Need advice for Astrophotography beginner

[perry314181]

Recently, I purchased a Celestron Omni XLT 120 refractor scope with tracking motor. I also bought the appropriate T-adapter and T-ring to mount my 40D on the scope. After hours of reading on how to polar align, I picked Orion Nebula as my first object to shoot. The result is disappointing..

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This is a single exposure, ISO 200, 30s exposure, shot through the telescope.


396381

This is done by having my 40D + 70-200 f/4L IS piggyback on the scope. ISO 200, 7 exposures, 1 minute each, stacked in DSS, adjusted in PS CS3.


My frustration with the result is they are just too blurry. I am thinking this is due to bad polar alignment. I have ordered a polar finder for the scope and hopefully once I get a more accurate polar alignment, things will get clearer. Something to add, I live in the heart of San Francisco Bay Area, light pollution here is very severe.

I need some major advice on how to clear up the photos a bit. Should I take shorter and more exposures or the other way around, longer and fewer exposures? And should the ISO be bumped to 800 for optimal result? Thanks for any tips in advanced!

[Nighthound]

This is a very good start. Nobody has complete success starting out so don't be discouraged.

First, yes up your ISO to 800 and in cold weather even 1600.

The first image is what I would expect at ISO 200 and 30 seconds.

Critical focus can be difficult at first. There are tools available to help you with that(see mask below). But keep in mind that there will be some nights that even the best focus won't be all that effective due to atmospheric conditions.

Bahtinov Style Focus Mask can be purchased:
http://www.lumensa.com/Lumensa/Focus-Masks.html

..or made(free template):
http://www.focus-mask.com/Free_Templates.html

A down and dirty way to help with focus is to use some black nylon fishing leader(30-60 pound test). Take two pieces and criss cross them in front of your telescopes main lens. Masking tape will do fine to hold them in place. These will create visible star spikes on bright stars in the camera viewfinder. Once you see them get very crisp on a bright star you can leave them in place if you want spikes on stars or remove them to image star-spike free. Just be careful not to touch the scope focuser once you have those star spikes sharp. It's a good idea to do a test shot before removing the fishing leader if you don't want spikes.

In addition to higher ISO you're going to need longer exposures than 30 seconds to one minute when imaging nebula. I realize you're restricted by your tracking capability but that's the nature of the beast. Stacking 300, 1-minute exposures will never resolve the same detail/depth as 60, 5-minute exposures. It's all about combining the most data(photons) per exposure in the stacking software. In other words, DSS as effective as it is, cannot produce data, only combine and output a file from the data supplied essentially raising the "existing" signal and lowering the noise in the process. If the individual sub exposures don't have the data, the end result won't either.

On a side note, something to keep an eye on when processing. You images show a clip of the black point. The histogram shows all channels all the way left on the graph. This means that much of the faint signal and detail has been deleted by the flood of black in the background. A lot of nebula detail lies at the very low end of the histogram due to the faintness of the light there. When the black point gets clipped, that detail is lost. Going pure black on the background hides noise and really makes things pop but it also scraps a lot of the signal that you're working hard to get. Space is not pure black, countless stars alone make sure of that.

There are light pollution filters out there that can reduce the bad light considerably. Hutech makes some good ones, I'm not sure if they make them for your set up though.

For CS3 I highly recommend Noel Carboni's Astro Actions. They help a lot with noise reduction and other issues with DSLR imaging.

Keep at it, you're doing great.

[perry314181]

Great response!!! Thank you for all these tips. I didn't know such trick existed in order to get the focus down. Even with live view, I was struggling to get the correct focus. Definitely going to give that nylon fishing leader a try. :D

Thanks once again! I will return with new results. :)

[spit]

orion's hard to do, the core easily blows out while the neb requires extended exposure- its best to start off with clusters and get your focus right along with polar alignment