M33/ modifed webcam, M33 Atik 314L

[Rocket Pooch]

yup, John noticed this the other week, but that's fine they are different chips and designs.

So the point is, its difficult but not impossible to be objective about the differences in performance of the optical systems, unless we fully understand what we are imaging with.

So with Rick's originals question we could work out what the differences are, including the sky background noise, but it would need some reference data, flat, flat dark, darks, bias, subs etc for each camera.  Also if we were using different scopes we would need to have an understanding of the optical differences, F Ratio. 

The sky background reading v's surface ADU's in reference between two images as a ration of brightness could be worked out using a reference no variable star as part of the image, this can be done using photometry, then we could measure the background sky reading after Bias etc are taken off the images, and then scaling them according to the duration of exposure etc.

So it is possible, but only if we take the reference images.

I have a set of images on M66 & M27 with all the BIAS Durations, Optics, Flats and reference items known, so I could probably do the same with the 314L and the 8" RC and work out what the effective difference is, but that would mean two nights imaging, and its cloudy and the wrong time of year.

Personally I think we have all practised a little more and we're just better.  We can focus better guiding is better and also the processing is better, and because we're not taking scientific images we don't do all the bits needed to do a proper evaluation.  But subjectively one is a hell of a lot better than the other.

For example, in mine and Fay Les Granges images we have not taken any flats, but I know we should have if we were to do a proper evaluation of the image. 

Also if Fay or I was to try to re-produce the same images here we would need to take flats.

So another pop quiz, then,

If we were to take two images of the same object with the same equipment one in france, background ADU Sky Count = 200, and one from my back garden background ADU Sky Count = 2400, why do I need to take flats at home and not in France?  The surface ADU reading of the object being imaged is 2600 ADU's?

This time the images we're being observed by a small horse called Monty who is playing a flute.

[Mike]

Because with the lower ADU in France the conditions responsible for making things such as dust bunnies, etc. show up on the image are far less prevalent, plus, I am guessing, the background IR radiation will be less (due to less refelctivity from droplets in the atmosphere, etc.) and therefore the overall noise level will be lower.

Monty must be very small indeed to enable him to get his hooves to press the keys on that flute. Either that or he is playing a very large flute. Does the size of the flute affect the background ADU somehow?

[MarkS]

To answer Chris's second pop quiz question regarding flats:

One of the main reasons I take flats (other than for dust bunnies) is to remove the effect of vignetting.  Vignetting means that with a high background sky count (as in your example) it will not be the same count across the frame but will be greater in the middle than in the corners.  This variation in the background sky count makes it tricky to remove.  Applying flats will compensate for the vignetting and subsequently makes the background easier to remove (it is likely to then be a simple matter of gradient removal).

There is a problem with flats - lots of buy-to-let investors have lost money on them - an event that they thought as being as improbable as seeing a horse called Monty playing a flute.

[JohnP]

If we were to take two images of the same object with the same equipment one in france, background ADU Sky Count = 200, and one from my back garden background ADU Sky Count = 2400, why do I need to take flats at home and not in France?  The surface ADU reading of the object being imaged is 2600 ADU's?

I'm guessing that it's something to do with the fact that in the image in France the S/N ratio is so much better (because background is a lot lower) so you don't need to process the image as hard to get to the finished image. With the one from home you have to push the image hard to extract the signal above the background so issues like dust bunnies & vignetting which lie down in the background become more obvious....

The thing that confuses me is that according to theory it is more important to take flats from a dark site than a light polluted site...??

John

[Rocket Pooch]

I thought it was darks not flats?

[Tony G]

I still prefer Fay's second image. 

Tony G

[JohnP]

Sorry mate - yes you are correct (smart Arse) - I meant to say darks from a dark site than a light polluted site...

John

PS - Tony - I hope you are following all this for the next image you take at DSC....

[Tony G]

John,

Following all what? I was lost from about the third reply, and when I looked again, I thought that I had stumbled onto an Open University Course in  mathmatics and advanced physics in F ratios, I logged off three times before I realised I was on the right site.

Tony G