Plane to see

[MartinB]

Dithering does give cleaner images. Certainly sorts out hot pixels and seems to work better than just using darks, not quite sure why. Dennis will know! The big problem with dithering is that you have to increase the delay between each exposure whilst the guiding gets itself back on track. If you are running short subs then this can cut into your overall exposure time quite a bit. So I tend to restrict it to narrow band or when I'm using active optics( when the delay is minimal)

[Tim]

Nebulosity, which I have been using, doesnt have the option for dithering.

How does maxim deal with the guiding? Does it choose a new star? Or use the same star on a different part of the camera?

It'd be a pita to do it manually I think!

[Psychobilly]

to dither or not to dither that is the question....

I know I have done it deliberately a few times as it also helps to smooth out senor carp when i cant be bothered to do proper flats...which is about 90% of the time...

It really aint that bad TJ... turn off the guideng quick random slew turn on the guiding again...

Billy...

[Tim]

Well, I always try to do proper flats. Even though pixinsight will do a pretty good job of evening out the pic, nothing gets rid of dust blobs that are a bane with CCD's. Never had that problem really with the dslr. Flats also sort the vignetting too though, and help with contrast. Darks on the other hand, I have let myself slip a bit on them. With the dslr they were vital to get anything decent, but the qhy8 is fairly low on dark noise compared to a dslr, and I've hardly used darks at all, getting lazy....

Out of interest, how does a gamma ray burst manifest itself???????

[barkis]

When you reach 10,000 posts TJ. :laughing2:

Ron.

[Tim]

hahaha! Good one Ron!

[George]

Looks like an intergalactic war theatre.

A great looking pic though.

Ron.

I was just thinking Starwars myself

Interesting image BTW TJ.

[MartinB]

I don't bother with darks with the QHY8, I'd end up using up valuable light exposure time. Dithering can help here.

In maxim you can dither via mount or via guider. Not sure what the difference is, I've tended to use the mount but all is explained in the help file.

Gamma ray bursts show up as a small white splodge on the sub.

[roundycat]

TJ, Dithering is simply moving the mount between exposures. The only real proviso is that the mount should be moved in every possible direction and different amounts each move. The idea being to keep the movements random so they will cancel out more effectively when you use a sigma reject type combine.

All noise is random but some is more random than other types. For example sky background and thermal noise in the chip. With a half way decent camera with proper temperature control you can keep a library of darks and periodically update them. They will take out nearly all of the cameras' thermal noise as it is pretty constant. If you look at a dark subtracted frame up close you will find odd pixels here and there that were not affected by the dark subtraction. You can take these out with a hot pixel removal tool (Maxim) or rely on the dithered frames to get rid of them.

With dithered frames the stars are in the exact same place in relation to each other. They are, however, in a slightly different place with respect to the frame edges due to the movement between exposures. Hence the odd noise spikes stand out and this is what helps Sigma reject algorithms to find and eradicate them. I would suggest moving the mount a distance in pixels equal to your arc sec/pixel scale. Typically 2 at 1200mm or 4 at 500mm. Or anything in between.

Dithering thus helps to clean up frames considerably, catching both hot and dark pixels. It should not be confused with anything at all to do with Flats or muck in the optical train as this will always be in the same place with respect to the frame edge and will not be affected in the slightest.

Contrary to your thought that sigma reject summation will do away with faint signal I have always found the opposite. It always makes faint signal clearer as it lowers the noise floor and the signal thus stands out.

I always dither by autoguider as dithering by mount requires an ASCOM connection or similar so your computer and Maxim can talk directly to the mount. If you dither by autoguider the amount of dither is set up in the Sequence dialogue and Maxim simply picks a new place for the (same) guide star and the mount then aligns itself to it. The whole process can be a bit slow if you are using a big dither value and a longish guide exposure. It is recommended that you allow a delay for the mount to settle on the new position. Typically 4 or 5 times the guide exposure plus, with Maxim, the fixed delay that is always there.

With my current setup of FSQ106 and Artemis with 9 micron pixels I am imaging at 3.5 "/pix and set a dither of 4 pixels and a delay of 30 seconds between frames. Cut this time too short and your camera will start exposing before the mount has zeroed the guide star.

With my TMB at 1.5 "/pix I use a 2 pix dither and a shorter delay.

If your guiding is a bit iffy for any reason you can gamble on a shorter delay but with my AP I am regularly looking at a guiding residual of around 0.02-0.04 pixel and see no sense in spoiling that by rushing things. Long exposure do mean that your delays are not as big a part of the overall imaging time as if you were only exposing for, say, 30s per sub. Then the delay is 100% of each sub exposure time.

Manual dithering could be achieved but would be a bit hit and miss I think. Simply switching off the tracking would allow you to move the mount but the guide star would have to be re-aquired. In fact anything you do whilst guiding is set to end in tears unless you re-set the guide star after each move.

Dennis

[MartinB]

That's a great summary Dennis

[RobH]

Out of interest, how does a gamma ray burst manifest itself???????

I believe there's a slight tingling as the neutrinos hit, followed a couple of seconds later by the shockwave.

Then.......nonexistence

[MartinB]

[Psychobilly]

Out of interest, how does a gamma ray burst manifest itself???????

I believe there's a slight tingling as the neutrinos hit, followed a couple of seconds later by the shockwave.

Then.......nonexistence

Bit like a heavy night on the Vodka Redbull then....

Billy...

[RobH]

[Tim]

Dennis, that is very helpful and clear to understand.

I do find Maxim to be clunky and bloated though, especially with freezing fingers, but maybe i'll have to give it another go. Either that or nag Mr Stark to include dithering in nebulosity.

Is it something you would always include without fail, or could you take it or leave it, ie, is the effect on the result so dramatic that it is worth the effort? I can see how it really would have helped with my dslr photos.

Cheers

TJ

[roundycat]

TJ, my ST10 is extremely well behaved, summer or winter. I have a library of darks for it, all at the same temperature. Simple dark subtraction works well with this camera to clean up the background although some hot pixels always get left behind. It seems to attract cosmic ray hits but all the noise comes out during calibration. If any hot pixels are left, and there aren't many, the sigma reject takes care of them. I sometimes dither with this camera but mainly only when I forget to turn dithering off.

If you use a camera that has questionable cooling or none at all then I would think dithering is essential as you will have hot pixes of varying magnitude all over the place. Hot pixel removal can leave a great many hot pixels for some reason. Tests I did showed without doubt that non-calibrated but dithered frames came up cleaner than calibrated but non-dithered frames when stacked using SD Mask.

You can do a simple test. Take, say, eight frames that have been dithered and calibrated and combine them using SD Mask and then do the same again using an average output. Look at the results at 100% magnification at least. You will see little groups of hot pixels all over the place in the average but none at all in the sigma reject combination. Remember also that there will be little groups of dark pixels that may not be easily seen until you start stretching it all.

By the way, I strongly recommend doing it all in the warm.

Dennis

[Tim]

Thanks Dennis. I posted this pic as a bit of fun, but it has been one of the most informative threads for ages!

Just out of curiosity, I took an old dslr image of M31 that was plagued with noise, and stacked it using SD mask. Lo and behold, noise gone or vastly reduced. Hmmm, that means that even if we get 3 months of cloud, I'll still have more images to re-work on than I have time for.....