Focussing for Prime Focus Photography- The Hartmann Mask

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

Astrophotography Exposure Guide

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

An AAAD & TWAN Presentation

New Nebula Discovered by Amateur Astronomer

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

Season Opens for Stargazing

Hooray, Summer seems to have finally dwindled away, and soon there will be no more dust, clouds over Delhi skies. Winter is almost here, and its time to take out those dust covers off your scopes because there is plenty of stuff to watch for and too little time

Winter is probably the best time for Delhites and Bohemianz in particular to go out stargazing. Nightlife seen after 2 am in the morning in Delhi may include drunk revelers, barking dogs, thiefs, call center execs, tired cops, and if you’re lucky enough you might run into a AAAD member hypercaffienated and focussed onto some dot in the sky!

That apart…some serious stuff…

Enter winter and the first thing you have to do is to get your bearing back on the night sky.

So where are all the planets now?. Lets start with my personal favorite wonder, Saturn. Saturn over the past year has visually moved quite a bit. Currently it is in between Leo and Virgo. Remember both are zodiacal constellations and the ecliptic passes through both constellations

Location of Saturn, Nov 2008

The other gas giant Jupiter at about -2 magnitude almost in the foreground of the Milky way in Sagittarius. Unfortunately you wont be able to spot it easily because of the proximity to dawn.. Similar sorry tales for Mars too.

Apart from planets, the winter is time for some of my favorite Deep sky objects. Its probably the best time to find greats like Andromeda (M31), mascot of the bohemians M45/ the Pleiades, Crab Nebula (M1) and the King of the winter skies M42- The great Orion Nebula. . Plus jewels from the summer milky way are not gone yet. They’ll keep you entertained till the wee hours of dawn.

This might also be a good time for meteor hunters, with the Leonids peaking mid November and the fabled Geminids in late December. Personally I’m not a big visual meteor chasing fan pretty much because of my inability to prevent my arteries from freezing.(Two attempts at Geminids…and I was through with it!)

I am contemplating lesser painful and more accurate techniques…namely radio meteor observation, i.e. using VHF antennas to listen to short wave backscatter from a meteor entry
If anyone has done this before please let me know. We can collaborate together on this project. Sanath…what do you say?

A simple 6 element Yagi Radio Meteor Antenna at Marshall Space Flight Center, AL

Looking forward to put up a project page soon here with plans updates, tips and more very soon

The best part of the winter season in Delhi is the February- March period, best known for you know what….a Messier Marathon. Yes last year we pulled off a great marathon, with Ajay actually photographing those elusive objects on the way! Thats something. Yeah thank god we’ always seem to have good weather during this time of the year.

Soon it will be all hot and dusty again next year, when there will only bt 3 stars seen from Delhi skies and we’ll eagerly wait for the monsoons to get over and train our optics to the heavens again!

Clear skies

SKY EVENTS for November 2008
Thanks to Ajay for this neat calendar that is handy for all significants sky events for November. For earlier versions go to