a half decent focal length for M45?

[joshyouarebaker]

I am wanting to go out this evening before the moon comes up (around 19:00) and take some shots of M45. I have a nikon D5000 and the highest focal length i can use is 200mm. If i were to take sufficient exposures (either lots of smaller ones or a few 10 second ones or so), would 200mm and 5 mins exposure be enough to capture the nebulosity that surrounds it? Or is 200mm not capable of this?

Cheers 

[Knight of Clear Skies]

Yes, 200mm gives enough magnification to see the Pleiades clearly and it's possible to pick up the nebulosity with short subs. Here's my quick effort at 135mm and f4.5 or f5.6 using 2 minute subs.

(10 minutes total, 5 x 2 minute subs. There is a bit of a gradient as it was low down when it was taken at the end of August.)

Edit - Sorry, re-reading your post again, are you shooting from a fixed tripod or using a tracking mount? I read your post as 5 minute subs, but looking again you are talking about 10 second exposures so I guess you are using the former. You won't get 10 second subs at 200mm due to trailing, something like 1.5 seconds is more realistic. This will show the bright stars clearly but picking up any nebulosity may be a struggle.

[joshyouarebaker]

stunning picture! thanks for that.

Without a tracking mount and just a sturdy tripod, if i were to take about 100 x 3 second exposures (giving me 5 mins), would this be enough to capture anything that slightly resembles your picture!?

Thanks 

[Knight of Clear Skies]

Thanks, I hope to re-shoot it at 200mm sometime this winter.

3 seconds at 200mm would give quite bad star trailing I'm afraid. I did have a go with my 50mm lens once from a fixed tripod and got this:

It's about seventy 3.2 second exposures at f2.5 or f2.8 stacked, the final frame has been cropped heavily due to coma. I could probably have pushed the exposure time a bit longer at 50mm. There is a tiny bit of nebulosity showing around Merope.

Hope that gives you a starting point. You may get better results with a shorter lens than the 200mm from a fixed tripod as this will allow you to increase the exposure time.

[joshyouarebaker]

it sure does! I will try at 120mm with about 50 x 5 second exposures. Just a quick final question.. Is there a minimum ISO one should use to capture any nebulosity? The nikon i have (well, it's work's) does have high ISO settings, but i dont want so much noise that my image appears gritty. Would 1600 be OK, or would  3200 be better if i can get away with it?

Thanks again. I'm hoping the clouds clear :/

Cheers

[Willi1972]

Hi,

I'm by nor means an expert but by using enough images in the stack, you will get rid of noise. I regularly use ISO 2500 on my D7100 and get no noise after stacking. Obviously, the higher ISO you use, the more subs you need. Also, don't be afraid of stacking what seem to be ruined images. I have had several sessions where my subs appeared very overexposed to the point where they just looked pale blue. However, stacking enough images will remove haze due to high clouds, as it appears as noise to DSS. Read and experiment!

Regards

Richard

[dph1nm]

Is there a minimum ISO one should use to capture any nebulosity? The nikon i have (well, it's work's) does have high ISO settings, but i dont want so much noise that my image appears gritty. Would 1600 be OK, or would  3200 be better if i can get away with it?

Despite what many terrestrial photo magazines would have you believe, changing ISO doesn't affect the sensitivity of your camera, it just changes the gain, so it doesn't really matter what you choose - the signal to noise ratio in your final image will be similar and you don't need to change your total exposure time.  So the only two considerations are that at very high ISO you lose dynamic range, and at very low ISO you risk quantisation of your image (as it takes several incoming photons to register one count). So stick to something in the middle!

NigelM

[ollypenrice]

200mm is excellent for M45. I used it to go after the two extended arms of reflection nebulosity.

Focal length dtermines field of view but not the depth to which you can reach. This comes above all with time.

Olly

[bluemaxroe]

Olly, what lens / scope dis you use to get that result? 200mm with 10min exposures? maybe 20-30 or so? Would be great to know....its stunning.

[laser_jock99]

Longer exposures are the key to getting results like Olly's - but to achieve that a half decent tracking mount is a must. The nebulosity immediately surrounding the Pleiades is quite bright and you should pick up something even from a light polluted site with say 2-3 minute exposures. To start showing the faint outer wisps as in Olly's picture you'll need to shoot from a dark sky site and use much longer exposures (say 10 minutes or even longer). Unfortunately with blue reflection nebulae it's not easy to use a filter to get rid of light pollution (although some kind of general LP filter will help a bit).

As for the focal length 200mm+ is a good starting point as it's a large target.

At 1000mm FL you start to loose the cluster's outer stars! 

EDIT - This image is a stack of 5x 3 minutes exposures (unguided)

[hjw]

I did one with a Pentax K-30 and a 300mm lens on a tripod a few days ago (my first attempt) Its 73 frames f/5.8 ISO 6400 and 10 sec exposure time each. I had to push the image pretty hard to get the nebulosity. I reckon another 73 frames would get me there . Hope this helps.

[bruno]

Have a look at this it may be of some use http://www.astropix.com/HTML/I_ASTROP/LENSES.HTM & give you a general idea.