Decisions decisions

[Kenny]

Samsung Galaxy Tab 3 8.0" bought last December for £179 broke. Amazon unable to repair so giving a full refund. I could a) buy a new one now only costing £134 or b) invest in a DSLR Camera for astrophotography. Hmmmmm. Comments (particularly re option b)?

p.s. Carole - reading your DSLR guide. I can get a 450D for £138...

[Carole]

Well Kenny, it's certainly a good way to get into deep sky Astro-imaging and cut your teeth on it all.  The only thing is you would have to get yourself a mount like the HEQ5 to enable long exposure, so this bumps the cost up somewhat.

Also what scope would you be using?

You could get the Moon on your Dob with a DSLR but that's about all really, planets are too small. 

Carole

[The Thing]

You could get the Moon on your Dob with a DSLR but that's about all really, planets are too small. 

It's relative, if the DSLR pixels are the same size as the planetary camera (and they are generally ish) and you can get the same optical magnification (i.e. a barlow or two) the the image scale in arsecs/pixel can be the same, you just have to throw away the surrounding empty frame on the DSLR image.

Using AstroPhotoTool there is a Planetary function that takes either a movie or high frame rate JPGs using LiveView (depends on the Canon Digic processor version) and you can use the LiveView 10x feature which effectively give you a region of interest ROI function like a CCD, i.e. it chucks the surrounding pixels away and just gives you the important bit with the planet.

Does a 450D have LiveView? I know a 305D doesn't.

[Kenny]

Would I also be able to use a DSLR for wide field photos without telescope magnification?

[Carole]

Would I also be able to use a DSLR for wide field photos without telescope magnification?

You can Kenny, but unless you have a tracking mount you won't be able to take exposures for more than about 20secs before trailing starts to show, unless of course you want to actually take star trails.

you can get the same optical magnification (i.e. a barlow or two)

Forgot about using a barlow, but I still think they would be quite small.

Carole

[RobertM]

A small portable tracking mount for wide field is a very useful thing to take on DSCs and holidays.

With the small pixels in these cameras you will see trailing well within 10 seconds (without tracking) depending on where in the sky the camera is pointed and field of view.  I remember seeing trailed stars with Fays 650D and a camera lens after 5 seconds (zoomed in).

Don't forget star trails - they don't need any tracking and are useful practice.

Robert

[The Thing]

Forgot about using a barlow, but I still think they would be quite small.

Carole

Only relatively to the size of a DSLR frame, the planet can actually occupy the same number of pixels in each case - been there, done that. However the webcam route seems to produce better images at the end of it.

[MarkS]

Samsung Galaxy Tab 3 8.0" bought last December for £179 broke. Amazon unable to repair so giving a full refund. I could a) buy a new one now only costing £134 or b) invest in a DSLR Camera for astrophotography. Hmmmmm. Comments (particularly re option b)?

Very difficult to get apps to run on a camera

[Kenny]

Camera won.

Canon 450D digital SLR with original Canon 18-55mm kit lens plus a Sigma DC 18-200 mm lens.

[Carole]

Well done Kenny, look forward to watching your progress.

Carole

[Kenny]

May be a slow start. I haven't used an SLR in over 20 years, and that one had film in it!

RTFM.

[The Thing]

YouTube - bound to be some stuff on there.

[Fay]

in general with a lens at 10mm  you should be able to do 60 secs without trailing, 15mm lens = 40 secs, 20mm lens= 30 secs. push the ISO and you can cut down the length of subs

[Kenny]

Thanks Fay. That's very useful. I'd heard of rule of 600 which is similar. I'll post some of yesterday's attempts.

[Mac]

Your sort of correct Fay, but it can be as low as 4 seconds before bluring,
Heres a copy of a post I made on another forum, just adjust the maths a little,

You are nearly correct in that its 600/fl as a general rule not 500, also you forgot one very important thing,
That general rule is for a full frame camera, the 7D isnt a full frame,.
Some maths to help you get your head around why.
Your sensor size is 5184 x 3456 pixels on a  22.3mm x 14.9mm chip,
which gives a pixel size of 4.3um or 0.0043mm

if you take a star as being a point source then it will produce a point source on your chip,

if we assume that that the star completely fills one pixel and to create blur has to move onto the next pixel,
this equates to a movement of one pixel.
¦....¦....¦....¦
If the above image represents three pixel, if the star as a POINT source is on the far left of the pixel,
all the time the star is in that pixel it will be detecting its light, once its moved onto the next pixel,
you can declare that the image is now blurred as the star has now moved.

Your field of view on the camera is 96 deg x 73 deg @ 10mm, so each of the pixels on your camera covers,

73 / 3456 = 0.0211

96 / 5184 = 0.0185 degrees. (assuming movement only on long side)

The stars in the sky rotate (well we do technically) 360 degrees in 24 hours (technically 23H56m) which is 86400 seconds.

so how long in seconds before one star moves 0.018 degrees

in one second the star moves 360/86400 0.004 degrees

the number of seconds take to move 0.018 degrees (one pixel) is 4.4 seconds.
THIS IS FOR STARS ON THE CELESTIAL EQUATOR. i.e. 90 deg from the pole
You then need to add some trig equations to calculate exactly how much SLOWER the stars will move depending on where you are
pointing your camera.
i.e if you are pointing your camera at the celestial equator then you will get maximum movement, i.e. 0.004 deg per second
The higher up towards the pole star the slower the stars appear to rotate,
If you are pointing your camera at the pole star. i.e. the pole star is centered exactly on your chip, then the diagonal of your camera will have a FOV of 120 degrees corner to corner, The corners being 60 degrees from the polestar, with 90 degrees being the celestial equator.
These stars move  @ 360/86400Deg * sin(60) = 0.0036
Time = 0.0185 / 0.0036 =
At this distance the stars will take slightly longer before they move one pixel, this is 5 seconds.
So after 30 seconds your star would have moved about 6-7  pixels.
Hope this helps.
Mac.