Image size

[jaygpoo]

I read alot online that using max image size as in 14mega pix etc does not result in better images of stars / deep space or planets. When I take an image of saturn my gut reaction is to crop it and get a larger image but all I get is a smaller one. Using max pix I was under the imppression that one could blow up an image or zoom in to an area an get a better definition . I am only at this point using the editing in iphoto on my mac. I do have photoshop but not enough usage as yet to get anything meaningful out of it. So can anyone advise on pix size for planet images using my canon 500d.

[mgirdwood]

hi, i am a raw beginner with the astrophotography but a couple of questions might help others to explaint he intracacies better than i - what equipment are you using, Scope ? DSLR type, Web Cam, Afocal or prime photography ? Also most of the shots on planetary for us beginners are stacked web cam avi and prob a 2x barlow to gain more magnification.

i also found that theres a lo of info in the forum areas if you try some key word search either here or in the Beginners section.

Theres a lot to learn but most around here are truly helpful.

[JamesF]

There are a few issues here. The first is that a given telescope can only resolve so much detail and if the smallest detail you can resolve covers dozens of pixels of the image sensor then you're not really gaining much by having them.

The second is that to get a good image you need to get enough photons landing on each pixel. If you make the pixels smaller that means you need more photons per unit area of sensor, which means longer exposures. However, where planetary imaging is concerned you really want very fast exposures which makes larger pixels (up to a point) more desirable.

It's really a bit of a balancing act between the capabilities of the scope and the camera. Ideally you perhaps want the diagonal of one pixel to be half the size of the smallest detail your telescope can resolve. The maths to calculate that isn't hard. It's a combination of the formulae for plate scale and the Rayleigh limit of your scope, but it will vary depending on the aperture of your scope.

James

[JamesF]

There are a few issues here. The first is that a given telescope can only resolve so much detail and if the smallest detail you can resolve covers dozens of pixels of the image sensor then you're not really gaining much by having them. In fact you're probably making your life more difficult.

The second is that to get a good image you need to get enough photons landing on each pixel. If you make the pixels smaller that means you need more photons per unit area of sensor, which means longer exposures. However, where planetary imaging is concerned you really want very fast exposures which makes larger pixels (up to a point) more desirable.

It's really a bit of a balancing act between the capabilities of the scope and the camera. Ideally you perhaps want the diagonal of one pixel to be half the size of the smallest detail your telescope can resolve. The maths to calculate that isn't hard. It's a combination of the formulae for plate scale and the Rayleigh limit of your scope, but it will vary depending on the aperture of your scope.

James

[jaygpoo]

Once again some helpful advise to this old fart who is having alot of fun with my gear. For the record I have a 127 skywatcher cassegrain and a 100mm Tal refractor. I have been using a barlow x2 to take some of my planet images but will I be wasting my money going for a x4 or even a x5 barlow both to use with my Canon an the telescopes?

[JamesF]

The answer to this is probably "it depends"

To get a decent image of, well, anything really, using a camera you need to get a sufficiently large number of photons landing on the sensor pixels to give a good range of colours. By enlarging the image with a barlow you're effectively "spreading the photons out" over a larger area. By doing that you may find that you just don't have enough to get a decent image. For example, last year because Saturn was higher in the sky and (I think I'm right in saying) closer to us I had no problem imaging it with a 2.5x barlow and extension on my 127 Mak, giving a total increase in image scale of about 3.5x. This year I really struggled to get good contrast in the image even with just a 2x barlow.

Given a suitably bright target I think the Mak will stand a 4x or 5x increase in image scale. It will be interesting to see how that works out with Jupiter this coming apparition. There are some other technical problems however, in that many Maks and SCTs suffer from "image shift" when the focuser direction changes and it's easy to lose the target from the sensor when that happens at this kind of scale. The only real fix is to use an external focuser. With a webcam at 4x focal length I find that changing the direction of travel of the focuser on my Mak becomes a juggling act between moving the focuser and moving the scope to compensate for the mirror shift. Where this might not matter too much is imaging the Moon or Sun because you probably won't lose the target altogether.

With the Tal I think you'd struggle even more because of the smaller aperture.

Where planetary imaging is concerned I think it helps to have a range of barlows/imagemates/powermates to allow you to step up and down image scales to find what is most effective on the night or failing that, a couple of the smaller scale ones and an extension to use with them.

James

[NickK]

Resolution limit:

With light there is a limit to how small you can see (in arc seconds). This is caused by an effect called defraction and is the basis of a well known formula called Dawes Limit.

Dawes Limit basically gives you the smallest thing you can see for a specific aperture.

Under this limit, things start becoming more and more blurred. So putting more pixels at the back of the scope isn't really going to help.

Pixel size and sensitivity also play a part here.. not much point in having a super high resolution system if the resulting image is so dim that it requires a week for a single photo! The smaller the pixel, the less light it gets compared to a larger pixel on a camera in the back of the same scope..

Overall field of view

This is down to the optics, the focal length and the number of pixels you can place under the light coming out of the back of the telescope.

[jaygpoo]

Hell, alot to take in but thanks to all for your thoughts and time. You mention extension with regards to barlow. Can you explain. Are you talking about the tube that it comes with that the E/P fits in or something else. You also refer to external focuser , is that something added to the cassegrain 127? The Tal has a crayford focuser that is a smooth as silk.

[JamesF]

I took the lenses out of the 2x barlow that Skywatcher supplied with my ST80 and put just the empty tube between the barlow and the camera. Adding it effectively increases the image scale beyond that of the barlow alone. It's possible to buy purpose-made extensions too, but I think they're expensive for what they are.

SCTs and Maks suffer from two particular problems with mirror movement because of their design. The mirror has a hole through the middle and mounts on a carriage that slides up and down the baffle tube inside the OTA when the focusing knob is turned. The baffle tube extends from the centre of the backplate up towards the secondary mirror and light travels down it from the secondary mirror to the camera or eyepiece. Obviously there has to be a little bit of play between the carriage and the tube, otherwise it would be too stiff to move, but unfortunately that allows the mirror to "rock" ever so slightly on the tube. This can happen when the weight distribution changes as the scope moves around the RA axis and that's often called "mirror flop" -- the mirror carriage rocks slightly under its own weight. It can also happen when you change the direction of the mirror movement by turning the focuser in the opposite direction. I've seen this called "mirror shift", but I don't know if that's a widely recognised term.

The way to deal with the latter problem is to fit a "normal" style focuser to the back of the OTA. You can then wind the mirror into a position where the focus is about right using the standard focusing knob and leave it there, achieving fine focus (which typically involves lots of moving back and forth) with the second focuser. There are also modifications to some SCTs to allow the mirror to be locked in position so it can't move, but they're not common to the best of my knowledge.

James