Size of objects in a photo

[Jose Auger]

I am starting in astrophotography. Because part of my equipment is not ready, I have been helping a friend who is also new to astrophotography. We went with a group of friends to a dark place and took several photos of a nebula and later a galaxy. However, the galaxy and nebula appear small in the photo. My friend is using a 130mm refracting telescope (a triplet). One member of the club, who has an 80mm refractor placed a photo in the club's Facebook page of the same galaxy we were photographing and it covered almost all the photo. What are we doing wrong? Do we have to increase the size of the photo itself after it is processed?

[Leveye]

You can always slightly crop the pictures a bit.

[wxsatuser]

They most likely cropped the image.

Here is a whole frame of M97 and M108 from a Canon 60Da, scope SW 80ED

Underneath that I have cropped out M108, both images reduced to 800pixels wide.

As you see M108 appears to be larger, it's not, it's still the same number of pixels

[Cornelius Varley]

The focal length of the telescope and the dimensions of the sensor can also affect the final size of the image. Play around with this FOV calculator and you will see what I mean http://www.12dstring.me.uk/fov.htm

[Langy]

The apertures you have mentioned will only govern how much light gets through the scope to decide on exposure times, so for image size they have no effect.

The focal length will firstly have a very big impact in the same way if you use an 18mm lens or 300mm lens on a camera. Then assuming that it was prime focus there could have been a Barlows in there to double, triple or even lengthen the focal length even further.

Finally as mentioned the camera sensor plays a big impact on how much of the receiving light is being recorded. For example a webcam has a very small sensor compared to a DSLR. If I use a webcam in my scope it captures about 0.5 degrees (basically about the width of the moon), now I have to use a 2x Barlows if I use my DSLR prime focus and that gives me the same FOV coverage.

Also as mentioned you can crop and resize images in PS or other editing programs just the same way as you would enlarge a section of a print in the darkroom. I've had images from normal daytime photography where I only used about 20% of the original image and enlarged it to print on a 6x4 sheet of paper. Looking at the print no one knows that the original covered a much bigger area.

[ollypenrice]

The size of the image formed on the chip depends only on the focal length.

The amount of sky captured by that focal length depends only on the size of the chip.

The amount of detail captured by a given telescope depends on the pixel size (small has more resolution) until they become smaller than the errors introduced by the atmospere and then the limit is imposed by this.

You can model a particular combination of  scope and camera on a map of the sky in free software like Stellarium.

Olly

[Jose Auger]

Thanks to all. We will try cropping the image as mentioned.