Exposure-How do you determine it?

[NorthernLight]

me again,

last night the weather here improved so dramatically that I had clear skies until almost 3am.
Everything worked perfect after I hooked the whole shebang to the laptop, so I could sit back, relax and enjoy a nice bottle of shiraz.

I chose the sculptor galaxy and decided to push the limits and expose for 10min. To my suprise I found today that after I stacked them all in Deepskystacker the resulting image was completely white and featureless. Tweaking around in Photoshop helped the situation but I am sure that I have totally overexposed the images (vigetting was awefull too). See below the best I could do so far.

So what is the key for correct exposures and how do you determine the amount of shots to be stacked?

Shot details:
150mm f5 newton
Canon 450D no filters
8x10min
no darks, no flats, no bias (was so excited that I forgot about it)
Median Combined in Deepskystacker
PS CS2: levels, curves, crop, downsample (for posting here)

Your guidance is as always much appreciated.

Cheers!

[cybereye]

Max,

I get the same thing in DSS where my resultant stack is "whitish". I remember seeing a thread about that somewhere but can't find it. Maybe someone else can give you, and me, a hand.

However, I do like your image and the thought of doing the imaging with a bottle of Shiraz is making me thirsty!!! (No, nott the pretzels...)

Cheers,
Mario

[cybereye]

Max,

I found this tutorial which might help with the DSS issues (whiteness)...

http://flintstonestargazing.com/2009/06/26/my-quick-deepskystacker-tutorial/

Cheers,
Mario

[NorthernLight]

Hi Mario,

thanks for the link and the flowers regarding the picture. I will study the intro to DSS and see what I can improve. To be quite honest I sometimes struggle with all the options that one can choose from and it happens that I choose one that damages the result. But I suppose in a few years I know exactly what "convolutive gaussian debayerization with separate channel interpolation offset value anticipation" - or so - is good for... gosh, Kants critique of pure reason is a good night story compared to all these methods.
But one thing is for sure: the more I learn about the difficulties, the more I admire great pictures of others.

Cheers!

[cybereye]

Max,

Don't tell my wife!!!

Cheers,
Mario

[bmitchell82]

Hey max, nice crack of the whip,

10 Min exp's are the goal for DSLR's you will capture the most amount of anything without the noise ruining the party. With that said you need dark skies to do this, if you have light pollution.. GAME OVER 10 min photos might not be available to you.

:) Keep up the good work things get easier over time!


Brendan

[NorthernLight]

Hey Brendan,

if 10 Min is the call, what is the maximum? And do I get it right: there is no such thing as too many images in a stack?

thinking about dark skies: Auckers is really an improvement to Melbourne but still pested with street lights and the like. Next time I will try using a CLS filter with custom WB and see how that goes.
Sure, perfect dark skies aren´t too far away. Its more like Perth then Melbourne. But its the lack of a proper location and the convenience factor that keeps me home.

Hey Mario,

don´t take it from the far side but there is nothing wrong with being gentle to a man.

Thanks guys!

[Alchemy]

Maximum exposure is when it's burned out on your camera, load it into your cannon program and have a close look, it also depends on your iso setting too.
Never too many images in a stack
A lot of deepskystacker problems are the result of applying the stretch that comes as a default in the program, it's helpful for ccd users given the well depth but for dslr users turn it off and make sure it's not applied when transferred into your tif export image. That's all the white problem is... Easy to rectify.:)

[NorthernLight]

Hey Clive,

sounds good but I have trouble finding the check box for this feature. Where is that one buried?

[ZeroID]

Hey Max, where abouts in Auckland are you ? There a few dark sites not far from the city ( 30 -40 mins ). I've got one I want to use up in the Waitakeres by the TV transmitter mast. Gives awesome Sth to West horizon view and some Northern sky with a hill behind to kill most of the city glow in the east. There is also a group that meet up Kumeu way which I'll find the contact for if you want. I keep meaning to get in touch with them.

My back yard in Mt Albert has a 4 meter square concrete slab with a fence around. Blocks most of the low level glow and gives me quite good views high SW to West and some NWest. I'm lucky in that we're up a long right of way with no close neighbours. So away from the local street lights and late at night everyone has gone to bed so very little local glare. On a nice night I get quite good views from my 4.5" newt. Can't wait to see what my 10" build will see. And then get it to the Waitakere site.

Then all I got to fix is the cloud cover !!:shrug:

[bmitchell82]

By default i believe that DSS doesn't apply these settings to the saved Tiff, if you bring it into PS you should almost have a black image. This is where you start applying a iterative Levels and followed by curves to bring your data to the full 16 bit gamut well as much as you can in any case.

As for blowing out or Too much exposure Stars generally blow out on DSLR sensors that is the pixel well depth has been filled. You can see the effect on the stars of this happening where the center of the star is White and then as you move away from the centroid you get color back. You can move to a HDR method of stacking different length exposures (avandonk does these extremely well.) or alternatively buy a camera with huge pixel wells allowing more data to land in there before saturating.

Hope that helps

Brendan.

[NorthernLight]

Hi Brent,

I am living in Bucklands Beach near Howick. I am having no clue where that place with the Radio tower could be but from the map I reckon it isn´t more than one or two hours drive. Lets go there when the clouds eventually move away. How can I contact you?

[NorthernLight]

Hi Brendan,

my last stack arived almost white in PS too but I haven´t done anything in DSS. But the initial overexposure came as you mentioned earlier from skyglow that was probably amplified by the stacking so either I use filters or move to a dark site. - hopefully soon.
Oh well, for now I have to stick to theory as there is no thinking about going out with a telescope. I hope at least the farmers are happy with the weather.

[bmitchell82]

Interesting it should arrive almost black. because your shooting with a camera that has i think 12bit color range, where as 16 bit has more space in the histogram. Ide reset everything to defaults and see if that helps you out at all.

[Alchemy]

Should be to lower left of stacked image, bunch of sliders, I think you slide all upper to the right and lower to the left.... Play with it and it will become obvious, then save the settings.

[p1taylor]

Hi Max, I get same thing 10 minutes is most I can get and get similar effect in Maxim wish there was a way to calculate exposure.

peter

[Alchemy]

In maxim when the screen is displayed, use the screen stretch window... Ctrl H, and set the value to ( max Val) this will display it without any stretch applied.

[ZeroID]

Transmitter Tower is up in the Waitakere Ranges, West Auckland on the scenic drive. Google Earth plot is 36.55.44 Sth 174.34.05 West. It's a public viewing spot on a corner on the side of the road. Maybe park about 8 cars there. Very little traffic. About 1.5 hours across Auckland for you I guess, probably less.

Have you thought about going out along Music Point which is down the road from you. Not sure if they allow night access but you can get away from the local street lights and probably have a protected eastern view away from the city lights. Even if you can only get up to the caretakers house before the golf course entrance where I think the put a barrier across at night at least you will be elevated and should be able to find some wind/light protection.

Another option for you is out along the Whitford Road towards Beachlands/ Maraetai. Just get out into the countryside a bit. Omana Beach carpark, Duders Beach toward Clevedon. All close to you ( 20 mins ) on good road. You could almost have horizon skies to the east and north.

Just have to wait for some clear skies. Keep in touch.

[[1ponders]]

Just out of curiosity Max are you using the Autosave file that DSS automatically created in PS or are you working on the final image that you save as a Tiff in DSS as your working document.?

[NorthernLight]

HI Brent,

thanks for the insider tips, I´ll certainly check that out.

Hi Paul,

yes I am using the Autosave.tif-file. So I guess I am doing something wrong, am I? what is the difference between the autosave and the final saved tiff?

After quite a number of adjustment layers I got the below, which actually looks better than the picture in the "ABC Universe" but still suffers from chromatic noise and a bit miscollimation (or maybe field curvature?) on the left.

[NorthernLight]

well its actually on the right- I mix that always up...

[Alchemy]

I think the autosave tif is a 32 bit file and the tif save function is a 16 bit file. Suffice to say I have only ever used the 16 bit file....... On thenleft of the screen there is a save image button there somewhere, that will give you the saved tif, my version of Photoshop only works with a 16 bit file.

[[1ponders]]

I was just checking to see that you were using the autosave.

I convert the 32 bit file in PS to 16 bit and work from there.

[p1taylor]

Hi Max
that looks better,

Hi Clive

when I go to max-value all except very britte stars disappear or am I doing some thing wrong.

peter

[Alchemy]

No that's what happens when you remove the "factory settings" stretch that is applied, I personally just use curves in Photoshop to get it to a point where I can work with it, at least where the image becomes faintly visible, then do a color balance and work from there.

There are many different ways to work with images, I'm sure others do it differently.

[p1taylor]

that definitely looks better.

peter

[bmitchell82]

You will always get a good dose of color noise when your talking about DSLR's this is due to the Thermal noise exciting the CMOS sensor.

The best way to control it is do 1 of 3 things i know of.

1. Use your camera in temps closer to zero
2. Use post processing noise reduction algorithims (Noiseware, Noise Ninja ect) or Gaussian blur a copy of the image and re introduce it back to the main image though Layer masking. this will be one of the hardest to implement! but can do well for dark areas with little to no details.
3. Get yourself a Cooled CCD/DSLR :)

Good crack of the whip though!

[Octane]

A good guide to exposure on bright extended objects is to try and expose for a histogram that peaks somewhere between 1/3rd to 2/3rds across the x-axis.

Given that you use a fast scope, and, a relatively sensitive imaging device, somewhere between 5-10 minutes would be ideal.

When I imaged with the 350D, I never went beyond 5-minute exposures at ISO-400. When I got the 40D, I pushed to 10-minutes at a slow f/7.5 and ISO-400, ocassionally flirting with ISO-800 on cold nights

Take as many exposures as you can; aim for an SNR of around 4-5. SNR is achieved by taking the square root of the number of exposures. Make sure you take dark frames. Dark frames with a DSLR are an absolute necessity. Again, aim for an SNR of 4-or-so. I have had success in taking a bunch of darks before I begin, during a meridian flip (or, half-way through my planned run of images), and, afterwards. Combining them will give you a good master dark as it should cover the variances in exposures. You don't need bias/offset frames as the bias is contained within the dark, anyway. Bias frames are only required when scaling non-matching (exposure length) dark frames. I think this is a bad idea.

You should also flat frame your images, too. Use the 1/3rd- to 1/2-way across the x-axis histogram peak rule and you should be right. Ensure you take flat darks, too. Again, SNR of 4-5 for both.

If you follow these simple rules for calibration, your images will shine.

H

[Bassnut]

Auckland?, nah, the wobbly CHCH effect will wreak havoc. Emigrate.

[NorthernLight]

Thanks Brendan and Humayun for your advice. I´ll start working on it as soon as the weather permits it. But one question:
a flat is what can be done with an evenly illuminated t-shirt. is a dark flat a t-shirt-without-light flat? and what is the difference to a normal dark frame?

Hi Fred,

what is the CHCH-effect? I just moved here two months ago from Melbourne and am quite happy with the backyard in terms light pollution and free view to south and everywhere.

Cheers

[Octane]

Hi mate,

You are correct.

Think of your flat frames like a light frame. You're exposing a subject. Therefore, think of your flat dark frame, as you would a normal dark frame.

So, say, your flat frame (shot at your lowest available ISO -- typically ISO-100) requires an exposure of 1/30th of a second to give a peak histogram on the x-axis of about 1/3rd- to 1/2-way across, then, simply cover your telescope with its dust cover, and take 1/30th of a second exposures as a dark frame (matched to your flat lights). Easy peasy.

Any other questions, feel free to fire away.

H

[p1taylor]

Am I right do you have to do normal dark at say 300 secs and flat dark at same as light frames, or am wrong.

peter..

[stanlite]

normal dark is meant to be the same ISO and exposure time as your lights. And your flat dark is the Same ISO and exposure time as your Flats. All should have the same ISO. Well this is what i have been told.

[Octane]

Peter,

Example:

Light frames = 300s @ ISO-400;
Dark frames = length of light frames = 300s @ ISO-400;
Flat light frames = 1/30s @ lowest ISO capable; typically ISO-100;
Flat dark frames = length of flat light frames = 1/30s @ lowest ISO capable; typically ISO-100.

H

[p1taylor]

Hi H Another thing is Bias frames and Flat Bias frames do you do these as fast as possible, do you need all these sets of frames or only sum of them.

peter

[Alchemy]

Bias frames are the fastest frames you can shoot with the cap on, I just load a bunch of these into deep sky stacker..... I presume other programs are the same.

Flat bias ?????? Never done any as they would be the same as a bias. ( cap is still on)

[Bassnut]

As in Christchurch ;), get it?

[Octane]

Peter,

Please read my reply a few posts above where I mention I don't use bias frames as the bias is contained within the dark already.

H

[NorthernLight]

Alright, thanks Humayun for that explanation. I hadn´t heard of the dark flats before. But now it makes sense.

CHCH - ok, I get it. I thought it was yet another obstacle like a naturaly but only in NZ occuring permanent stratospheric turbulence or whatever....

No, alll good so far in Auckland-didn´t even feel the slightest tremor.

[p1taylor]

Thank you very much for your patience, I think the Penny as dropped,it makes sens now.

peter