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Maximum exposure time for untracked mounts

Dyptorden

By Dyptorden
in Getting Started With Imaging

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lensman57

lensman57

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Hi,

As the title says, could you tell me from your experience what would be the maximum exposure time for an unmotored\untracked mount so that I won't get star trails at all?

Thank you

Untracked and un motored means means very wide angle lens and very short exposures , I think that you'd be safe with 5s~10s MAX  at 50mm with a cropped DSLR sensor if you do not enlarge the image for printing.

A.G

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RichM63

RichM63

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You don't say what kit you are using. Telescope, camera, lens, mount???

I've not been doing this for long, but a more experienced guy put me onto a general rule.

500 divided by the focal length of the lens. Thus 500/18mm = 27seconds maximum for widefield milky way shots.

But more info' is required for a better answer.

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MartinB

MartinB

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500/focal length in mm will give you an approximate max exposure time in seconds.  This is only a rough guide, it also depends on pixel size and also whether or not you want to present your images at full size or reduced (as is usually the case over the internet).  It's best to experiment.  

Unfortunately, even a short focal length scope is only going to allow very short exposure times.  However, a DSLR with a wide angle lens opens up a world of opportunities - TWAN

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Dyptorden

Dyptorden

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On my 1100d cropped sensor using a 55mm lens it is about 2 seconds only before star trails are evident if pixel peeping.

One more thing is that I don't speak about "evident". I don't want star trails at all !! :)

So probably I will have to stick with the 0.41 seconds...

If I shoot through eyepieces, what would the 500/fl formula become?

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lensman57

lensman57

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I own a Nikon D40 and a 8" Dob with a 1200 Focal Length... so the maximum exposure length would be 500/1200 = 0.41 seconds ? :o

Martin correct, unfortunately unless you use a DSLR with a wide angle lens you'd be wasting your  time with anything else. Another alternative is to guide by hand using some sort of guiding EP but in your case and the DOB being an AltAz mount and @ 1200mm of FL I wouldn't bother with it. You may wish to have a read of this article but even here the writer assumes that a blur of 8.5 pixels for a star is acceptable, whether this works for you or not is another matter. http://photo.stackexchange.com/questions/30263/what-is-the-rule-of-600-in-astrophotography

A.G

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hjw

hjw

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I own a Nikon D40 and a 8" Dob with a 1200 Focal Length... so the maximum exposure length would be 500/1200 = 0.41 seconds ? :o

I just got a HEQ5 Pro GOTO mount and at the best polar alignment and tracking I can achieve 30 sec on a 900mm scope. Nor even close to where I would like to be. If you have no means of tracking use a camera lens (200-300mm or so). Many DSOs are really big!

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nicks90

nicks90

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Its not really going to make any difference. If you can't track / guide, then you can't do long exposures with out trailing.

As a guide, I took some pictures of the moon using my 12" DOB - 1500 fl - and 32mm ep and my camera at 18mm stuck on the end. This is called a-focal photography.

The moon worked, but NOTHING else did. Nothing.

post-26554-0-88672700-1420309790.jpg

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lensman57

lensman57

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One more thing.Since my Nikon has a 1.5 crop factor, i guess the 0.41 seconds become 0.27 seconds of maximum exposure before getting star trails?

Please forget all the nonsense about crop factor and all that. These are meaningless terms invented for terrestrial photography. I was a pro for over 25years so I know what I am talking about . In AP what matters is image scale. A DSO target does not get any larger or smaller if you change your sensor, what changes is the FOV. A short APO will give you a wider FOV for a specific sensor size, a long FL scope will give you narrower FOV. This is exactly true for normal photography. A tree at 300 Meters does not change size if you changed your camera or the lens what changes is your FOV and in AP what matters is image scale. In the final analysis if you want to do meaningful DSO AP you'd need a driven and guidable EQ mount, for planetary imaging a driven AltAz mount is sufficient .

Regards,

A.G

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zcdawson

zcdawson

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What is a good exposure time for an unguided but tracking scope. I have a nexstar evo 8". I am waiting on the atik 414ex, hyperstar and wedge. I live in moderate light polluted skies (in the yellow right next to the green) and have a 2" narrow band LP filter and a 1.25" OIII filter. I was really thinking about getting a focal reducer for one end of the scope and the filter swapping attachment for the hyperstar. Any thoughts? Filter advice?  

Thanks,

Z

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Dyptorden

Dyptorden

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For stars near the celestial equator, where star trails are the longest, try the following formula:

star_trail_length_in_pixels = focal_length_in_millimetres * exposure_time_in_seconds / pixel_size_in_microns / 15

Mark

Your formula sharkmelley :

star_trail_length_in_pixels = focal_length_in_millimetres * exposure_time_in_seconds / pixel_size_in_microns / 15

Is the same with :

exposure_time_in_seconds = star_trail_length_in_pixels * pixel_size_in_microns * 15 / focal_length_in_millimetres

My Nikon D40 has a23.7mm x 15.6 mm sensor size and 6MPixel

According to the formula : Pixel_size = 1000 * sqrt (sensor length * sensor width / number of pixels) , it results that my camera has a 7850 pixel size.

The telescope has a 1200mm FL

So the exposure_time_in_seconds in order to get a 1 pixel star_trail_length_in_pixels :

exposure_time_in_seconds = 1 * 7850 * 15 / 1200 = 98.125 seconds?

I can't really believe it.... where's the mistake?

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RikM

RikM

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These tables always miss out a very important caveat. It assumes that you will not crop the final image nor zoom in and you must view both the APS-C and Full Frame images at the same size. They were originally set for an 8x10 print. They are a rough starting point and nothing more. I use typically 6 seconds at 50mm and there is still slight trailing visible.

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Charic

Charic

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......like you said, rough guides. Like everything else, trial and error to get the best results. Good wide angle fast lens, High ISO and depending on the 'Rule scale (500/600) / focal length to get the most ammount of exposure without the sensor detecting any movement. Even programmed modes can give strange and eerie results. 

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