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jgs001

M3 - Rough edit - New Restack added and a question

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I had a go at M3. My first attempt at a glob. I used the Skymax 102 mounted on the HEQ5 for this and used the CLS filter, there appears to be a green cast left in the image. I had intended on getting at least an hours worth of data on this, but my powertank gave up after only 14 subs... ho hum. Roll on the mains power supply, then I'll be able to use dew strips too. This was my first attempt at imaging Deep Sky with the Mak, and it's worked far better than I expected.

14x60s @ISo1600, 5 darks, 7 flats and 7 flat darks, log sqrt stretch and a small neaten in GIMP. I'm on the lappy which doesn't have PS on it, I'll give it a proper edit later.

I've no idea why I have a -ve vignette, I can only think that it's down to the numbers of subs. A restack with less flats may be in order.

m3.jpg

Restack with only 3 flats and flat darks. Same processing in GIMP.

m3restack.jpg

Edited by jgs001
Restack Added

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Very nice John. By coincidence I also had a go at M3 last night but haven't processed it yet. With a bit of luck I may do by the end of the day.

Dave

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Thanks Dave. I've had a quick restack with less flats and that seems to have helped with the vignette a bit. Good luck with your data.

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If you are taking the shot prime focus without an EP ie: then there shouldn't be any vignetting. Likes more like banding which occurs and signal noise. Try dropping you ISO to 800 and apply the same number of darks and flats as you have subs.

Cheers

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Trevor, it's vignetting. It's a 1.25" fit on the Skymax, same as configuration as I used to use on my ST80 alike and I always had vignetting on that too. I don't see the vignetting on the C80ED with a 2" fitting. I've also frequently used ISO1600 with darks and flats just like this, only much more lights and not had the vignetting or noise in the end result.

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I've just restacked without the flats. They introduced those vignetted corners as far as I can tell, also the colour cast. I don't get it :thinking:...

I always got vignetting using the same camera attachments to my Konus, yet there's none evident in this image from the Mak, only when combined with the flats. Anyone have any ideas ?

I took flats in the same way I did for the M44 image... and for all my images previously.

m3restack2.jpg

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Cheers Jamie.

Some more info.. The flats clearly show vignetting that doesn't appear in the lights... This is getting more and more confusing...

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I've found the cause of this. The first flat, that was shot on Aperture priority mode, despite having identical EXIF information, shows the peak at a lower point on the histogram. I don't understand why there should be a difference between that and shots taken in Manual mode but that was it. The one's shot in manual show the peak on the 80% line, the Av mode shot shows the peak at about 70% across the Histogram. I need to reprocess it all properly now.

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Hi John.

Its a nice image, M3 is a great cluster, and you have processed it without ruining the core, lovely.

You really need to be looking at 20+ flats if you are going to use them (well worth it). I usually take 25 or so. Also, I think as you realise, they all need to be taken with the same settings. Having more of them, the average is a more accurate result. You dont mention taking BIAS frames though? I would drop the dark flats and go with BIAS, else the flats wont work correctly as I understand it. Just take 20 odd exposures at the usual iso and fastest shutter speed setting you have. The bias frames get subtracted form the flats and lights I think, as they show a signal that shouldnt really be there, helping to make the whole pic cleaner and enabling you to better extract the detail while minimising the noisy signal.

Keep at it, these pics have real potential!

Cheers

TJ

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Thanks TJ. I'd been hoping for at least 60 light frames, but lack of power wiped out that plan :).

What get's me, the flats were all taken with the same settings, yet the histogram was varied from Av to Manual. I don't understand that, but hey ho, the only thing I can think that might have affected it is the metering mode, but that really shouldn't make a blind bit of difference whether the camera is averaging across the entire frame or spot as I understand it for flats.

I didn't take bias frames, I read, I can't remember where, might have been the DSS help, that you only need either bias or flat darks. As the darks contain the bias information already. I did shoot bias frames once, but they caused a horrendous green cast across the image, when I dropped them from the stack that cleared.

It's another learning curve, capturing data through the Skymax compared to the Frac's, just when you think you've got it sussed you change the gear and back to square 2.

Thanks for the info, I'll try that instead next time.

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Hmm, i'm not certain about that John. Could well be wrong, but I was under the impression that the dark flats are to remove noise from the flats, and would therefore theoretically have to be the same exposure length as the flats to be accurate. In which case, there woud be BIAS signal + some (albeit limited) noise signal from the longer exposure.

I bet Dennis (Roundycat) knows the answer! Either way, I found that my images from both dslr and ccd camera improved no end with proepr flats and bias applied.

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I'll give the bias frames a try next time around.

The flat darks were the same exposure length. I do basically the same thing as my lights, set the camera up then use the remote. Shoot the flats, cap and shoot the darks. It's what I've been doing since about october last year with the Konus images, then the C80ED images. This is the first one, new scope... that's done something weird since.

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Trevor, it's vignetting. It's a 1.25" fit on the Skymax, same as configuration as I used to use on my ST80 alike and I always had vignetting on that too. I don't see the vignetting on the C80ED with a 2" fitting. I've also frequently used ISO1600 with darks and flats just like this, only much more lights and not had the vignetting or noise in the end result.

Sorry I wasn't aware of your setup of course using a 1.25 adapter will induce vignetting.

The school of thought generally amongst Canon camera users is that there is nothing gained by using ISO1600 and infact more noise is induced into the image at higher film speeds.

Cheers

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np Trevor, sorry I didn't reply earlier. I've used ISO1600 in the past to overcome some of the limitations of using AltAz tracking. Get enough subs and the induced noise doesn't seem to be a problem, ok 154 subs on a cluster may be a bit excessive :).

Thanks also Mark.

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John, I just looked into this thread for the first time, I tend to ignore anything about DSLR's and DSS because I cannot offer any help on either topic. Except for one thing, I do know how light meters work. How are you taking your flats? Getting a different answer from auto metering compared to manual is no surprise. Are you photographing a white board or the twilight sky. Stick with manual metering whenever you use a camera unless you really are shooting from the hip.

Flat darks (same exposure time as your flats) are the best method of dark subtracting your flats but it is only really important if your flat exposures are long. I think it is a bit of a lottery anyway with a camera that is not cooled because the signal uncertainty (or noise) in your flats is hardly likely to be exactly the same as your flat darks. You should subtract bias from the flats using a master bias of about 30 frames. Unfortunately those bias frames will vary a bit because of the lack of temperature control.

If you get odd results when subtracting bias from a one shot colour camera try using a hot pixel filter on each flat before stacking them. Also try stacking them using a sigma reject type method as that will control random hot pixels very well. Such an algorithm will not adversely affect your flat quality.

Strictly speaking, for good raw frame calibration you should have at least 15-20 darks taken at the same temp and stacked using a sigma reject type method, that is your master dark for a particular exposure time. Subtract it from each frame first of all.

Shoot 15-20 flats but they must be pictures of an EVENLY lit target or the twilight sky. Those flats should be dark subtracted but if they are all short exposures you can get away with just subtracting bias from each flat before combining them. Keep the flat exposure up so they are out of the noise, half to two thirds max. Apply the flat to the dark subtracted raw light frames.

Align and combine the calibrated light frames to make a master and save it as a TIFF. Work on it in Photoshop.

Dennis

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Thanks very much for all the info Dennis. My flat taking I'm sure isn't ideal. DSS suggests using Av mode to get the flat frames. What I tend to do is take one to get an exposure length, then set the camera to manual and shoot a load. They are typically quite short about 1/3s. This was also the first and so far only time I've used the Mak for deep sky imaging, previously I've only used it a few times for the moon and Saturn, so there's a few (probably loads) things I need to work out.

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Nice shot John, I'm sure you'll iron out any problems.:)

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Thanks Gaz. It's that old chestnut of changing things... always messes you up :)

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