Thursday, March 30, 2006

Telescope Mounts For Astrophotography

Once again we will start with the basics of telescope mounts and the advantages and disadvantages of each type. We will then explore the particular features which are important for astrophotography.

A telescope mount's purpose, in simple terms, is to hold the telescope firmly and to provide a way to smoothly move the telescope. However for astrophotography purposes the mount needs to be more stable and be able to track the object you are trying image accurately. There are basically two types of telescope mounts though each has different variants. They are the altazimuth, and equatorial.

Altazimuth Mounts

These are the simplest, most basic styles of telescope mounts. They move in two directions for tracking, altitude (up/down) and azimuth (side to side) thus the name, AltAzimuth. Due to their simplicity they are cheaper (except in hi-tech computerized models). These mounts are fine for visual use, especially when using a short focal length telescope, but they are not very good for astrophotography except for planetary/lunar imaging. However the computerized mounts can be used for deep sky astrophotography if you are using a digital imager and keep your exposures short. More on this below. The three major variants of the altazimuth mount are:

Dobsonian; The in the 1960's. A very simple design, with a flat platform on the ground, with a box on top of it where the scope is mounted. The box rotates on the platform for the azimuth motion and the telescope, which has circular 'hubs' attached to it, rotates in semi-circular cut outs into the top of the box for the altitude motion. Due to the simplicity and low cost of materials this style of telescope mount is the least expensive. You can spend more on a Newtonian telescope, with a large to very large mirror. Great for visual use but not at all usable for astrophotography. There are electronic tracking motors available for 'Dobs', even equatorial platforms, but this also increases the cost and thus you lose the price advantage.

Tripod Mounted Altazimuth; A more complex design than the 'Dobs' but still rather simple. Camera tripods are a very basic form of this mount type. There are also models made especially for telescopes which have slow-motion controls allowing you to track the object you are viewing fairly smoothly. Again the are fine for low-power, wide field visual use but they are not conducive to long exposure imaging. There are now also computer controlled mounts of this type which are fine for planetary/lunar work, but none that I have seen are usable for deep-sky astrophotography.

Fork Mounted Altazimuth; There are some computerized marvels in this design type but they are expensive. However with the electronics now available in this design you can get excellent results on planetary and lunar imaging especially with web cams. And you can also get good results on deep sky targets when using digital imaging equipment such as the Meade DSI, StarShoot Deep-Space CCD Color Imaging Camera, or a DSLR camera. You need to keep your exposures to 15 to 30 seconds, take multiple images, and then stack them. Again these mounts are costly.

Equatorial Telescope Mounts

These are the best telescope mounts for astrophotography bar-none when motorized. When accurately polar aligned you can take long exposures (although you will need to do some guiding corrections). The scope will only be moving on one axis (azimuth or right ascension) so it will track very steadily. It is also better for planetary imaging as the target will remain in the field of view for longer allowing you to take many images and then stack
them or if you are using a film camera to pick the best shot (when the atmosphere was nice and steady). There are two styles of this type of mount generally available commercially.

Fork Mounted Equatorial; Usually used on (physically) shorter telescope designs such as Schmidt Cassegrains and Maksutov Cassegrains. To make the fork mount equatorial you need to purchase a wedge so the scope can be polar aligned. I own a computerized model like this and get excellent results. However there is a drawback. When you start adding weight to the set-up with a guide scope or piggy-back camera, or your imaging equipment, you need to
add counter weights to achieve proper balance. This is more difficult with this design than with the 'GEM' which is reviewed below.

German Equatorial Mount (GEM); The best mount for astrophotography (in my humble opinion). There are two advantages to 'GEM's' in comparison to fork mounted set ups. Firstly achieving balance is a lot easier as this design already has counter weights included in the design. Plus the telescope body itself can be moved forward or
back on the mount to get the best balance. The second advantage is that you can mount different scopes on the mount where has with a fork mounted set-up you are stuck with the scope which came with the mounting.

A final tip, get the most stable tripod you can. The less vibrations and movement you can achieve the happier you will be with your astrophotography.

Clear Skies.

Telescopes For Astrophotography

Wednesday, March 22, 2006

Telescopes For Astrophotography

Telescopes For Astrophotography

OK, we are going to start from telescope basics and work our way forward. You will learn about types of telescopes, the advantages and disadvantages of each
type for both astrophotography and visual work. We will also research types of mounts and the limitations of each type for astrophotography. We will also
look into telescope manufacturers such as Meade, Celestron, Orion, Vixen, Skywatcher etc. and what they have on offer.

Be sure to keep checking back as I will be adding more information all the time as well as reporting any breaking news or new telescope technology especially as it regards astrophotography.

So lets start with telescope types.

The job of a telescope is to collect light, not to maginify an image (the eyepiece does that job). The larger the objective (the part that collects the light) whether it be a lens, in refractors, or a mirror, in reflectors, the more light the telescope will collect. The more light you can collect, the more detail you will be able to capture, and also important for astrophotography, the shorter your exposures will need to be to capture this detail.

Refractor Telescopes

The type of telescope most people visualize when they hear the word telescope is the 'Refractor'. This is what Galileo used for his break-thru discoveries. A refractor has an objective lens at the front which passes the light straight through to the back of the tube, focusing this light at an eyepiece or for astrophotography a camera.

refractor telescope


-No central obstruction (see more in the reflecting scopes), giving higher contrast.
-Due to the simple design they require little maintenance.
-Excellent for planetary and lunar viewing and photography.
-Excellent for wide field viewing and astrophotography especially in shorter focal lenghts (more on this later).
-Because the objective is permanently mounted and aligned there is no need for collimation (again more on this in another article).
-Excellent color in apochromatic and ED (Extra Dispersion) designs.


-Costlier per inch of aperture (objective) than reflectors and catadioptric telescopes.
-Can become bulky and dificult to manage, especially in larger lens designs.

This is a good example of a starter astrophotography telescope in a refractor design:
C80ED-R GT Refractor

Newtonian Telescopes

This design was invented by Sir Isaac Newton (he of the apple on the head fame). Instead of a lens at the front of the tube this telescope design uses a concave, parabolic mirror to collect light reflecting it back towards the front of the tube to a flat diagonal mirror which reflects the light out the side of the telescope to the eyepiece or camera for astrophotography.

newtonian telescope


-Lowest cost per inch of all the telescope designs.
-More light gathering power per dollar because of the lower cost design.
-Absolutely perfect color rendition.
-More compact design compared to a refractor of similar light gathering ability.
-Excellent contrast for planetary and lunar astrophotography and viewing in longer focal lengths.
-Can get excellent wide-field astrophotos and short exposures in shorter focal lengths.


-Slight loss of contrast due to the central obstruction (the flat secondary mirror) as compared to a refractor.
-Requires more maintenance, such as colimation (discussed in another article) which is vital for great results in your astrophotography, although you will learn how to do this quickly with practice.

Meade now has a hybrid version of the newtonian called a schmidt newtonian. It is an excellent starter telescope for astrophotography.
Telescope, Meade LXD75 SN-8" with UHTC

Schmidt-Cassegrain Telescopes

This is a very popular design, with a high tech look. Also known as a CAT (Catadrioptics). They use a combination of lenses and mirrors to collect and focus the light onto the eyepiece or camera. The light enters the telescope through a thin 'lens' called a schmidt corrector plate, goes to the back of the scope to a spherical primary mirror which relects the light back towards the front. Here the light strikes another mirror, the secondary mirror which is mounted on the corrector plate. This secondary mirror then reflects the light back towards the back where it is focused onto a hole in the primary mirror where the light is collected by an eyepiece or your astrophotography camera.

schmidt cassegrain telescope


-Compact and portable.
-Low maintenance although once again colimation is required for top performance.
-Many, many astrophotography accesories available.Cheaper per inch of aperture as compared with refractors.
-Excellent all-round telescope, good to very good for both visual and astrophography.Very good for planetary and lunar viewing and astrophotography.
-Very good to excellent for DSO (Deep Space Object) astrophotography with a caveat (see the disadvantages).
-Very good to excellent optics, both Meade and Celestron are putting out excellent optics on a consistant basis.


-Costlier per inch of aperture as compared with Newtonian telescopes.
-Loss of contrast due to the central obstruction which is even larger than that in the Newtonian scopes.
-Due to their longer focal lengths the field of view is smaller and longer exposures are required for astrophotography, although a lens known as a focal reducer is available which minimizes or removes this problem. The longer focal length is actually an advantage in planetary and lunar photography.

This is my main scope:
C9.25-S GT Schmidt-Cassegrain w/XLT Coatings


The Maksutov-Cassegrain telescope design is basically the same as the Schmidt-Cassegrain design except it uses a meniscus lens at the front instead of a Schmidt corrector plate. The main advantage to the Mak telescope design is you will get sharper higher contrast planetary and lunar images when compared with the Schmidt design.

In our next article we will discuss different mounts and their advantages and disadvantages.

Come back soon for more articles on Telescopes For Astrophotography

Tuesday, March 21, 2006

Telescopes For Astrophotography

Welcome to my telescopes for astrophotography blog. Here you will learn how to use and choose astrophotography telescopes.