Useful hints for the amateur:

This page contains hints that will make you love your hobby even more. It will be practical hints about observationtechnique, phototechnique, image prossessing and more.
The list will be extended in the future.

Contents:

Polar alignment 17. Dec.98
Fieldsize in the eyepiece 17. Dec.98
How to avoid dewing 17. Dec.98
Notes for astrophoto 3. June.99
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Polar alignment

This is a problem many amateurs struggle with. A good polar alignment is not very difficult, but can be some timeconsuming.
If you intend to do only visual observations, aiming the polar axis towards Polaris will be enough. You can use a compass but be aware that mounts of iron can influence the compass.

If your mount will be used for long-tracking photography you need a moore precise method. A common way is the driftingmethod. You will observe a star drift over the telescope field, this gives an idea of how you must adjust the polar axis. With all polar alignement it is og great advantige to level the base of the mount. This will make all further adjusting much easier.

The method is as follows:

1. Aim the polar axis roughly with eye and/or a compass.
2. Put an eyepiece with reticle in the telescope.
3. Aim at a star near the meridian in the south.
4. Let the motor of the telescope run.
5. Observe if the star is drifting north or south.
6. If the star is drifting to the north, the polar axis is pointing to much west.
7. Repeat point 5 and 6 till you are satisfied.
8. Aim at a star on the west hemisphere.
9. Observe if the star is drifting north or south.
10. If the star is drifting to the north, the polar axis is pointing too low.
11. Repeat point 9 and 10 till you are satisfied.
12. Go back to point 5 and make a new adjustment.

What is north and south in the eyepiece?? Look in the ocular and move the telescope towards the pole star. The stars enter the field in north and leaves in south.

Note: These directions are written for the northern hemisphere. If you live on the southern hemisphere reverse these directions.

There are more precise techniques than this but they are more timeconsuming and little convenient for portable telescopes.

Fieldsize in the eyepiece:

To know the size of field you have the eyepiece is very convenient when you are looking at new places in the sky. The size of field is always given as an angle. If you look at the sky as a sphere, the distance around the horizon will be 360 degrees. The distance from the horizon to the zenith (right overhead) is 90 degrees. The moon has an apparent size of 1/2 degree, or 30 arcminutes. One degree can be divided into 60 arcminutes and one arcminute can be divided into 60 arcseconds.
You can find the fieldsize of a given eyepiece by two methods:
In the field by the telescope or by calculations based on techniqal data.

In the field by the telescope:

1. Aim the telescope at a star near the celestial equator.
2. Put in the eyepiece you want to messure.
3. Stop the motor if you have any.
4. Use a stopwatch and time the stars drifting over the field.
5. By multiplying the time in minutes by 15, you get the fieldsize in arcminutes.
Example: The star uses 3 minutes to drift over the field.
3x15=45 this is 45 arcminutes or 3/4 of a degree.

Calculation based on techniqal data:

In ths case we need the focal length og the telescope and the eyepiece. We also need to know the aparent field og the eyepiece. This is not always marked on the eyepiece but we can find it in the manufacturers cataloges.
1. Find the magnification by dividing the focal length of telescope by the focal length of eyepiece.
2. The apparent field is then divided with the magnification.
3. The fieldsize will then apear in degrees.
Example: A 25mm eyepiece with aparent field of 60 degrees, is put into a C8 with 2000 mm focal length.
2000/25=80 that is 80x magnification.
60/80=0.75 that is 3/4 degree or 45 arcminutes.

Was the calculations difficult?? Take a look at my telescope calculator.

You can now make a ring that match the field in your star atlas. I have made a overlay myself with circles for all the eyepieces I have. By laying this over my star atlas (Uranometria 2000.0), it's easy to see how much I have to move the telescope to reach the place I want. It is also easy to see which stars and objects I can expect to find in the same field.

How to avoid dewing:

Dewing is something most of us has experienced out at the telescope. This often leads to an early end of the observing session. You take the telescope inside so it can dry out, when it is dry is is too late to go out and start again. You have to be patient and wait for the next clear night. Here in Norway that can take a while.

Dewing occurs on a surface that is colder than the surrounding air. Water will condense on this surface, in real cold the water will freeze. How can a surface become colder than teh suroundings? As you you know, the coffee gets cold but not colder than the room temerature. To explain this phenomena let's think of a fine summer day. Temperature of air is about 20C. If we move into the direct sunshine we feel warmer. All surfaces exposed to the sun become warmer than the air, even if the sun is located some 150 millions km away.
The same is happening at night but with opposite sign. Out in space the temperature is near absolute zero. If we now look at the sky as an object, it will try to cool the surface to its own temperature. As in daytime it is only the surfaces who becomes influenced.

When you understand how dewing occurs, it is easy to fight the problem.
One way to prevent dewing is to heat ut the surfaces you want to keep dry. This requires energy i.e. batteries or normal current. If you observe out in the field it is not very practical to bring out so much battery power.
The other way is to isolate the critical surface from unnecessary sky exposure. The method is very simple. Take a isolating material like a sitting-pad from a sport shop, and make a cylinder of it. The pipe is then mounted over the telescope objective. The pipe is recommended to be 2.5-3 times as long as the apertude. The sitting-pad is easy to glue together with a hot melt gluing gun.
I have personally used the latter method over several seasons now. The experiences have been very good. Only one night have been so moist that the objective became wet. I have seen many times the dewshield become completly white of frost on both outsiden and inside, but objective - dry as gras.

Notes for astrophoto

When you're in the astrophoto-business there are a lot of elements that affect the resulting image. The gear train of the mount must be very good to make the telescope follow the stars exact. The focus of the image is also crucial. When you get the pictures back from development, you sit and admire the results. Then you think: How did I make this picture?
After a few years of experice with astrophoto I have made my own form. I use this form for all kinds of astrophoto. You can find the form here and shuld be easy for everyone to use. Here comes a more deepening desciption of how the form is used.
Common data: Some data are the same for the whole film. Primary it is the type og film and the speed (ASA/ISO). I also notes the date and place of development. If you develop on your own then you note type of developer, time, temerature etc.
Object: This is maybe the most important thing to write down. The resulting picture isn't always what you expect. Therefore it is useful to have a record of what the attemt was. There are many ways to do this. If you use the popular name i.e. Orion nebula, or M42, or only the position is up to you.
Number: Here you write the number of the picture. Use the number of the camera counter.
Date and time: Date does not need any further explanation. I use to write the time of start for the exposure. All times are given in universal time (UT). In most cases minute accuracy is sufficient. If you're working with occultasions you note the scounds too.
Exposure: Always the first question when you show an astrophoto: How long is the exposure? I make the note after the exposure is done. Is the exposure shorter than one second you note the adjustment of the camera. Up to one minute you note the exposure i seconds i.e.15sec. If the exposure is longer you only note the minutes. The seconds ar not important in theese cases.
Focallength and f-number: Since we are dealing with photo here and not visual observations magnificatinos is irelevant. Focal length and speed of optics (f/) are eccencial. This makes the records directly comparable with all other kind of photo. If I do photography through a Celestron8, I note2000mm f/10 Doing it this way ensure you to have the data either you working with piggy-back og eyepiece-projection.
ST-4: I use this columm to remember what I do with the autoguider. If you doesn't have an autoguider, you record the magnifiction og the guidetelescope. I use to note the exposure time of the ccd, the gain-level and if I use a faint or averange star. This might look something like this: 2sG2A. That means 2 secounds exposure, gain-signal 2 and averange star.
Marks: This columm is used for all other kinds of information. It could be notes about filters, eyepiece with eyepieceprojektion, interrupt of exposure etc. etc.
Filing: When the pictures has arrived from the developer you file the negatives in a binder. The form you filled out is filed together with the negatives. On the backside of each print you write down the particular data according to each picture.

Now, everrything should be ready to take good pictures. You can compare the notes for each time, and you don't have to do the same mistake twice.

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