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getting a good view?



hi i wish to get a good view of M31 "The Andromeda Galaxy" ive seen a few pictures where they have the galaxy covering the full frame of the photo but not sure how they get it like that? ive got a celestron c80ed and a canon 450d but when i try and focus on the galaxy it seams a bit small how do i get a better magnification or a tight view of the galaxy?  

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A telescope has a fixed focal length, so with a given camera the field of view is fixed.  You have limited options...


1) use a telescope with a longer focal length

2) use a Barlow lens to multiply the focal length of your existing telescope

3) crop and expand the image in post processing

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ok this make a bit more sense now my telescope focal length is 600 mm (24 in) standard,  at the moment ive got a extension tube on my camera that is about 7inch long so if i remove the extension tube and use a 3x barlow lens infront on my diagonal then attatch my camera i should get a better result? can i use multiple barlow lens?

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Your field of view is just over 2 degrees wide.  Andromeda should fill that field nicely. In fact, with a width of over 3 degrees, it is too large to completely fit.


In your original note you said that you were having problems focusing on the galaxy.  That may be your problem.  All objects in the night sky are essentially at infinite distance from your camera, so for best results you should focus on a bright star that is near your target and then once you are in focus you can slew to the target.  For M31 I would suggest Alpheratz as your focus star.


M31 will appear small in short exposures because you are capturing only the bright core.  The outer spiral arms are dim enough that they will only show up in longer exposures.


In order to frame the galaxy you may need to rotate the camera so that the galaxy goes diagonally, from corner to corner.  If you cannot rotate the camera without affecting focus, then you may need to re-focus on Alpheratz before returning to M31 to begin imaging.


Good Luck!

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No, that would be without the 3x Barlow...


Andromeda is BIG and Near, and actually presents a Field of View that is almost 6x the size of the Full Moon!!

But, only the Core is bright enough to be readily visible in single Short Exposures - so you are quite likely seeing only the Core and thinking that the entirety of M31 is "too small".


Follow Astroman133's advice on Focusing - Using a nearby Bright Star.  (Any Star will do even if not located "Near the Target" - just not a Planet...)


Then, see how long an Exposure you can perform before Tracking Problems causes the Image to start Star Trailing or Smearing...


Then, back off just a bit on Exposure, and take a good number (20+) RAW Exposures at that Setting, and make an effort to use DeepSkyStacker (DSS) or other Stacking Software in order to process your final AP Image...


Let us know how it turns out...

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No, I did not use a Barlow in the calculation.


The formula is:  sensor width (in mm) * 3438 / telescope focal length (in mm) = field of view (in arc-minutes)


So for you the calculation is 22.2 * 3438 / 600 or ~127 arc-minutes.


If you add a 3X Barlow, you would triple the focal length and the FOV would drop to 42 arc-minutes.


The "Night Sky Observer's Guide" lists the width of  M31 as 185 arc-minutes across.

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A good tool for visualizing what your field of view will look like with various well known objects (and free!) is here:




Be sure to get the extra set of images.


You can also add your own or NASA, etc. images by copying them into the install directory (they should be 360x360 pixels, one degree square) 


Planetarium programs and apps often have a similar feature, although perhaps with "artwork" of objecs vs. actual images.


By the way, you can use plate solving, in particular AStrotortilla, to "goto an image". E.g. let's say like you like the CCDCalc framed image of M81 and M82. You can have astrotortilla goto that image and then you will end up with the same framing (pointing mid way between M81 and M82).

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