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Old 16-11-2005, 05:14 AM
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iceman (Mike)
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Focus and settings for widefield astro shots (350D)

Hi guys.

I like to do widefield milkyway shots while the camera is sitting on the EQ platform, and obviously hope to do the same with the 350D now that I can do bulb exposures longer than 30 seconds (30 seconds was the limit of my Sony DSCP100).

With the Sony digicam, I'd use the following settings:
- ISO 400 (the max)
- 30s exposure (which would also take a 30s "noise reduction" shot)
- Focus: infinity
- 10s self timer (to avoid camera shake when I pressed the shutter)

And I'd take 3-5 or more of these shots in a row, and stack them in registar or similar, and then process in photoshop. If all went well, I'd end up with results like this which I was fairly happy with given the equipment.

So now, to do the same thing with my 350D, what settings should I choose?
- Manual or Program mode?
- ISO 800 or more?
- Bulb mode, obviously I need a remote (one is coming soon) to hold the shutter open for 1 minute or more
- My widest lens is the 18-55mm, and I've also got the 75-300mm.. I don't know that my platform will track accurately enough for anything at the tele end of the 300mm, but we'll see.
- What aperture value should I set? Widest open for the least exposure time?
- Manual focus mode? Do I just turn the focus ring while looking through the viewfinder to try and see when it looks focused?
- Noise reduction on?
- What else?

Appreciate any help you can give!
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Old 16-11-2005, 05:59 AM
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RB (Andrew)
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OK Mikeee,

Well done mate on that Milky Way shot with the other camera, it's awesome in itself.

OK.

These are the settings I use on my Canon for wide field.

ISO 800, sometimes 1600 but I try and avoid this due to noise.
I use a "Cable release" and set the camera to fully Manual mode AND SWITCH OFF AUTO FOCUS on the lens. !!!

I would experiment with your aperture settings. In my case is stop it down 1 or even 2 steps to get better star points. Leaving it wide open you may get bloated stars. But see how you go with your lens.

Focusing is a little tricky but not as bad in wide field shots say at 18mm but as you go through to 300mm focus will be critical.
At wide field I usually take it to infinity and back it off just a par sec or two *you know what I mean* (just a touch).
Check it out in the viewfinder but it's not always acurate.
In deeper field shots ie 300mm I use a right angle finder and set it at x2.5 which gives a mag view of the star. I try and remember the best focus point and mentally set it each time but it's best to put a small mark for future focusing.

In your model I would set Noise reduction on and also MIRROR LOCKUP

Also I recommend the cable type remote not the wireless remote for astro because later you will need DSLR Focus or similar to control the cam via laptop.
The wireless control is great for family portraits but for astro I prefer cable control.

Hope it helps, I'll think about what else and post again.



BTW does the 350D have the old 300D style cable plug which is just a 2.5mm stereo plug or is it the proprietry 20D style 3 pin plug?

Last edited by RB; 16-11-2005 at 06:11 AM.
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Old 16-11-2005, 11:56 AM
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[1ponders] (Paul)
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Ahh Mike, you've got a fun time ahead of you.

For imaging point sources, aperture is more important, for extended objects then f ratio is more critical. So if you consider a widefield shot as basically an extended object then the faster the lense the better. Others might argue th point that a widefield is a collection of pointsources then so be it, but from my limited exprience I've found the faster the better, and then stopped down a half stop or two.

Focusing is fun especially with the zoom lenses as the infinity setting doesn't really focus at infinity you do need to back off a bit. (that and finding someway to keep the zoom ring from moving when your focusing. I use a lump of blu-tak to hold it in place) Check out the image here on John Drummonds site for focusing using longer lenses. For me focusing through the view finder is very hit and miss. My astigmatism makes the view look closely focused but when I take a test shot it can be way out. Hence my dependance on DSLR Focus. Test shots and viewing on the LCD does help if your doing it manually, but I'm an automation junkie

http://www.possumobservatory.co.nz/a...5-focusing.htm

Have you checked the accuracy of your platforms tracking Mike? It might be worthwhile downloading something like K3CCDTools V2 with the free monthly trial and using the drift explorer to get an idea of its accuracy. Can you manually guide your platform in RA? You're DEC is non-adjustable once your latitude is set isn't it? It may be a lot of mucking about but with the K3 you could do a drift align, certainly for your altitude ajustment (I'm assuming it has the capability of fine adjustment and not just notches for different latitudes), not sure how you would do it to get it fine enough for your Az ajustment though.

JM2CW
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Old 16-11-2005, 12:19 PM
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Striker (Tony)
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Settings will differ depending on your lense....it will be all trial and error.

For example my setting differ tremendously just between a F2-F4 lense for the same time exposures.

Just start with some 30 second exposure at say ISO 800-1600 then experiment.

Yes remember to turn everything off auto focus...yes I did this once, left my lense on auto focus and couldn't work out what was wrong....lol

I dont have the focus problem with the added zoom function on the 20DA....sorry thats being snobish....lol
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