As you can imaging when it come to
photography focusing is extremely important. With astrophotography
even more so. Good focus allows you to image fine details of
To accomplish good focusing there are a number of
tools available, Auto focusers, motor focusers and manual focuser.
Auto Focuser use a program to control the motor
focuser by sampling the image and determining if the focus needs to
be adjusted. This sometimes is needed during long imaging session where
the ambient temperature varies greatly. Unfortunately these devises and
associated software can get rather pricey.
Then there is the motor focuser. This device usually
replaces the focuser that comes with the telescope or
controls the existing focuser with a small motor. The great
benefit in having one of these devices is that you do not need to
touch the telescope to focus it. Simply touch a button
to drive the focus in or out. Touching the focuser with your hand
causes all kinds of jitter which has to quiet down to
see if you actually are turning the thing in the right
direction. I feel that a motor focuser is a must for
The manual focuser is what you usually get
on the scope when you buy it and requires you to grab
the knob of the focuser to make any adjustments. OK for visual
observation but a real hassle when using a camera to watch the
Focusing a camera while doing normal photography is
rather simple if your eyesight is good. But focusing a star or other dim
objects in the night sky can be challenging at best. To assist us
old folks in getting good focus we use focusing masks . Probably
the most common is the Hartmann Mask.
This mask usually consists of three holes or triangles
cut out of some material and placed in front of objective lens or
opening of the telescope. This mask causes spikes on bright stars and
the object it to get these spikes clear and aligned across
from each other. This is sometimes easier said than done. IF you
want to read more about the Hartman mask click
If you want to create a Hartman mask, this
LINK will get
you to a mask generator that you can customize
for your scope.
I have been using a Hartman mask for a while until I
out about a new mask designed by a Russian named Bahtinov. The Bahtinov
mask uses a series of slits, instead of holes or triangles, that
generate an spikes around a star that resemble an X with a line thru it.
As you adjust the focus knob the center
lined moves up or down depending on the direction that you
are turning the focuser.
When the center line is in the middle of the X
your in focus. It's that simple!
(click on the picture to enlarge)
You can buy masks on line for $50 to
$150. But I found it easy to make myself for peanuts.
This mask is the best focusing tool I have found yet.
The job of getting near perfect focus now takes a few seconds where
using the Hartman mask focus was somewhat iffy and could
take a few minutes
I created my mask out of 1/4 inch foam core board. It
is Styrofoam sandwiched between heavy poster board. It's quite rigid and
easy to cut with a craft knife. It is mounted on an
embroidery hoop and easily slips over the front of my SN8 scope or
over the dew shield.
Others had good results in printing the mask on transparency film. You
can get that done at you local office supply store ( Staples,
Office Max) that has a copying service.
If you are interested in making your own goto to
http://astrojargon.net/maskgen.aspx where you will find a mask
generator. I used Firefox to print out the mask. MS Internet
explorer doesn't do a good job. Another program that you can
use to printout the resulting mask is called INKSCAPE. It's free
and can be downloaded from
Improved Bahtinov mask :
I didn't think that there was an improvement to the
Bahtinov focusing mask but I was wrong.
Recently a friend suggested making a Bahtinov
focusing mask using thread to create the gradients instead of
trying to cut them out of cardboard or paper. I thought that
he had a great idea since the thin threads allow more light thru thereby
allowing you to use the mask on smaller stars. He built one
and showed me the image through it. The points are very sharp and well
He images a lot using an H-alpha filter which requires longer exposure.
The new mask lets him focus using the H-alpha filter in place because it
allows more light to reach the camera.
As an example the picture on the right shows the
a typical Bahtinov mask cut out of heavy paper ( image on the left) and
the string Bahtinov mask ( image on the right)
(Click on the picture at right to enlarge)
These two shot are of the same star at the same
exposure using and 7nm
He convinced me and I set out to make one. All
it took was a piece of 1/4" plywood a couple of machine screws a piece
of aluminum angle and a spool of sting.
If your interested in making one click
HERE for the directions.
More notes on focusing:
Above I have
mentioned a number of different tools that can be used to achieve
good focus. I have tried all of them with some success. However,
for the better part of the year I have been using the fine focus feature
in Nebulosity. I have found that after getting the best focus I
could with the masks, the stars had tails or were otherwise
I have been using
Nebulosity for quite a few years to capture and process my images
but had never thought to use the fine focus feature . In a effort to try
and improve the images I decided to give it a try. To my surprise it
worked very well. I now use it exclusively and don't bother with the
masks any more.
The process is quite simple and
works in conjunction with the Frame and Focus function:
To use the fine focus I would proceed as follows:
- With Nebulosity running and the OTA pointed at a
star field, I set the exposure time to 1 second and get an
image using the Frame and Focus option.
- I then abort that process and click
on FINE FOCUS.
- I click on a star that's not saturated. The
program will now display that star in a small window as well
as a graph and numbers representing the HFR and Max
values of the star's profile.
- Now I adjust the focus until the
shape of the star is as round as I can get it and the HFR number is
at its minimum. I have found, on my setup, that a nice round
star does not always mean the lowest HFR number so I go for the
That is basically the process. Since I
have a motorized focuser controllable by the computer and frequently
control the setup form inside my house, this option works great for
me. There is no need to go out in the cold to put a mask over the
OTA then adjust the focus. It's a simple matter of going thru
the above steps if I think the Focus needs to be adjusted.
If you are not currently using Nebulosity and want
to try it you can
get it at STARK
LABS for $60 a copy. I have used it for quite a few years and
swear by it.