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Old 11-12-2009, 06:43 PM
bobneil
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Imaging for an amateur

Hi,
I'm looking to get into photography/imaging. I have a celestron nexstar 8se.
What devices do you recommend for:
a) Planetary imaging?
b) Deep space objects?

Keep in mind that I am an amateur.

Thanks in anticipation,
Bob
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Old 13-12-2009, 08:47 AM
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netwolf
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Bob, with the Telescope you have at present you can do planetary imaging.

For Deep Space you will be limited to Short Exposures.

1. The mount you have is Alt/Az (up down and left right) whic is not in Sync with the natural rotation of the earth. While this can track objects, the earth rotation will cause the field you are imaging to rotate over time and this will be worse in some parts of the sky than others. So this limits you to a short exposure. An option is to add a Wedge between your tripod and Fork mounted telescope. So you turn your mount in to an Equatorial Mode RA/Dec.

Look here for some illustrations of Field Rotation
http://www.todman.id.au/astronomy/alt-az_vs_wedge.htm
and here
http://www.cloudynights.com/documents/nexstar8i.pdf
In the second link is a review that shows how the Wedge is used.
The Idea of RA/DEC is that you aligh the RA axis witht the axis of the Earth (the angle of this is equal to the Latitude of your location).


2. The scope you have is natively F10 so short exposures will not catch much light. But you could use a smaller sensor (DSI I or II or even III or cameras with similar size sensors) and use a Focal Reducer to get your scope down to F6.3/~F5 even, or F3.3.
http://www.isomedia.com/homes/cvedel...l_reducers.htm
Look at this page, there are good illustrations of the light cone, reducers, and chip size. A bigger chip will suffer Vigneting (dark areas around the edges where less light is getting in).

3. Another option if you want to do DSO with the current mount/Optics is to ad what is known as a Hyperstar. Essentially the secondary mirror is removed and a camera is fitted here in its place and this gives you a FL of F2. Looke here
http://www.hyperstarimaging.com/
and here
http://www.cloudynights.com/ubbthrea...l/fpart/1/vc/1
By reducing to F2 you are catching more light in a shorter amount of time and hence can catch it before the "field rotation" kicks in. However I am not sure with a Single fork arm how sturdy your mount will be to support this method.

4. Another option to consider is getting the best Equatorial mount you can afford. Your currnet mount allows you to easily remove your Optical tube and add it to another mount. This is what I like about the Nexstar single arm mounts.

5. This above feature of your mount also means that you could get a Small refractor telescope and mount that in place of your C8. This gives you a scope with a shorter FL, and it can be reduced using better reducers. Also the light cone of these refractors is quiet larger hence less vigneting on larger chip sensors.

There are many options. But I suggest you start with Planetary imaging. Your scope's native Focal length is quiet well suited for this and field rotation is not an issue with this type of imaging.

Last edited by netwolf; 13-12-2009 at 09:22 AM.
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Old 13-12-2009, 11:16 AM
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Fahim,
I like the article you are mentioning below about focal reducers
This is how the review of optical component (including lenses) should be done: hard facts, no "poetic" language, clear conclusion.

Quote:
Originally Posted by netwolf View Post
2. The scope you have is natively F10 so short exposures will not catch much light. But you could use a smaller sensor (DSI I or II or even III or cameras with similar size sensors) and use a Focal Reducer to get your scope down to F6.3/~F5 even, or F3.3.
http://www.isomedia.com/homes/cvedel...l_reducers.htm
Look at this page, there are good illustrations of the light cone, reducers, and chip size. A bigger chip will suffer Vigneting (dark areas around the edges where less light is getting in).

However, you will not catch more light by reducing F number (as you are suggesting below), the amount of light will be exactly the same.
The difference between high and low F number is, the same amount of light from distributed objects (like nebulae or planets) is spread over the smaller area of the sensor (because it is FL that is reduced by reducer), so the illumination (amount of light per pixel) will be increased. The image of the object will be smaller but exposure time can be shorter for the same surface brightness.
This does not apply to stars, they are point-like sources, and their light will be focussed on 1-2 pixels in any case (unless the optical system is not ideal and/or resolution is decreased because of diffraction)

Quote:
Originally Posted by netwolf View Post
3. By reducing to F2 you are catching more light in a shorter amount of time and hence can catch it before the "field rotation" kicks in. However I am not sure with a Single fork arm how sturdy your mount will be to support this method..
Otherwise, ditto

Last edited by bojan; 13-12-2009 at 04:12 PM.
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Old 13-12-2009, 01:21 PM
bobneil
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Thanks for taking the time to respond. All useful information. What camera/imaging device do you recommend for a novice?
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Old 13-12-2009, 03:38 PM
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Webcam for planets, DSLR for DS (Canon for example, but any will do, as long as it has manual mode).
And of course, film camera.. I know now there will be many eyebrows risen.. but film still works :-)

You may also browse older posts on this forum, you will find everything that might be of interest for you.

Last edited by bojan; 13-12-2009 at 04:02 PM.
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Old 13-12-2009, 04:43 PM
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Hi Bob, if you have a DSLR already it's a good start. You can do piggy back on your scope and widefield until you get more time for prime focus. Webcam's good too for planetary. It's not easier, just different.
I started with a Nikon Coolpix, then slowly moved to a second hand pentax then a dedicated CCD. Start small with what you got and it'll take you naturaly in the direction you prefer. Have fun in the process.
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Old 13-12-2009, 04:55 PM
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Quote:
Originally Posted by netwolf View Post
field rotation is not an issue with this type of imaging.
This largely depends on how long you capture for and it does take effect when imaging in monochrome with an alt az mount. Fahim it pays to qualify what you are saying. Planetary imaging is not as easy as some might think.
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Old 13-12-2009, 07:50 PM
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Bojan thinking on it i understand what you have said and it makes sense to me.
Paul i did not think FR affected planetary, can you explain how this happens?
Not that i dont belive you but i would like to understand how this happens.
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Old 14-12-2009, 12:13 PM
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Fahim, if you run for 40 seconds each colour and then a couple of seconds for dampening down between colours, then that will give you field rotation with an Alt Az mount. In a colour camera going over 120 seconds will mean your images will be blurred by the field rotation as well as planetary rotation.
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