taking pictures afocally

[gurey]

So I connect my dslr to my telescope with a x2 Barlow. If I try to photograph anything but the Moon (i.e planets) they come out very small. I think using x3 Barlow would make it slightly bigger, but are there any other ways without zooming in?

[jambouk]

Are you imaging at prime focus or afocally?

My response in this thread contains a pdf I made to try outline the absolute basics about imaging with a telescope:

http://stargazerslounge.com/topic/253120-eyepiece-projection/?fromsearch=1

What scope are you using and what is its focal length? Planets look very small in all but the longest focal length scopes.

James

[Alien 13]

That generally is the prefered method for imaging Planets but as you have found out the image is very small unless you have a scope with a very long focal length. The way around it is to use a web/planetary cam with a tiny sensor this gives a bigger image scale.

Alan

[gurey]

Are you imaging at prime focus or afocally?

My response in this thread contains a pdf I made to try outline the absolute basics about imaging with a telescope:

http://stargazerslounge.com/topic/253120-eyepiece-projection/?fromsearch=1

What scope are you using and what is its focal length? Planets look very small in all but the longest focal length scopes.

James

Yes, sorry, it is at prime focus. I will check it out. It is a 76mm 700 fl.

[gurey]

That generally is the prefered method for imaging Planets but as you have found out the image is very small unless you have a scope with a very long focal length. The way around it is to use a web/planetary cam with a tiny sensor this gives a bigger image scale.

Alan

Thanks

[jambouk]

Yes, the planets will look small with that set up; the moon will be framed nicely.

Plug your scope and camera into this site and you'll be able to see how things will look:

http://www.12dstring.me.uk/fov.htm

There are very few [if any] black and white answers in practical astronomy, but I'd say you have the wrong set up currently for planetary imaging. The relatively short focal length of your scope lends itself to DSOs, which love being imaged at prime focus

Good luck with it all.

James

[Alien 13]

That scope is a little longer FL  than mine Gurey but it hasnt put me off having a go a Jupiter with a DSLR and cheap skywatcher barlow

I too must get a webcam.

Alan

[jambouk]

Impressive Alan.

[jambouk]

My attempt with a C11, 2.5x Powermate and an ASI120MC.

[gurey]

That scope is a little longer FL  than mine Gurey but it hasnt put me off having a go a Jupiter with a DSLR and cheap skywatcher barlow

jup3.png

I too must get a webcam.

Alan

Very nice, Alan. How did you do it?

[gurey]

My attempt with a C11, 2.5x Powermate and an ASI120MC.

Very nice image. I doubt i'd be getting anywhere near that in the foreseeable future.

[gurey]

Yes, the planets will look small with that set up; the moon will be framed nicely.

Plug your scope and camera into this site and you'll be able to see how things will look:

http://www.12dstring.me.uk/fov.htm

There are very few [if any] black and white answers in practical astronomy, but I'd say you have the wrong set up currently for planetary imaging. The relatively short focal length of your scope lends itself to DSOs, which love being imaged at prime focus

Good luck with it all.

James

Cheers for the link, Interesting

[miafey]

Very nice image. I doubt i'd be getting anywhere near that in the foreseeable future.

Don'g worry, there is nothing wrong with your set up, just not suitable for these kind of targets. 

Basically you need to take field of view into consideration when framing your target. 

Field of view is decided by the aperture of your scope, the f value and the size of the sensor of your camera. When you use your camera for daytime imaging, you will adjust your lense to get closer or further away from your target, and you will notice your field of view changes with it won't you? it is the same concept here. 

Sometimes when you are not familier with things up in the sky you have little idea about the size of an object. At least I was like that before I spent a fair number of time reading things on this topic. I thought planets were of course bigger because they look bigger in the sky. Deep sky objects were surely smaller as they look tiny up there. But no it's not really true... A lot of deep sky objects are very big. Your camera sensor is actually rather big compared to sensors in most of the astronomical cameras. With the same set up, if you use a camera with a smaller sensor, your field of view (the canvas if you like) will be smaller and the object will then appear larger on it. You are effectively zooming in. 

For imaging planets, webcams (or planetary cameras) are actually more useful than DSLRs as they have smaller sensor and higher fps value. Alternatively, you could increase the focal length of the scope to get a smaller FOV. You can get a fairly good idea of FOVs from this website:

http://astronomy.tools/calculators/field_of_view/

Play with it and you'll start getting the idea!

[jambouk]

With your set up, you can take video with the dslr of the planet in question, with a barlow, then stack the video with free software registax (or others). The stacking software breaks the video up into frames and picks the best ones to stack - i suspect that is what Alan did. The problem with dslr video is that there is some compression of the data which results in losing some of your data. You can achieve frame rates of 20-30 fps with most dslr cameras.

The other option would be use a astro-modified webcam like an SpC900. These have a very small field of view so it can be diffixult to get the planet on the sensor; and you need good tracking to keep it there. These cameras can achieve about 10fps else you start to lose data, but at 10fps there is, i believe, less compression of the data compared to the way a dslr records. I don't know how the compression of a dslr compares to the slower frame rate of the spc900. The other thing to consider is the pixel size; someone else will know about the importance of pixel size. An spc900 will cost £40-50 second hand, and you'll need to use a laptop and various free software.

The step up from the spc900 would be a dedicated planetary camera, like one from the ZWO ASI series; £100 plus. Again you'd need a laptop and dree software.

One problem with all of this is the fact the planeta are all going to be reasonably poorly poaitioned for a couple of years. Jupiter should be ok this year; saturn past its best; mars is hard; uranus and nepture very small and very hard to capture with shorter focal lengths.

James

[happy-kat]

Alternatively, you could increase the focal length of the scope to get a smaller FOV.

.

Could you achieve this by leaving the telescope cap on and removing the smaller middle cap when imaging the Moon?

[Stu]

.

Could you achieve this by leaving the telescope cap on and removing the smaller middle cap when imaging the Moon?

No, that changes the focal ratio (making it much longer), but doesn't change the focal length of the scope which is what matters in this instance. It would also reduce the resolution (by reducing the aperture) so you would have less detail available.

A Barlow is the best option, people use up to x5 Barlows for planetary imaging.

[GeoffB]

Thank you Big Sumorian and others for your explanation as to why my photos seem so small with my dslr. As many know I am a complete novice using a newtonian for my Astrography only converted to a bigger scope in the past couple of weeks. Used a refractor for the best part of a year but mainly on the moon just moving to the deeper space objects. My cousin in America has the benefit of playing with kit from Harvard Smithsonian and Nasa she makes me really jealous but then shes viewing high energy stuff millions of light years from us.

[ollypenrice]

That generally is the prefered method for imaging Planets but as you have found out the image is very small unless you have a scope with a very long focal length. The way around it is to use a web/planetary cam with a tiny sensor this gives a bigger image scale.

Alan

I don't think this is correct. You can't make the image larger by making the sensor smaller. Thinking about it a step at a time...

1) The size of the image projected onto the chip is totally dependent on the effective focal length of the optics. The longer the effective focal length the larger will be the image projected onto the chip. (Adding a 2x Barlow will make it 2x wider and 2x higher than at native focal length.) A bigger chip will catch more sky around the planet (which you may well not want) but if you use a smaller chip to remove that unwanted sky your projected planet doesn't get any bigger.

2) When your captured data appears on the computer screen at full size one pixel from the camera will be awarded one pixel on the screen. But if your camera pixel count makes that image too big to fit on your screen then most software will default to reducing the size of the presented image to 60% (or 40% or whatever) of full size to make it fit on the computer screen. However, the real image in the data is not affected so if you zoom in on the image to 100% and centre on the planet you will see it at full size. You could crop the unwanted sky and then the image would be opened by default at full size on the screen.

To look at it another way, if you take two chips of the same size, one with smaller, and therefore more, pixels than the other, then when presented on the computer screen the small pixel camera will give a larger image than the small pixel camera. I see this every day. I run a 9 micron pixel camera in one half of a tandem rig and on the other half I run a 7.3 micron pixel camera with the same sized chip. The scopes are identical but the small pixel camera gives a larger image - simply because one chip pixel gets one screen pixel on the PC.

Conclusion: you can get a bigger planetary image by having more focal length or smaller pixels or both but you cannot get a larger planet by reducing chip size. You can also, if you are over-sampled, get a larger planetary image by using drizzle stacking and fast frame cameras in long FL optics really benefit from this hugely.

Olly

[dph1nm]

A few years ago I think webcams did have much smaller pixel sizes than DSLRs. But DSLRs have shrunk a lot and many new cameras now have pixels sizes of 4um or less.

NigelM

[Alien 13]

Very nice, Alan. How did you do it?

The image was a series of 6 stills cropped and centered in PIPP then registax there was a fair bit of noise and colour offset so I split the channels into seperate RGB and processed each seperately I then combined and aligned them for the final image.

Alan

[GeoffB]

Thanks for the discussion on newer cameras and pixel sizes on the DSLR I think youve just helped me decide to use my really old D50 with only 5mp on its sensor for astrophotography, so my pixel sizes would be bigger but obviously not able to print above A3. I feel like the world is going full cicle with regards the arguments in the photographic world may be its time for full frame Digital rather than a 1.6 crop factor for me lol. Don't tell the wife though, she'd go spare if I bought a new camera especially for Astro Imaging.

Geoff