Focusers:

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 astrophotography.

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 process.

Focusing Aids:

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 HERE. 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 found 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 www.inkscape.org/ .

Have fun!!

10/14/2010

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 defined.
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 difference between 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 Ha filter.

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.

12/5/2011

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 somewhat misshapen.

 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 round star.

 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.