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Karlos

Unguided/non-tracking imaging

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I am thinking of attempting some widefield shots with just my Canon DSLR on a tripod, of some of the brighter objects in the sky such as the Pleiades, without any means of guiding/tracking. Which is probably the simpliest way of doing it, but with a number of subs, I am hoping its possible to start to grab some reasonable images in a partially light polluted area? There are a few questions I have about it as follows:

What are the longest exposures I can take before trailing becomes apparent, is there some kind of calculation to work this out?

If manually moving the camera with rotation of the Earth when needed, can separate shots be stitched together and processed just the same as you would with say Lunar shots and Registax?

I've heard of some software called DSS, would this be suitable to process these kind of images?

Any help would be great, it feels like a big stepping stone moving into the DSLR world :)

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Thanks Steve, thats very useful. Of course, best results come from experimenting, but its great to have some guidelines. Would LP also be a limiting factor for exposures?

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When imaging unguided, it's advantageous to select targets at high latitudes. ie. near the celestial pole. This will help keep any trailing to a minimum. If you study star trails that are centred on the pole star (Polaris), you will note how much longer the trails are, the further away from the pole' If you could see the ones near the celestial equator, they would be extremely long indeed

Practicing with trial exposures, noting the times, and the results, will give you a good idea of what you can achieve.

Very nice widefield unguided exposures are certainly possible, as mentioned above, re JGS001

Ron.

Edited by barkis

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Thanks Ron, I noticed the difference in distance from the NCP and the amount of trailing, so I've found it best to let M42 rise later on to try and get as much exposure as possible. The best I got was about 0.8 secs at FL of 900mm, which trailed very slightly but is hardly noticable. So here is my first DS attempt of "photography".

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Hey,

I started out this way and I found at 200mm I could manage 2.3 seconds,I also used 6400iso and stacked several hundred images in DSS. It's a long and time consuming process but you'll be surprised what you can achieve. WWW.12GSuper.deviantart.com/art/Orion-Nebula-269998164 (thats qhat i managed on Orions nebula) I then bought a 200mm scope on an EQ5 mount started hand tracking shots. Feel free to have a browse on my page and see what you could do.

Edited by teh_feckerer
link error

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Hey Ted, I've tried your link but its not working "Page could not be found", error maybe?

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Try this link...

12GSuper on deviantART

I found that about 25 seconds between overhead and the pole, down to about 10 seconds near the celestial equator (orions belt).

Startrails are good fun, especially if you can get something interesting in the foreground!

Have fun

Ant

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Karlos - I just recently got an EQ mount. Only used it once so far. But before that I had the same set up you have and used that for about 6 months. I only had 2 lenses. 18-55mm and 100-300mm. For exposure lengths I found that at 18mm I could easily get between 30-40sec subs, at 55mm I could get 15-20sec, 100mm I could get 5-10sec and 300mm I could get 2-4sec. And that was pointing at M42 which is just past the equator.

I also tried capturing NGC 188, which is about 4 degrees from the CNP. I only tried a couple times but at 100mm got 30sec subs and 300mm got about 10-15sec subs. So the closer you get to the CNP the exposure time goes up quite a bit.

Also I see you mentioned that you waited for Orion to get higher in the sky to capture better exposures. From my experience it doesn't help that much to wait. I will when taking longer exposures but when we are taking such short exposures it doesn't make that much of a difference, for me anyways, to wait till 2am to get it at is highest. Also 900mm is super long FL for non-tracking. I wouldnt go past 300mm. I'm at work but I post what I did with my Canon and 300mm lens later.

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Thanks for the link updated ant!

Those images have truely inspired me! The only problems I'll have is getting any exposure longer than even 1 sec at 900mm, which is no way suitable for this type of photography I know, but I think I may just give it a go anyway.

Nmoushon, your post is extremely helpful, there really is a big difference in exposure times usable regarding distance from the NCP. I'll look forward to your post about your Canon 300mm lens.

I'm very tempted to buy a faster small refractor for imaging even if I use it unguided or tracked, obviously the longer the exposures the better but surely many smaller exposures stacked and process similarly could achieve the same or close to same results?

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I've been experimenting myself with taking hours of time lapse photos of the night sky with the EOS 500D on a fixed tripod (no guiding/movement).

I've learn that star trails become quite obvious somewhat before 15 second exposure.

Here's a crop of two pictures of a 4 hour time lapse I did a few nights back. If you zoom in you can easily see the star trails. The blue one (before it was dark) is a 15 second exposure, the orange/brown one (later in the evening) is a 25 second exposure. Both are with the kit lens set to 18mm.

So 8 seconds sounds about right to me.

post-32860-133877729153_thumb.jpg

post-32860-133877729157_thumb.jpg

Edited by Cath

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Ok so finally got home. Just to clarify I got a Canon 450D and I used a 100-300mm lens. Though I can't remember if I shot at 200 or 300mm. Exposure time was 3sec. ISO 1600. That with just a regular camera tripod. I'm sure if you had a better camera you could get more since they can have ISO3200 or higher. The second image is what I took with my mount. 300mm, ISO1600/800, 30sec exposure. If your really getting interested in it this hobby there are some not-so-expensive computerized mounts. Definitely should get that before a telescope. Which is the situation I'm in. And since you have a 900mm lens you can get plenty objects with that.

Edit: Sorry forgot. First image is 120 stacked exposures + darks and second is 20 stacked exposures no darks.

post-29384-133877729272_thumb.jpg

post-29384-133877729277_thumb.jpg

Edited by nmoushon

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Oh found my image of NGC188. Thought I show what you can get the closer you get to the NCP. Shot with Canon 450D, 100mm, 30x30sec exposures, ISO800, plus darks (forgot how many).

The bright star in the center is the North Star and NGC188 is near the top left corner.

post-29384-133877729284_thumb.jpg

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Slight trailing you can process out. This is a little extreme, but you do have round stars now.

If this was a stacked tif, there would have been more data to bring out and far less noise - but you get the idea!

Cheers

Ant

I've been experimenting myself with taking hours of time lapse photos of the night sky with the EOS 500D on a fixed tripod (no guiding/movement).

I've learn that star trails become quite obvious somewhat before 15 second exposure.

Here's a crop of two pictures of a 4 hour time lapse I did a few nights back. If you zoom in you can easily see the star trails. The blue one (before it was dark) is a 15 second exposure, the orange/brown one (later in the evening) is a 25 second exposure. Both are with the kit lens set to 18mm.

So 8 seconds sounds about right to me.

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Thanks cath, I found the same with my images I've been keeping it at 55mm to make it as fast as possible but even so exposures are short!

Nmoushon, that is a lovely image. Sounds silly but I've not yet looked into darks , I understand they are use in the image calibration process but just what are they? :)

Thanks Ant, I think the odd stars shapes are also partly because I've been clicking for exposures manually with just 2 seconds delay not enough for my wobbly tripod :icon_salut:

I guess next step is to start putting multiple images together, I downloaded DSS awhile ago will give it ago.

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You welcome Karlos.

Ant: What did you do to make the stars round I wonder?

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Karlos - To put it simply darks help clean up your image of noise.

To take them you simple put the lens cap on and start taking pictures with the EXACT same setting as your light exposures. If you take different sets of exposures with different settings( ie. one set at 2sec exposure and another at 5 sec exposures) you do darks for both sets. You will want to about half as many darks as lights. Thats what I did and it helped a lot.

When you take long exposure images the camera starts to heat up and will produce bad pixels. (Not sure of the exact science someone else will have to help explain that part) Those bad pixels are the random color pixels in your image. When you take a dark exposure it will show only those bad pixels and everything else will be black. When you put those into DSS the program will subtract those bad pixels and help clean up the image.

DSS is a pretty simple program. You can find tutorials online and im sure in this forum. Just search it through the forum or google.

Hope this helps.

Nik

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Oh my, that's a brilliant technique. I knew darks were used in subtraction, but I assumed you'd just take a much shorter exposure which would have less noise. I will do this later on this evening as the weather forecast looks set to be good. I think I'll use DSS having had a quick look around the interface looks like it makes a difficult processing job much more intuitive. Thanks for all your help! May post some results if they are not too embarassing :)

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