Beginners photo critique

[bonox]

Thanks mostly to this forum, i've been encouraged to take up a little imaging.

Last night, after much reading, I started my first attempt at drift aligning a telescope. Much to my surprise, after about an hour, I was able to get a mount that took about a minute to move off the 9mm reticle line. Not too bad I thought for someone who can't see closer than 40 degrees to any horizon.

So, whacking the camera on the scope, I proceeded to do my first manual guide photo over sydney city, and ran into my first interesting problem.

http://i3.photobucket.com/albums/y77...0885Medium.jpg

I was pleased that even with a modest 85mm lens, I was able to keep the stars as circles on this f/2 shot at 90 seconds with little correction. Question number 1: is that well and truely sky fog in that shot, or have I done something else wrong? I don't have any filters at this point, so is there anything else I can do? Shoot at a smaller aperture, take many 30 second shots and stack them? Just give up until I can get to a dark sky site?

The second attempt was more challenging. Mounting a 500mm mirror on the camera, I found the next challenge being to focus the thing. A bunch of 10 second exposures got me this one (full size 500pixel wide crop of original frame):

http://i3.photobucket.com/albums/y77...901cropped.jpg

Obviously, Jupiter itself is overexposed, however any less and the moons don't show up. Is the solution to this to mix long and short exposures?

This was taken at 9pm last night (after a 29 degree day at an elevation of no more than 30 degrees over the building opposite) so the seeing wasn't exactly great. It is however a great inspiration to me that I've managed to produce an image of something!

So, what am I left with? Are these things that will improve with practice or should I give up now? Can I reduce the effect of sky fog by using longer focal lengths (narrower fields) or does this have no effect at all?

At this point, dark sky sites are out of the question due to other commitments and i'm taking the opportunity to practice even in the lights of a big city centre so that I might know what i'm doing if I ever do get out to a dark sky site. Thanks in advance for the criticism.

[h0ughy (David)]

yes it is sky fog due to light pollution, you will need a uhcs filter or other light pollution filter to get any success. I feel for you, that much light pollution is shocking.

[g__day (Matthew)]

The other two things you can reasonable do include:

1) stacking shots (Deepskystacker - freeware) take shorter duration shots and sum them up to make a better than equivalent longer shot (providing your target isn't so faint you are stacking only noise - not signal)

2) re-set your dark point higher (levelling) and boost / amplify the lumonsity difference between lighter and darker regions of the sky (non linear curves). Programs that do this are generally available - although I only know of expensive ones that do many other things - e.g. Photoshop CS2.

I'll expand on the second point a bit. Imagine sky glow causes a 10 minute shot to have 1 billion photos recieved per pixel in a dark area of the frame, whereas a real star that is faint boost the pixel it falls on to 1.2 billion photons and a bright star boost the photon on its pixels to 25 billion. All this data is recorded and sent to software that works out what to display - software that is generally written for day time use. You end up getting the image you see.

What you'd like to do is say any pixel lower than 1 billion photos - make it pitch black, and above 1 billion is a star, and if one is 25 times brighter - don't make it so bright the air around it glows and you get too much bloom swamping your picture.

So in a raw astro image you might get intensities of light that vary by a hundred million levels of intensity - you wish to throw away the junk and allocate what remains in a sensible way - that changes reality - by boosting very interesting faint details - like planetary nebulae e.g M42 Orion and toning down over bright areas - major stars. So for Orion the end effect might be altering the data of stars and glowing gases so instead of having a billion levels of light intensity between very bright and very faint objects - you end up with only 10,000 levels of light intensity.

You are cheating - boosting what you wish to see and dimming what would otherwise swamp shots. Welcome to astroimaging techniques!

Recommend reading the Zone System for Astrophotography by Rod Wodaski.

[bonox]

Thanks for the info g_day.

Back to the books I guess to learn how to use photoshop.

[g__day (Matthew)]

Simple level and curves will do most of what you initially want. For instance you shot was a bit tricky - signal to noise was poor - so it took 2 minutes (5 iterations of levels then curves - each round less aggressive adjustments) to do this with your first shot:

BTW - Jupiter's moons look like your focus is out. I think Jupiter is often a combo of 1/30 of a second shot - say best 50 of 200 - stacked and aligned. You can then do a 2-3 minute shot - subtract out Jupiter leaving just its moons and add this to your earlier shot after you remove teh moons from it if need be.

[bonox]

That's not as bad as I thought. Seems to be a lot of hot pixels as well though. Thanks for the demo of what can be achieved.

I finding the focus thing is still hard. It was taken with a 3" camera based SCT which doesn't give any spare light for a hartmann mask. I guess the best way to do this is taking test shots? Adjust, photo, check, adjust?

[Zuts]

Originally Posted by bonox

Thanks for the info g_day.

Back to the books I guess to learn how to use photoshop.

Hi Bonox,

Here's what I have to put up with in Glebe light pollution here is horrible. My photo is with my 400d on an eq6 at about 18mm. I can only really go for about 30 seconds before things get to bright.

1st photo is the original, 2nd photo is photoshopped, but only moved the black point a bit.


Paul

[bonox]

Hi Zuts - it would appear that what you can do with digital stuff would make the film photographers of yore start cursing. Mind you, they probably didn't have the same pollution problems as now.

I'm in Alexandria, so we are fairly close together in terms of the same light sources - have you tried any filters here?

[Zuts]

Originally Posted by bonox

Hi Zuts - it would appear that what you can do with digital stuff would make the film photographers of yore start cursing. Mind you, they probably didn't have the same pollution problems as now.

I'm in Alexandria, so we are fairly close together in terms of the same light sources - have you tried any filters here?

Hi Bonox

I have used an Orion Skyglow, Astronomics UHC. Both are Ok but the Orion gives reflections around bright stars while the astronomics is far higher quality but you have to image for a long time as it cuts out lots and lots of light. I have just ordered a Badder Contrast and UV/IR filter and plan to use them when they come. I am hoping the contrast booster will cut out a bit of light pollution and still let in lots more light than the Astronomic.

I have uploaded a shot of M42. This is 300 seconds at prime focus on my ed80 and 400d with the Astronomics filter. No processing in photoshop at all, just used irfan view to convert from raw to jpg at 70 % quality.

Paul

[ballaratdragons (Ken)]

I applaude you guys for getting anything in amongst all that Light Pollution



I really do take my dark sky for granted!

[bonox]

Originally Posted by Zuts

I have uploaded a shot of M42. Paul

Thanks for the filter feedback - i've only used a UHC visually, and had no idea about what the filters can do photographically.

I'm impressed by that last shot - we can obviously get something to work with, which is a start I guess. Beats not being able to see the milky way!

[Zuts]

Originally Posted by bonox

Thanks for the filter feedback - i've only used a UHC visually, and had no idea about what the filters can do photographically.

I'm impressed by that last shot - we can obviously get something to work with, which is a start I guess. Beats not being able to see the milky way!

Thanks Bonox. Yes you can get something but i mean... 300 seconds and I cant even see the running man nebula. Stacking sometimes helps so you should try that as well. Anyway i am waiting for my autoguiding cable to arrive then i may be able to go longer. When i have sorted that out and the new filters i mentioned i will post some more photos.

Here is the previous image after 2 minutes work in photoshop, levels only.

I have a C11, but the sky is so bright here i dont even bother looking through it in Sydney, so to keep the astronomy habit i spend my time imaging through the ed80.

Paul