How to CCD Image higher than 72 dpi?

[zakkhogan]

How do you image at higher resolution? 

Im using the ASI120mm but can only produce images at 72dpi ? is this normal or am i doing somthing wrong

Thanks

Craig

[JamesF]

The camera has no concept of dots per inch.  Where are you getting the 70dpi figure from?  That's more like the sort of figure that would relate to a display.  Is it from Photoshop or something similar?

If you really want more resolution from the camera, you need more focal length and/or more aperture.

James

[zakkhogan]

Hi James,

Yeah im getting the dpi from photoshop once i open the tiff file.

How do i ad focal length? like using a barlow?

thank you 

[JamesF]

If it's in Photoshop then it's just an indication of how the program will handle its internal conversions.  I shouldn't worry about it.

Generally if you want more resolution in an astro image you need to increase the focal length either by using barlows or changing scopes, or decrease the pixel size by using a different camera.  It's important to remember however that the OTA can't resolve detail above a limit fixed by the aperture size.

There is a point at which one can be said to have matched the resolution of camera to the potential resolution of the OTA.  It's not that hard to work out if you're comfortable with a bit of trigonometry, but someone suggested the other day that as a rule of thumb you'd be fairly near the mark if you aimed for a focal ratio figure of five to six times the camera pixel size in um.  For example with a camera such as the SPC900 which has a pixel size of 5.6um, you'd aim for a focal ratio of f/28 to f/34, or for the 3.75um ASI120, f/19 to f/23.

James

[zakkhogan]

Thanks james, 

Maths is sadly not my strong point but i think i grasp it. 

So, my CPC800 has a focal ratio of f/10 so combining that with say 2.5x barlow should increase the focal ration to f/25 ?

does thank mean that at that magnification i will not be able to achieve pin sharp images?

Thanks

[JamesF]

Everything gets harder (including focusing) as you extend the focal length I'm afraid.  And if the atmosphere isn't into playing ball that night then that will become more obvious too.  The bonus is that on those nights when everything comes together you know you've probably got the best you could hope for.

There's a fun little wrinkle with SCTs and Maks relating to focal length.  Basically because they contain two optical surfaces (I'll ignore the corrector plate) the total effective focal length depends in part on the distance between the two surfaces.  That's just the physics of optics.  Because focusing moves the primary mirror, that means the effective focal length changes as you try to focus the image.  Therefore your OTA is only actually f/10 when the camera is in one specific position (which Celestron presumably knows, but we don't).  So, adding your 2.5x barlow will certainly multiply up the focal ratio as you expect, but you won't necessarily know the exact figure you end up with.  I usually work backwards from my image sizes to calculate the focal length I'd need to get that image size and thus the actual focal ratio, but if you're using FireCapture and you tell it the target you're working on it will actually estimate the current focal length and save it in the data file with the recording, allowing you to get a rough idea of what the focal ratio actually is and attempt to adjust your imaging train accordingly.

If you put the 2.5x barlow straight into the visual back of your OTA and then the camera into that, I'd not be entirely surprised if you were only getting about f/20 I think.

James

[zakkhogan]

Thanks James. 

Youve been very helpfull, Just one more thing to ask if i may, you mention placing the barlow \ camera directly into the visual back ive been placing the my it camera, filter wheel etc into the diagonal,

am i doing it wrong?

thanks 

craig

[JamesF]

I wouldn't say it was wrong, particularly.  It's just another bit of kit to deal with that might get misaligned or otherwise cause problems, especially as you increase the weight in it.  And of course it does rob you of a few photons as it's not a perfect mirror.

James