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Hi everyone, I would like to share my very first astrophotography picture I took today.
I only own a 50mm f1.8 lens so i decided to try to shoot the orion nebulae (I was also thinking of the andromeda galaxy however the galaxy is near the horizon unlike the orion nebulae).
The location where I took the picture has a lot of light pollution (class 5) and I don't have any filter. (I am aware that this is not ideal at all furthermore the fullmoon didn't help)
The picture was made using 111pictures of 5s each, ISO 1600 at f2.8.



-Even with an aperture of 5s with a focal lenght of 50mm, star trailing is still visible on raw images.
-I find the manual focusing hard to do because the focus ring can be slighty turned and the focus would seem to be the same.
Also, what makes stars appear as big spheres on the picture and how do you reduce this effect?

Welcome! Wow, it looks like you're well on your way already!

Usually if the stars look bloated in astrophotos, it's because they're overexposed. But 5 seconds, f/2.8 at ISO 1600...seems unlikely here. What stacking software did you use, and how did you align your individual frames? A little bit of misalignment can definitely make them look bigger. What are you using to process your images? There are a few techniques to reduce star sizes that are fairly specific to the software. For example, in Photoshop you can use the Noise | Dust & Scratches filter to essentially eliminate the stars from a (copy of!) the image. For an image like this the process would go something like:
1) Duplicate the background layer. Name the duplicate "starless".
2) Use the dust & scratches filter on the duplicate to get rid of the stars (apply as many times as needed).
3) Set the blending mode of this layer temporarily to Difference (i.e., with the original, untouched layer underneath). Suddenly it's nothing *but* stars! Create a clone layer of it (Mac: Command-Option-Shift-E; PC: Ctrl-Alt-Shift-E). Name this layer "stars". If you like, try using the "Minimum" filter to reduce the size of the stars.
4) Set the blending mode of "starless" back to Normal, and drag it to to top of the stack. Add adjustment layers however you like to stretch and enhance colors.
5) Once you're happy, drag the "stars" layer to the top of the stack (hiding "starless"). Give it a layer mask. Paint black on the layer mask with a soft brush to let the "starless" layer show through anywhere you want.

5) Creating a star layer - Imaging - Tips, Tricks and Techniques - Stargazers Lounge

I'm willing to bet that the lens (exactly which lens? exactly what camera?) still has some residual CA (chromatic aberration) at f/2.8. That results in somewhat larger stars and, in this case, some blue fringing. You likely also have less-than-perfect focusing. The trick to better focusing is to use Live View at 10x magnification on a bright star such as Rigel.

If I'm right about the residual CA, stopping down to f/4 (one more click) should prevent it. But then you will get an even dimmer image and you will have to take and stack four times as many exposures (subs) to get the same signal-to-noise ratio (SNR)! You can't simply lengthen the exposure time of each sub to compensate because you have already gone past your limit, which is why you see trailing. The old rule of 500 says you can do up to 10 seconds with a 50mm lens (500/50) but that rule was based on film and full-frame cameras (and frankly wan't all that great to begin with). You can see it doesn't work with your camera. You are getting significant trailing at half that limit. I would say a maximum of three seconds.

So now at three seconds exposure and f/4, you must stack nearly seven times the number of exposures to get the same SNR. If that seems like too much work, you need a tracker OR, if your camera is compatible, use an O-GPS1 to enable Astrotracer. (I'm assuming you don't have a K-3II or K-1 or K-1 Mark II, which have Astrotracer built in, or you would have used it already.)

Thanks!
I used deepskystacker and then I edited the resulting image with Lightroom. The stars were already blurred and rounded in the Raw pictures.


The camera used is the pentax k70 and the lens is pentax KAF 50mm f1.8 (I thought it would show it with the picture but it didn't)
I used the live view with the zoom, that's why I find it a bit hard, you can't really know if you have perfect focus or not.
Next time, I'll try to shoot it at f4 and try to make a better focus.

Another method is to use what is called a Bahtinov mask along with Live View. You can make one out of cardboard.

Improve Your Astrophotography Focus With a Bahtinov Mask
Bahtinov mask - Wikipedia
Make Your Own Bahtinov Focusing Mask: 7 Steps

There's a gadget called the O-GPS1 that can slide into the hot shoe on your K-70 and turn it into a poor man's tracking mount by sliding and rotating the sensor via its shake-reduction actuators. Could probably pick one up used fairly cheaply. I used one with some success for quite awhile. 50mm is right in the sweet spot for it. You could also make a "barndoor" tracker. Or, you know, drop fully into madness and buy telescopes and equatorial mounts and guidescopes and dedicated astro cameras...My God, what have I done? ("My question exactly", says my wife tartly.)

Considering the size of the stars seen through a 50mm, would a Bathinov mask work?

Thanks, it didn't know it existed! However it relies on diffraction which is proportionnal to the focal length.
Consequently, a cardboard model can be make when using important focal lenght. However with smaller focal length, such as 50mm, the size of the split needs to be very thin which is not really convient for a homemake carboard mask.

Yeah, I've already heard of it and I'm even planing to buy one...at some point...when I'll have enough money. So many things to buy, that's why I want to keep it simple for now.

I like to cheat. Pre-Focus in the afternoon, put the body in to Manual Focus and then tape down the focus ring on the lens with some tape, and you are good to go.

Then make it out of plastic. You'll note that a picture in the first link shows a cardboard mask used with a zoom lens with a maximum focal length of 42mm.

Good idea. If you want something you can return to while using the lens for daytime work, then as an alternative to tape you could mark the lens barrel and focus ring with thin indelible lines when the lens is focused at infinity (image on something as far away on the horizon as you can find, or on the moon in the evening sky). Now just align the marks when imaging at night.

I will note that Live View works perfectly for me when I image with an autofocus-capable lens. I haven't needed to use any other technique. But I prefer using manual-focus lenses, all of which I have confirmed with a homemade knife-edge focusing aid actually focus at infinity when the focus mark is at infinity (which is the lens's hard endstop, which is not the case for autofocus lenses).

Thanks for your suggestions.
I tried to make a bahtinov mask but it didn't work. However I was able to get a pretty good focus, I guess I just needed some training.
I went with a lower aperture time and I went to a darker area. As a result the images I got were better than the first one I took.
Here is what I got: (pentax k70, Pentax 50mm f1.8, 100*3s ISO 3200 )

Keep on trying it takes practice. Did you edit the stack? If not try stopping the lens down to 2.8 again as a lot of the detail got blown out in the Orion nebula. Editing stacked astro images is it's own monster. I have a link to some youtube tutorials at home that covers some of the basics of astro image editing that Pete_XL pointed out a while back that I can post for you to get you hopefully pointed in the right direction there too.

I fell like I already made some progress. Also, yes, the stack was edited. I used Gimp and I played with levels, color saturation, exposition and curves.
A tutorial for image processing would be very handy indeed!

Nice shots!

Astroblog: Using The Gimp for Astrophotography (Part 1)

(When will Darktable or Rawtherapee support image stacks?!?)