DSLR mirror-up

[Dave_D]

while pondering the issue of the horizontal gradient which appears at the bottom of a lot of my images, i noticed this:

mirror in down position:

mirror in up position:

is the mirror housing obscuring part of the light cone from the primary to produce the gradient? as the mirror is at the top, and the images produced are inverted, the gradient would appear at the bottom.

this led me on to wonder if it is safe to completely remove the mirror and it's mechanical components as this camera is purely for astro work.

[Xplode]

Mirror can be removed without problems, there's a guide here http://www.markshelley.co.uk/Astronomy/Projects/Mirrorless/canon550mirrorless.html

[Dave_D]

Mirror can be removed without problems, there's a guide here http://www.markshelley.co.uk/Astronomy/Projects/Mirrorless/canon550mirrorless.html

well it even answered my question about messing the light cone   or should that be /facepalm???

ahh the joys of t'interweb.

[nmoushon]

You should be fine rmeoving it. Its only for looking through the view finder on the camera itself. So you wont be able to use that if you remove it. Does the 1100D have live view? Not sure if it does or not but you will need that if you remove the mirror.

[Dave_D]

and done... and a lot less traumatic than i imagined 

thanks for the link Ole and thanks Gina for the reassurance 

[Guest]

Thats a good idea plugging the kettle into the back of the camera

[IanWatkins]

Top work.

Does the kettle work off the camera battery?

Seriously, does doing this make a difference for imaging? I ask as I used to suffer from a nasty gradient on my 20D (way back) that I put down to amp glow.

Cheers

Ian

[Dave_D]

Top work.

Does the kettle work off the camera battery?

Seriously, does doing this make a difference for imaging? I ask as I used to suffer from a nasty gradient on my 20D (way back) that I put down to amp glow.

Cheers

Ian

just waiting for a clear night to try it out, but from the link further up the thread, it should help as that was exactly the same effect i was having on my subs

[Alien 13]

I get gradients at the bottom but only when shooting at 4000th sec (mirror speed too slow) mirror lockup fixes it.

Alan

[russellhq]

just waiting for a clear night to try it out, but from the link further up the thread, it should help as that was exactly the same effect i was having on my subs

I would think you should be able to see a change in your flat frames?

[sharkmelley]

Do you use your camera on a particularly fast imaging scope?  Most people don't notice this bottom gadient unless using a fast scope like the Takahashi Epsilon or a Hyperstar. 

The other problem that mirror removal cured is the extra pair of vertical diffraction spikes on bright stars near the bottom of the image - again caused by the mirror edge interrupting the light cone.

A flat frame will definitely tell you if you have cured your problem - no need to wait for a cear night.

Mark

[Dave_D]

i've no provision for doing flats at the moment... trying to build a light box at the moment. it's an f4.8 and the gradient was quite noticable. i often wondered where the extra diffraction spike came from as well, but it wasn't limited to just the bottom of the frame

hopefully i've killed 2 birds with 1 heated scalpel blade 

[russellhq]

How about a simple white sheet over the end of the scope (T-shirt, pillow case, A4 paper) and point it at a window during daylight? It's quick and dirty but should work.

[Dave_D]

just did a very rough-and-ready test (white shirt over scope with it pointing out the window) and the difference in shape of the vignette is huge. keep in mind the scope isn't collimated, in a warm bedroom and the focus point isn't where it would be for imaging.

pre removal

post removal

[Dave_D]

full frame sub from tonight (imaging at this very moment) and NO gradient

[Alien 13]

Nice result, it looks like an effective mod for an astro only camera.

Alan

[sharkmelley]

Good result!  Glad my guide was helpful

Regarding diffraction spikes - yes you will still get an extra pair of diffraction spikes on stars all the way around the frame edge - the spikes will be perpendicular to the edge.  But a greater area is affected by this on the edge with the mirror, until the mirror is removed.   It's because the CCD on a DSLR is at the bottom of a rectangular opening and each edge of the rectangle chops the light cone to some extent.

So far I've removed the mirror from my 350D, 400D and 550D - it's the same procedure for each.

Mark