Exposure time with SLR on plain tripod

[jah79]

I am going to be in South Africa for a couple of weeks in areas with great Milky Way views. As much as I'd love to take my 6" Newtonian it would probably be a slight bit too bulky to fit in the overhead compartment and don't want to think what the collimation would be like on the otherside! So I will settle for taking my bins, SLR (with cable release) and plain camera tripod.

What would be the max exposure times I'd be looking at for taking wide-field shots without the stars having motion blur? Would I also be able to take multiple shots and then stack them for added effects or does this just really bring out detail for DSOs? With 18-35mm and 200mm lenses are there any Southern Hemisphere objects which would look especially good photographed this way?

Thanks!

Johann

[jgs001]

Johann, you can just about getting away with 25-30 second exposures at 18mm. As the FL increases so the exposure time comes down. At about 50mm, you're looking at around 8-10 seconds for example. However, you can still get some nice results from a bunch of 30 seconds. Have a look at

http://stargazerslounge.com/imaging-tips-tricks-techniques/73737-basic-widefield-camera-tripod.html

[arad85]

Motion blur is a function of where you are pointing the camera relative to the centre of rotation and how long the focal length is. I have some images I took of Orion rising over a hill in Spain a month or so back and they show motion blur at full resolution in 20 seconds with the lens set to 18mm...

Unfortunately, I can't give you more info....

[Merlin66]

The Southern Milky Way, the Coal Sack, Eta Carinae, Magellanic Clouds.......

[astro_dt]

Hi Johann

I have some tables on my website (technical section) that allow you to calculate the exposure based on the focal length of the lens and the declination. For an 18mm lens you can get away with a minute at the celestial pole but 20 secs at the equator is a good guide.

Cheers

Danny

[arad85]

Easier to show than say... Here's a 20 second exposure at 18mm from a 450D of the Pleiades. This is without any resizing. Taken from around 42 degrees North

[neelam]

at 18mm i have exposed for as long as 40 42 seconds.trailing is visible after 15 seconds when viewed at large but resizing it and reducing its size will not show trailing even at 40 seconds for 18mm focal length

[jah79]

Thanks for all the tips, really helped me! I think I'll mostly try to get a load of 10s shots and then stack them. Can't wait to see the results!

[arad85]

but resizing it and reducing its size will not show trailing even at 40 seconds for 18mm focal length

I fully agree, but then you have to define resizing and reducing. I could shrink my image to 98% of original size (which would be resizing and reducing!) but I'd still have the trails.

I used the above to produce a 600x900 image and at full size, that showed the correct shaped stars (although you have to be careful to use the correct resampling algorithm (Bicubic sharper in Photoshop )

[neelam]

@arad i agree but i usually use ms paint to stretch and skew and also i think the initiator of this thread has a manual slr so it can be tricky

@jah79

10 sec exposures staked would yield no good and it wont increase the details as stacking is mainly done for reducing noise.if u want to play safe then make it 25 seconds at 18mm and 15 seconds at 35mm.

you can further take some single shots at increased exposure to find out how it looks like.