Astrophotgraphy, I dont get it....

[Astro-Nova]

Have used my phone holding against the EP and taken some nice shots considering the method.

I want to take some more photos of planetary but also nebulae and galaxy.

The thing I dont get looking on the internet what other people have done is 2 things.

1. Instead of EP(which obviously is needed for magnification) they screw in a webcam and still get same/similar magnification as you would have with an EP.

2. DSLR cameras attached to a telescope also seem to have great images of nebulae and galaxies........how does this work with magnification? When looking through an EP even with high magnification its still a blob and faint.

So my question is how does it work with DSLR and webcam to get magnified image without the use of an EP.

[Andy_E]

Quite a complex question to answer, so I’m going to explain it in simplified terms (forgive any inaccuracies guys!). the telescope itself has a focal length, which is similar to DSLR lenses. So here goes… imagining that the human eye as an FOV of around 50mm, then a 400mm camera lens will produce an image of roughly 8x magnification (this is of course not as it appears on the sensor). If your telescope has a focal length of say, 1200mm, then when attached to a DSLR is functions as a 1200mm lens – giving effectively 24x mag. Now, as ‘full frame’ DSLRs use a 35mm sensor, magnification is again increased (or more accurately, FOV is REDUCED) – this gets even higher with standard DSLRs, which typically have a crop factor of around 1.5 – 1.6. Hope this helps clarify things a bit!

[Tim]

But you CAN use an eyepiece with a DSLR too, a method called eyepiece projection and get extra magnification.

As noted above, its all about focal lengths, chip and pixel sizes, and arcsecond per pixel figures.

Cheers

[John]

I've edited Andy_E's post above to make it a little easier to see

[zainraza]

I've heard that some webcams act like a 6mm EP so thats why they have high mag and when you add a 2x barlow to that, you'll end up getting a lot of zoom (I think)

Im not sure about the rest though, I'm confused about the whole 'x' minute subs at 'x' iso/exposure etc etc

[SeedyF]

Don't forget as well that the images of faint objects will be taken over several hours so that they capture as much light as possible. Your eye just cant compete with hours worth of images all stacked together to make a really bright, detailed image.

As Zainraza says, a webcam gives a view like a 6mm eyepiece and my Canon 400DSLR is pretty close to a 20mm eyepiece. I don't really understand the equations but the only way I can get my head round it is that it's like strapping a 1200mm lens on my camera when I put it into my scope.

[themos]

The eyepiece does not form an image, it just enlarges the image that is formed somewhere inside the telescope by the primary lens or mirror. Think of it as a magnifying glass that you put between your eyes and the thing you want to see. Take away the eyepiece and an image is still formed. In fact, you should be able to see through your telescope without an eyepiece but you need to put your eyes much further back. So the flat sensor of a webcam or a DSLR, if it's placed at the spot where the image is formed, will record the image. In photography, we speak of the image scale: how many arcseconds fit in a pixel. That's given by a simple formula that involves only the focal length and the pixel pitch.

[Kevdan]

Ok heres my point of view of how to explain,

Scope = light bcket to collect as much light as possible, then using the focal point (which is the point at which an object becomes focused) projecting it onto a sensor/chip to turn light collected into a digital image, the longer the exposure the more light can be collected,

Imagine a bucket in the garden when its raining, the bucket being 12" dia, if you take the top off the bucket for a split second it will catch some rain, if you now take the lid off for lets say 10 minutes it will collect more rain,

however now onto ISO when the bucket lid was off for just a split second some rain now called "signal" came into the bucket but also a little dirt, when the bucket lid was of for 10 mins there was more rain but lots more dirt, this dirt is now called "noise" The ISO is the sensitivity of the sensor and depending on setting gets more or less noise (ISO 200 is less noisy than ISO 1600) but by losing sensitvity you also lose good picture data, so by collecting lets say 20 images 10 minutes long and stacking them you get 20x10 = 200 mins exposure length but then you have a low ISO of 200 but because of the number of exposures hardly any picture data loss. Which is know as signal to noise ratio.

Hope this helps.

Kev

[Ags]

When looking through an EP even with high magnification its still a blob and faint.

Magnification makes objects fainter. So a nebula viewed with high magnification is larger but fainter than the same nebula at lower magnification. It makes sense if you think about it - the scope gathers a fixed amount of light, and as you pile on the magnification, you stretch out the light more and more thinly.

Stars are an exception - they do not get fainter because they are point sources, so high mags don stretch them out at all.

Another point - the bright red nebulae you see in photos are shining in red Ha light. The dark adapted eye cannot see Ha light at all, so we are blind to a large proportion of the light coming from the nebula.

[Fandango]

I'm not sure I get all of this.

I thought that if you used a webcam you didn't use the EP. As I don't have a webcam I thought I could use the video function on my compact Canon 120 ixus. I focused on distant trees as a test, however when I took the EP out and fitted my camera into the focuser tube, which fitted perfectly, all I could see in the camera's view screen was the secondary mirror and spider. Did I just get the whole concept wrong?

[Tinker1947]

You can screw a DSLR with a T-Ring attached to Hyperion 68 Degree and Pentax XW EP's they both have a 43mm thread to take a Adapter Ring, there may be other EP's.....

http://www.firstlightoptics.com/baader-planetarium/baader-hyperion-zoom-eyepiece-t-adaptor-m43-t.html

[umadog]

I'm not sure I get all of this.

I thought that if you used a webcam you didn't use the EP. As I don't have a webcam I thought I could use the video function on my compact Canon 120 ixus. I focused on distant trees as a test, however when I took the EP out and fitted my camera into the focuser tube, which fitted perfectly, all I could see in the camera's view screen was the secondary mirror and spider. Did I just get the whole concept wrong?

Here goes: The primary on it's own forms and image at the focal plane. An image forming condition is defined as one where all rays leaving one point on the object converge onto one point in the image plane irrespective of the angle at which they left the object. Thus: http://www.antonine-education.co.uk/Pages/Physics_5_Options/Astrophysics/AST_01/Astrophysics_1.htm

As a consequence of the above, if you place a piece of paper at the focal point and you look at the paper you will see an image of the object. This is because light rays from one point of an object all converge onto one point on the paper.

The eyepiece changes things, however. The light rays coming out of the back if the eyepiece are parallel. See second figure: http://casswww.ucsd.edu/archive/public/tutorial/Telescope.html So there is no convergence of light rays and you won't form an image (at least not a very good one) with your piece of paper. Instead, you need another lens to focus the light. This is what the eye is doing in that diagram.

So! You can use a webcam with a built-in lens instead of the eye. Or you can take out the eyepiece and take out the webcam lens and use the webcam in place of the piece of paper (i.e. put it at one focal length from the primary). You placed your camera (with built-in lens) at 1 focal length from a primary mirror with no intervening eyepiece. That's not going work. All you will do is image the inside of the telescope.

[Fandango]

Thank you umadog. There is a lot if info to take in there, I will read through it properly in the morning. Just a quick further question:

I have a newtonion reflector, so I assume your explanation works the same. I know some people attach their dslr cameras to the telescope. What do they do differently to avoid imaging the inside of the scope?

Thanks for a very thorough answer. I really appreciate it.

[sologuitarist61]

I look at it this way - the DSLR is not part of the imaging process but merely records the image that the telescope focuses onto the sensor. As has been already said - just as if it is a big telephoto lens.

[Andy_E]

Hi again. If you’re planning on using a compact camera to take pictures, then you may well be best off using it in conjunction with an eyepiece (or ‘digiscoping’ as it is affectionately known). Alignment is the big problem here… i.e. the alignment of your cameras lens to the eyepiece and/or secondary mirror. This means you’ll only be able to take pictures of the brighter objects such as the planets or the moon as you’ll be needing fairly quick shutter speeds. As for the problems with inserting the camera directly in to the scope, it could well be that your camera lens is on too wide a setting, though someone else may be able to answer this better than me. Good luck with the photos! Andy

[umadog]

I have a newtonion reflector, so I assume your explanation works the same. I know some people attach their dslr cameras to the telescope. What do they do differently to avoid imaging the inside of the scope?

It doesn't matter if it's a reflector or a refactor: they work in the same way. If you remove the lens from the dSLR and place it at one focal length from the objective then you will be able to form an image of objects located at infinity (i.e. stars). That's all you have to do. Try pointing your scope at the moon, take out the eyepiece, and put a piece of paper over the end of the focuser. You will probably need to move it back and forth but you will find a point where you see a nice sharp image of the moon on the paper. When you do prime focus AP the sensor is in the location of that piece of paper and the CCD or CMOS chip records the image.

[umadog]

Hi again. If you’re planning on using a compact camera to take pictures, then you may well be best off using it in conjunction with an eyepiece (or ‘digiscoping’ as it is affectionately known).

Yes: you have to do it this way because the lens on a compact camera isn't removable. The technical term for this sort of imaging is "afocal" photography. It is so called because the recording device (the camera) is not placed at the focal length of scope. Instead, it captures the parallel light rays exiting the eyepiece and these rays do not form an image.

[Fandango]

Okay - I'm beginning to get this! Fanfare!

To summarise: The light coming thru an EP is parallel, it is the lense in the eye that focuses this parallel light to form an image in the brain. If I use a compact camera (with a lense) it is the camera's lenses that focuses the parallel light coming thru the EP to form an image in the camera. Just the same as an eye.

I didn't quite get the part about taking an image of the inside of the scope. I will read it again! Hey thanks. I'm learning astro physics