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Astro Photo Help Needed


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Hi,

I've got a Canon 400D unmodded.

Also i'm using a Meade LXD75 mount with an 8" SN scope.

I've seen others take some good pics with this setup so i'm hoping it's not the gear.

All my pictures of deep sky objects (the only thing i'm interested in) come out really grainy. I've tried various ISOs but not higher than 400 just yet (bad weather limited me.)

I've enclosed one image.

I was trying to photograph comet elenin last night and this is what i got.

I took about 8 shots of varying length from 5 sec to 2 mins.

I've tried to follow a basic tutorial on registax and also tried DSS.

Both with little success. Maybe the original images are useless?

I don't know much about the ins and outs of astro photo (clearly.)

There are no clubs here to join and no one to ask.

Does anyone have any ideas?

Or perhaps a link to a BASIC tutorial on astro photo?

Thanks.

Mark

post-25761-133877612243_thumb.jpg

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Mark, first off, the image is showing some trailing, which probably means DSS would only stack one of the frames anyway, so you're not getting the increase in the signal to noise ratio of the stacking. It also looks quite light polluted, you might want to look at getting an LP filter of some sort. Registax is not designed for and therefore does not do as good a job on deep sky images as DSS does, so you should only be using DSS. It's also pretty warm at the moment, so the banding could well be from the heat, I was getting that a month or so back as the air warmed up. I would suggest you go with ISO800, and try and work out the best exposure time to minimise trailing. You probably also need to look at setting up guiding, as I'm guessing the 8" has a long focal length, which taxes the tracking more.

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There are some really excellent primers in the Imaging section. Also, Making Every Photon Count should be a mandatory read for all imagers.

The comet that you are trying to image is very, very faint. 5 second exposures will not be anywhere near long enough. Have a look at this thread the total exposure was 35 minutes at ISO1600.

There is also some tracking errors on your shot, so your polar alignment is a bit out as well.

The very noticeable noise in your image is probably a result of the short exposures (read out noise). More and longer subs will help....especially as DSOs are very faint. Try and aim for about an hour minimum per DSO, though some will need a lot more (in my very limited experience, objects like M101 need a minimum of 4 hours). The noise won't be helped by the warm nights increase the heat of the DSLR sensor either.

Keep at it...it a difficult path you have chosen, but very rewarding. have a bash at a brighter target...try M13 or M92 in Hercules

<edit> John got there first!

Edited by Zakalwe
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Hi,

I thought the trails were that because I was tracking a comet?

Maybe i'm over estimating the movement in time?

No light pollution here at all. It pretty dark @ 11:30PM

And the comet was South East ish so the sky wasn't as dark as winter but it was pretty dark to the eye.

Guiding is something i need to read up on and see if i have any options.

I will try DSS now, so that's save me some time , thanks.

I'll try ISO 800 also.

I know it's a CMOS camera but it seems to be very speckled on even 15 second exposures, i didn't expect that really. Maybe 5 minutes...

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Hi,

While I was setup last night I took a few extra shots just in case elenin eluded me. I got M101 (faint) and The Ring Nebula.

Last week (when the cloud went for one day) I got a shot of Hercules.

But all suffer from the same problems.

I'll try that book but all the books i've bought so far assumed too much knowledge and are probably never going to get opened again.

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The camera can make, what you're eye takes as really dark, like day. I captured an iridium flare last year sometime, I thought it was dark, but the 30 second exposure of the camera made the sky bright blue. For comets, you still use sidereal tracking, so the stars should still be pinpoints... unless you're guiding and locked onto the comet itself.

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Hi,

While I was setup last night I took a few extra shots just in case elenin eluded me. I got M101 (faint) and The Ring Nebula.

Last week (when the cloud went for one day) I got a shot of Hercules.

But all suffer from the same problems.

I'll try that book but all the books i've bought so far assumed too much knowledge and are probably never going to get opened again.

You will need more subs to get rid of some of the noise. As others have said though, using an uncooled DSLR will result in more noise in these warmer nights. I could see my 50D was a heck of a lot noiser once the ambient temperature rose above 10C

Get the making Every Photon Count book...it really is superb for the starter.

As for the tracking, the comet is not moving that fast that you would track it.....what tracking does is track the apparent movement of the sky as the Earth rotates. At this point, I would suggest not jumping into guiding. Take some time, and learn how to get your polar alignment much better....there is clear evidence of poor tracking in your earlier shot. A guided image will still need good polar alignment.

Keep at it.....DSO imaging takes time. I have attached one of my M101 shots....there is over 2 hours of exposure in there, and that is nowhere near enough.

post-21666-133877612257_thumb.jpg

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The tracking of the mount, will be, as Zakalwe said, based on the rotation of the Earth, so you should be getting pinpoint stars if the tracking is accurate enough. The Ephemeris data gives the location in the sky.

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Steps that I would take (and have taken as I learn about imaging):

  • Polar Alignment: Have a look at Astro Baby's tutorials, or use a PC based PA application. The tracking is off, so this needs work.
  • Basic processing: Taking some darks would help with the very obvious banding that the DSLR is giving you.
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Yeah my polar alignment is a mess certainly. My problem is getting level ground. I'm working on a gravel yard and it's terrible. I'm going to build a concrete base and make sure it's dead level, this should help no end. Dragging it across undulating gravel is likely to be the culprit. It was alighned perfectly with the pole star but the levels may have been out, I found out later that the bubble in the level that came with the scope had actual come off, so i was working on faulty readings. Now i've ditched that and got a decent level from th shed it should help, might try again tonight, but these late nights are good for me !

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You are getting there. You are able to get the DSO in the FOV, something that I don't always get anywhere near!

The alignment looks better in that last shot, but there still is some trailing. Be ruthless when selecting the sub-exposures to stack...if you can see trailing, then bin them.

And there is a huge amount of noise in that image. more subs are needed, and probably some dark frames.

Once you have that lot sorted, then you have to wrestle with processing them (another story altogether!)

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One thing I don't know, is ...

What is a SUB, A DARK, A LIGHT and so on.

I'm beginning to wonder if i should have bought the DSI2 instead of an SLR.

I just thought the SLR would be handy for other stuff too.

I've seen some good images on here from unmodded dlrs.

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Just a test (to show i've been paying attention) & get the experts here to confirm :

Sub - individual picture of what you are taking

Dark - exactly what it says - I think put the cover on your scope (take on the same night, before you put your kit away).

Light - search for light box - it shows the dust bunnies & will (if i've understood right) remove them from your pic, when you stack

What i'm still reading up on, is how many to take of each & does it change with what you are taking ..............

Edited by Scarlet
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Actually,

sub = light = the actual data you capture

dark = as said, cover the scope and take a series 10 or 15 at the exact same settings as the lights

Flat = The light box... although if the aperture of the scope is small enough, notepad maximised on a laptop screen will surfice.

Bias = a dark of the shortest exposure length your camera can manage... probably 1/4000s at the same ISO as the lights/subs..

A Meade DSI needs these frames too... and you can't use a DSI for other stuff.

The help in DSS is very good for this.

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One thing I don't know, is ...

What is a SUB, A DARK, A LIGHT and so on.

I'm beginning to wonder if i should have bought the DSI2 instead of an SLR.

I just thought the SLR would be handy for other stuff too.

I've seen some good images on here from unmodded dlrs.

DSLRs are fine (the M101 image was done with a DSLR). You get a lot of bangs per buck, a huge sensor and the ability to do other things with it. They just are not as sensitive as a cooled CCD, but then again, you have to budget a LOT more for cooled CCD cameras.

Read up on the tutorials in the imaging sector. You will end up frustrated trying to work it all out by yourself. Imaging is a difficult, frustrating hobby (and expensive), but nothing beats the "Wow" moment when DSS takes a stack of faint images and chucks out something special.

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Don't worry Mark - deep sky imaging is a steep learning curve and there's plenty of valuable help on here. Don't expect to achieve great results to start with as there is much to learn and various conditions can make it all the more difficult.

Canons are great astro imaging cameras so don't think you've made a bad decision. I'm using the 1000D (unmodded so far) and I'm delighted with it although it's taken me 18 months to start getting the best out of it.

What you need to consider is that at this time of year imaging becomes far more difficult - it's only really dark enough for long exposures for a few hours - It wasn't really dark enough here in Chester until gone midnight. Also, as the camera is uncooled, electronic noise is worse in images in warmer weather. The good dedicated astro CCD cameras have built in Peltier cooling whereas the DSLRs don't.

How you can movve forward:-

1. Don't try to start imaging too soon for the moment - I'm not sure where you are in the country but I'd aim for post midnight.

2. Take darks - don't worry - they're easy - just cover the lense of the scope up and take exposures of the same length and settings as you light frames (your actual pictures) - these are best taken immediately before, after or during your imaging session as you are aiming to take them at the same ambient temperature as your light frames. These are then loaded in Deep Sky Stacker as Darks when you load you light frames and are subtracted from the image to reduce the nois.

3. By all means start to look in to guiding but I appreciate that cost may be an issue in the short term. I've built my kit up over 3 years to spread the cost.

4. Just treat each session as a learning experience.

Hope this helps

Regards

John

Edited by Sparrow
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