# Thread: CPC 1100 - expected DSO size

1. ## CPC 1100 - expected DSO size

Hi,
with my CPC1100 I really try to focus on finding DSOs.
Here is what I am still not sure about ... :
As oppose to planets, knowing what (relative) size to expect from a DSO isn't straight forward. I think I found the correct formulas for that, but please correct me if I'm wrong:
I figured I needed to find out the actual degrees (or arc minutes) I can see with my eyepiece, and then compare that to the size in arc minutes of the DSO.

The focal length (FL) of the CPC is 2800mm,
the diameter of the CPC is 11" or 28cm
the eyepiece FL is 23mm,
the eyepiece apparent field of view (AFoV) is 82 degree

That gives me:
a magnification of 2800 / 23 = 122x
and a True Filed of View (TFoV) of 82(AFoV) / 122 = 0.672 degree or (x60) 40.33 arc-minutes or 40.33'

Then I looked for webpages that tell me the size in arc minutes of DSOs, and found this one: http://astropixels.com/galaxies/galaxies.html
For Andromeda (M31) it says 178'x63'. That should exceed my TFoV.

Three weeks ago I had for the first time an acceptable, although not perfect night sky out there in the desert. Before I froze to death I'm fairly certain I looked at M31, but I don't recall it overlapping the view. (I will be out there this weekend, but I'm afraid the moon will be freakish bright). Maybe it was still too misty that night and M31 was too faint at the edges so I misread its size.
Or do you think my calculations could be wrong?

2. ## Re: CPC 1100 - expected DSO size

Hi Matthias,

This link to this website will give you all the answers you seek.
https://astronomy.tools/calculators/field_of_view/
Just put it in your scope and your eyepieces and the desired object and it will show you a picture of what you should see.

Cheers,
JT

3. ## The Following User Says Thank You to jaetea For This Useful Post:

Mttwtt (01-18-2019)

4. ## Re: CPC 1100 - expected DSO size

I assure you that M31 exceeded your field of view. You are just learning how to observe and sometimes you need more experience to properly interpret what you see. jaetea has recommended good tools. Keep on and your skills will improve with practice.

5. ## Re: CPC 1100 - expected DSO size

I made a similar mistake when I first observed M31. I could see the bright central portion and mistakenly thought that was the entire thing. I wondered why I could not see M32 near the edge. Then I learned the truth: I wasn't seeing the edge!

I came much closer to seeing the entire width of the galaxy one extremely dark night with 20 x 80 binoculars and was shocked at how much farther it extended than I had thought.

6. ## Re: CPC 1100 - expected DSO size

M31 is....... HUGE! 6 times the angular size of the Moon.

7. ## Re: CPC 1100 - expected DSO size

Even with a Tele Vue Panoptic 41mm the FOV is 1 degree. Even then you can not see all of Andromeda in a CPC 1100 GPS. I know I have one and wide field is not a great strength of that scope but finding the faint fuzzies of other DSO's is. My AT 152 Refractor with a Panoptic 35mm will see nearly all of Andromeda at almost 2 degrees FOV. So having a refractor as a compliment to the SCT is a good thing.
Last edited by dagadget; 01-18-2019 at 04:18 AM.

8. ## Re: CPC 1100 - expected DSO size

Hello !
You may look there and compare the Moon and Andromeda !
Clears skies
Arsene

9. ## Re: CPC 1100 - expected DSO size

Like others have said, I can assure you that M31 exceeds your field of view! It is enormous. It is a naked eye object in my skies, and, with averted vision, you can see just how big it is: the reports that it is bigger than the full Moon are true.

What happens is that magnification reduces the brightness of objects. A fixed number of photons are spread out over a large area of your field of view. The more magnification, the larger the area they are spread out over, and therefore the dimmer the object appears.

So all you are seeing is the bright core of M31.

10. ## Re: CPC 1100 - expected DSO size

Thank you everybody,
that makes absolutely sense. And it brings me to a followup question, though I rather open another thread for that.

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