M81 and M82

[Ant]

You have no idea what hopes I had for this image.

You have no idea how despondent I was after the first stack/process...

This is 75 x 90 seconds with the ED80 and Canon 300D. No darks (forgot ). ISO1600 with LPR filter in place.

I have processed this a little better now and have regained a little of the colour in the cigar - I am running Darks as we speak - the temperature outside is roughly the same as last night - so should be OK.

(click to enlarge)

Now the question I need answering is - Will more Subs give better results (like, much better results)?

Ant

[CELESCOPE]

wish i could help with that Ant not being a dslr user i,m not to sure , i would think the subs are ok , and i can see you have a lot of detail in there as well , so maybe as u say a few darks , and flats would have improved this image a lot , lets see what the DSLR boys think

Rog

[Ant]

Had another go ad stacking and processing.

This time I used Darks and Bias frames as well. I think there is an improvement... Although I was still disapointed when I looked at the image again this morning...

(click to enlarge)

Ant

[MartinB]

Don't be so hard on yourself Ant. The trouble is we have the image we would like in our mind and feel disappointed with what we get. Then the processing gets more and more brutal in an attempt to get to where we think the image should be!

You have a decent image there with the arms in M81 showing up well but the core detail in M82 not blown out. The darks have certainly cleaned up the background very nicely but, to my eye, you've just stretched the image a little too much. I preferred to more subdued process of the first one.

Roughly (lets not get into a pedantic techie discussion about the nicities of this) doubling your exposure time will double your signal to noise ratio. Another couple of hours will make a huge difference taking out a lot of noise allowing you to be much more aggressive with the processing. Depends whether you're after quality or quantity of images. Personally I think that image is well on track to being a beauty and just needs more time.

[Ant]

If I went out again tonight and got another load of subs - gonna try for two, two and half and three minute subs. How do I add them into the existing data?

I'm using DSS.

Ant

[MikeD]

detail looks good to me.

What were the skies like for you Ant, it could be down to the conditions, my images last night were a lot more noisy the i would normally expect.

Mike,

[trevboyd]

Nice image, Ant. I see we're currently trying the same thing! Do you know yet if more subs will help, and do they need to be longer exposures than the first set, or will a load more of the same length do the trick?

Trev

[Ant]

I've mist and fog for the last few nights... my NE is over westcliff / southend - so not good at all.

I've needed the LPR Filter for even 30 second shots - so that hasn't helped.

I think that the more the merrier Trev - I'm going to try for more tonight and wil try for longer ones as well. Certainly can't so any harm...

I've been chatting things through with Russ this morning and I think a loit of the problems boil down to not having enough money to throw at the problem.

The 300D is not a good match IMO for the ED80. Fov of view too big! SO I can change the camera (get a dedicated CCD) - or change the scope...

So my plan after talking it through with Russ is buy a 150PL OTA. This will give me the following advantages...

1. Smaller FOV

2. More light gathering power.

3. A scope better suted to planets and lunar.

4. Only a slight drop in F/ Ratio (F7 to F8)

5. A small investment

6. I can then image through one and guide through the other.

There is one disadvantage that I can think off.

It has a 1.25" focuser - this might be an issue with the 300D. A bridge that we can cross when we get there...

Ant

[trevboyd]

Sounds like a plan! My 350D is attached to a 1.25" adaptor, and seems OK. Not sure if there'll be vignetting if I go for something like M31 that will more than fill the FoV, but nothing obvious so far. Might end up getting a 2" t adaptor (and of course then a 2" LP filter to go with it!)

I agree that the more money you spend, the better the results can potentially get (though there's a huge amount of skill and experience involved too!) I don't have time to commit to really serious imaging with guiding etc, so I'm happy at the moment to "tinker" with the fairly modest setup I have and just have fun!

Its interesting to see what others are producing with similar if a bit more expensive equipment (you and Cosmos) and compare them.

Trev

[steppenwolf]

Ant, I use a DSLR and an astro CCD and both give much better results in proportion to the number of subs. To steal a phrase from one of my other pastimes, 'there's no substitute for subs!'

[Ant]

I completely agree Steve... The more Subs the merrier, but I'm guessing there'll be a point where more subs make no difference and it's longer subs that you need (or more light gathering power or a more sensitive chip).

I'm going to get more subs tonight - but I think that I need to consider if the 300D is the best match for the ED80.

Ant

[Ant]

Sent the image to a mate of mine, and he ran it through Ninja Noise (?) I think it's an add on for PS.

and came up with this...

(click to enlarge)

Again better.

Ant

[MartinB]

Yes, that's a good result.

With 2 hours you double the signal to noise ratio compared with 1 hour. From 2 hours you need go from 4 hours and then 8 hours, so yes it is the law of diminshing returns to some extent.

As for sub length - the reason for long exposures is noise. There are 2 sources of noise - r"ead noise" which is inherent in the system and is a constant amount regardless of exposure length. The other is "shot noise", this is the noise which accumulates as you increase the length of exposure (yes - noise increases with length of exposure!). Fortunately signal strength increase much more rapidly with exposure length than does noise. So, with long exposures you get a much better signal to noise ratio.

Stacking short exposures has exactly the same effect as running longer exposures as far as shot noise goes. The problem with subs which are too short is that read noise isn't reduced by stacking. You need to go for a long enough exposure to make the impact of read noise insignificant. In practice in the UK our ideal sub length isn't determined by getting rid of read noise so much as problems with light pollution. Once light pollution exceeds 2500-3000 ADUs in a 16bit image read noise is swamped anyway

That's a very round about way of saying that your sub exposure length should not exceed the point where light pollution starts to effect the image significantly.

[trevboyd]

That's very helpful Martin - makes sense and nicely put! Thanks

Trev