Dim image???

[Leonid]

Hello to all!

Recently went out at night for astrophoto session. Im a beginner in astrophotography and do not have sophisticated equipment, just the motor-driven mount and my Canon 1100D DSLR. I made a few RAW images, showed them to my friends in my local astrophoto community and they commented that pictures are way too dim for their exposures. Kindly comment if you believe the same and what may be the reason??? (auto WB setting)

1) Leo @ ISO400 f/3.5 177s exposure

2) Ursa Major @ ISO800 f/3.5 59s exposure

[happy-kat]

Hi

When taking your first image when you review take a look at the histogram. Aim for the peak of the histogram to be fully clear of the left hand edge.

You may find 800 or 1600 works for you, but your local light pollution and camera settings like exposure length and aperture  will dictate what ISO you use when aiming to keep the peak clear of the left hand edge. This protects you from clipping data from the dark side and not going past half way protects and light side of the image though half way for me is too bright an image.

White balance will only effect your jpeg, RAW files are not white balanced.

You caught a bit of satellite in your first image. Opening them both shows lots of them and a really high plane I think.

If using the kit lens it performs a little better if you stop it down a little bit and the chromatic aberration should reduce as well..

Great tracking and got good tight round stars.

Great start, hope you get to try again soon.

[LightBucket]

It looks like you have a filter on the camera, if so what is it, as the centre stars in the second image are very blue...  

[Leonid]

10 hours ago, happy-kat said:

When taking your first image when you review take a look at the histogram.

Here you can see the histograms for both pictures. Yes, it is a Canon 18-55 EFS kit lens. By stepping down you mean zooming in a little bit to increase the F value? This will eliminate bluish tint on the stars?

Also how do you suggests adjusting the white balance? Is it good idea to make a custom WB setting by taking a picture of gray sheet and setting it as custom WB?

I am not claiming that my friends in the local astro community are experts, but they are definitely more advanced and several of them commented that at this exposure (especially Leo at 177s) the image should look way more brighter and probably overexposed. Now im confused.

10 hours ago, LightBucket said:

It looks like you have a filter on the camera, if so what is it, as the centre stars in the second image are very blue..

It is a stock Canon1100D. I dont know if it has any built-in filter. I did not modified it in any way for astrophotography.

 

[happy-kat]

Your first image the histogram is ok it is clear of the left edge (could be still further away) but your second image is verging on being clipped, so potential of loss of data.

If you are taking single images to look good when viewed as taken then you'll want the histogram peak much further away from the left edge.

If taking several or many raw files to then stack and process you need to ensure you are getting all the dark and light data by not clipping either side of the histogram but notice how the white data spreads out much further so don't clip that side.

Stopping down means increasing the aperture, say going from f3.5 to f5. It is not changing the zoom.

When you take a raw file there is no white balance. WB is used on any jpeg taken. I have an 1100d and have never needed to fiddle with WB.

Edit: you should be using M mode.

And turn off in camera noise subtraction. This is a must. You want all in camera noise reduction turned off.

I find enabling back button focus is a great improvement on using this camera.

 

[Leonid]

Thank you, ill put my thought into it later. But my main concern now is are the images dim for their exposure? Should I be worried about my cameras sensor? Or does image brightness correspond to the exposure?

[happy-kat]

I wouldn't be concerned.

Image brightness is influenced by local conditions (dark sky or light pollution etc.) , aperture, ISO and exposure length.

But if your lens had dewed up then that would cause less light to get through though I think your stars would me messier shaped.

[Leonid]

Ok, thanks. No, the lens was not dewed. Next time I will try to reduce the apperture.

What would you suggest for WB adjustment?

[happy-kat]

When you shoot using RAW there is no White Balance applied to the RAW file on the camera.

What you see in preview is the in camera conversion to show you a JPEG.

I have never felt the need to adjust the white balance on my 1100d.

 

The smaller the aperture number the large the opening through the lens, but this can come with increased chromatic aberration and/or quality of star shapes. Only you can decide what you are happy with and whether you might stop it down a little.

[The Admiral]

Hi Leonid,

So far no one has asked how you processed your image, but that is fairly critical in the production of astro images. First of all, what most folk do is take a series of images and overlay them all in a process called 'stacking'. There are free programs around that you can use for this, such as Deep Sky Stacker (DSS). See http://deepskystacker.free.fr/english/index.html. It's not difficult to use. Following that, the stacked image has its contrast considerably enhanced, something we call 'stretching'. A single image such as yours will also need stretching. In order to get the image looking decently bright, you'll find that all the noise in the image will also be revealed, and it might not look so good. This is a constant battle for astro imagers, and it's the reason why they stack many images together, as in doing so the noise is reduced as the square root of the number of images stacked.

I see that you are using Lightroom. To stretch your single image sufficiently I doubt that the 'contrast' slider will give you enough, so probably you will have to resort to using curves and applying a steep transformation (i.e. the 'Tone Curve' on the RH panel). You can also play around with the colour balance to get the background sky neutral, and do a bit (lot!) of noise reduction. The stretching essentially pushes the top end of the histogram all the way to the right; the bottom end is adjusted so that it is just above black clipping.These images are not processed in the same way as conventional images, but much more aggressively. Hope this helps.

Ian

[Leonid]

Hello Ian,

I am glad you shared the info, but for now im quite far from understanding "stretching" and "clipping" etc. But I will do my homework!

What do you think about the brightness of the image???

Thank you!

[The Admiral]

OK, using your image from the post, never a good thing for quality but it can illustrate the point, here is what I get when I 'stretch' the histogram.

You will note that there are many more stars, and the two trails are clearly seen. For this example I've added a bit of brightness and then applied a steep tone curve to stretch the histogram, whilst at the same time tried to retain some minimal brightness in the sky. I've had a go at setting the colour balance with the dropper, but it is very dependent on just where you place it. But we have some other issues too. First I think is the problem with light pollution, which is turning the top, and particularly the left, orangey. You most likely also have vignetting, i.e. where the exposure isn't even across the frame, making the centre brighter than the periphery. This is not your fault; it's something we all have to deal with. This is where there is an advantage to specialist astro processing applications in dealing with these situations. Some people use Photoshop with useful astro plug-ins, some of us use StarTools, not a free application but very reasonably priced. There are others around too. Additionally, a closer inspection shows a lot of noise in the background.

This is by no means a masterclass , but I hope it illustrates the point. Astro imaging is fraught with difficulties owing to the low brightness of the subjects and our very light polluted skies. But, they are eminently soluble and I'd encourage you to keep at it, and read up on it too. There's a lot of fascinating stuff to learn!

Ian

[SteveNickolls]

Hi Leonid,

Thanks for posting. Do follow up on what both happy-kat and The Admiral have had to say. Astro-imaging has its learning curve but your images already show promising points, firstly you have nice focused stars and you manged to image for near on three minutes almost from the start. The light pollution problem which The Admiral noted you have (and so many of us do) will be an important consideration in your imaging but certainly not something that need defeat your attempts.  

I would suggest some reading to compliment and support your practical imaging and there are a number of tutorials in the, 'Getting Started With Imaging' Topic on SGL. Above all get outside when the weather permits and experiment with your imaging. Look forward to your future image posts.

Best wishes,
Steve

[Leonid]

Thank you all! Will definitely do my research!