomers typically use two methods for photography. The more easier method is called the AFOCAL PROJECTION method, and the more advanced method is called the PRIME FOCUS method. Most professionals with high end gear prefer to use the prime focus method due to the superior image quality produced by it.

AFOCAL projection is much easier, requires simpler & cheaper equipment and most amateurs begin with this method, although it has more optical components compared to the prime focal method

Determining the F-number

If you’re using Afocal projection method, your effective stop ratio is influenced by two optical components. The first being the optical tube and the other being the camera

To calculate the effective f/# of your afocal system

f/N ‘ = (f/N) * Camera FL/Eyepiece FL

So lets say we have a 114 mm f/11 Celestron scope with a 25 mm eyepiece and a 50 mm Pentax SLR at the eyepiece

We will see an effective f/# of (f/11)*(50 mm / 25 mm) ~ f/5.5

Hence you can see afocal projection makes the system ‘faster’ than before

In the prime focus projection system, there are no intemediary lens systems. The focussed wavefront of the incoming starlight is made to fall directly onto the film or CCD/CMOS sensor

So there are no calculations involved here. If I choose to have a prime focal setup for the same Celestron 114 scope, I will have the same stop ratio as the tube… i.e. f/11

Exposures and Stop Ratios

So what is all the relationship between stop ratios and exposure times.

If you notice the f/# on your cameras, they are each successive number is 1.414 ( which is square root of 2) times bigger than its previous one. DECREASING the stop ratio by one step i.e 1.414 times allows twice the light exposing the detector

So a system with f/4 is twice as fast as a f/5.6 system, requiring only half the exposure time as the latter

Note that modern cameras have stop ratios in half-steps. What that means is that between a f/4 and f/5.6 there will be a median value of f/4.8 also.

Courtesy: Wikipedia

This information is handy while trying to extrapolate exposure information of an object using different optical equipment. You might want to take a picture of Andromeda galaxy using your setup but the only exposure information may be from somebody else’s page

Absolute Exposure Scales for different objects.

Here is a crib sheet on guessing the right ball-park exposure settings for your imaging project. Choose an appropriate ISO and a stop ratio you are using, and look up the exposure times for the appropriate target

ISO 100	f/1.4	f/2.8	f/5.6	f/11
ISO 400	f/2.8	f/5.6	f/11	f/22
DEEP SKY OBJECTS, time given in minutes
Open and Glob. Clusters	10	20	40	80
Bright Nebulae	15	60	4h	—
Emission DSOs	22	90	—	—
SOLAR SYSTEM OBJECTS, time given in seconds>
Jupiter with Moons	0.5	1	2	4
Saturn	1/8	1/4	1/2	1
Naked Eye Comet Nucleus	6m	25m	100m	—
Sun w/ Filter	1/4000	1/1000	1/250	1/
Total Solar Eclipse – Corona	—	1/4000	1/1000	1/250
Moon 50% Phase	1/4000	1/1000	1/250	1/
Moon Full Phase	—	1/2000	1/500	1/125

* Double the exposure if object is only 8-14 degrees above horizon.
* Quadruple (4x) exposure if 5-8 degrees high, or for thin clouds.
* If ISO is higher, decrease exposure time by same proportion, i.e. if ISO is doubled (or if film is hypersensitized) reduce exposure time by 1/2. ( God people take so much pain with film!)
* Keep a record of your own exposure data, it will save time energy & film
* Use the f/11 exposure data if you are using an f/10 SCT.

The data presented above is (c) 1991, 1997, Jeffrey R. Charles

Happy Imaging
Vidur

An AAAD & TWAN Presentation

Image Courtesy Dave Jurasevich Astro-Physics 160 mm refractor @ f/5.7 or 912 mm focal length 12 ea x 1200 sec exposures combined
Imaged through a Tru-Balance 6nm Hydrogen Alpha filter with STL-11000M camera
Who says that the domain of new discoveries is only possible if you are at the helm of the Hubble or the Spitzer

Amateur astronomer Dave Jurasevich recently discovered a bubble like nebula in Cygnus, and submitted the discovery to IAU on July 17th 2008 after noting that the object was not listed on any chart.

Only a few days later the nebula was independently discovered again Keith B Quattrocchi and Mel Helm while photographing the Crescent Nebula from a robotic observatory in the Sierra Nevada mountains.

For the enthusiast
The object is located at RA: 20 15 22.16 and Dec +38 02 41.9
currently the object is still awaiting IAU designation of a name and a discoverer