10" RCT with 0.67x Reducer - severe vignetting

[ian_bird]

Hi Olly

13400 in the corner - 15200 in the center. Phew!

Thanks for that Olly. Very useful to know. Really appreciate it.

Love to Monique.

Looking forward to the Pizzas!

Cheers

Ian

[kirkster501]

What are ADU please and what that about?

Steve

[ollypenrice]

What are ADU please and what that about?

Steve

Analogue Digital Unit. It's just the brighness count on a pixel. This means that, in Ian's case, the darkest pixels on his chip are reading 13400 counts of brightness in the corner and 15200 in the middle. A perfect system would be a uniform value for a natural flat field but this variation is relatively trivial and will easily calibrate out using flats.

If you stretch your flats, or present a screen stretch of them, you are exaggerating this difference and plunging yourself into despair, thinking that you have terminal vignetting when really you don't. Ian has a non flatness of just 1800 counts out of 65000. Zilch. If you look at a flat that has not been stretched (and that is how they are appled to your lights) you will see that, in reality, you usually have a decent flat field anyway. (If you have a Takahashi FSQ, anyway!! :grin: )

Olly

[pixelsaurus]

How does one measure ADUs in an image?

[ian_bird]

Depends on the software you are using. In Maxim DL it's shown on the bottom status bar as a value after the i: - as you move the cursor over the image you will see it change.

Other software - don't know.

Cheers

Ian

[Buckk]

About back focus - take all the imaging stuff off (camera/filters/spacers/etc), point the scope at a largish brightish light source like the moon or a street lamp, move the focus fully in and move a piece of plain white paper in and out from the focusser until you see the image in focus.  This is where the ccd/cmos chip should sit.  Saves a lot of time winding in and out looking for an image on a screen in another room!  Cheers,  Bob.

PS just noticed how old this thread is - never mind, the method is still valid!

[ian_bird]

On 13/06/2016 at 13:31, Buckk said:

About back focus - take all the imaging stuff off (camera/filters/spacers/etc), point the scope at a largish brightish light source like the moon or a street lamp, move the focus fully in and move a piece of plain white paper in and out from the focusser until you see the image in focus.  This is where the ccd/cmos chip should sit.  Saves a lot of time winding in and out looking for an image on a screen in another room!  Cheers,  Bob.

PS just noticed how old this thread is - never mind, the method is still valid!

Thanks Bob. great idea. I love the simple ideas that are so simple they are actually brilliant!

Cheers!

Ian