dithering without guide camera

[tiziano]

Hi Robin
I have the Celestron Nexstar 6SE mount (Altazimutale)
on which I mounted Sharpstar 61DHP, with reducer (61mmx275mm)
in the photos I take I always see colored lines (the famous rain noise)
how do i make them disappear in sharpcap? , how do I dithering after each photo or group of photos.
I read on the Internet that you should move the mount after capturing the photos, this way these damn lines should disappear
can you help me

I don't want to use the guide camera

Thanks
Tiziano

[admin]

Hi,

you can set SharpCap up to use what is called 'dither only guiding' - this does not require a guide camera, but will just send a random small movement to the mount between frames while live stacking - see https://docs.sharpcap.co.uk/4.0/#!2!Guiding%20Tab and https://docs.sharpcap.co.uk/4.0/#Guiding%20Tab

cheers,

Robin

[oopfan]

Hi Tiziano,

Raining Noise is caused by insufficient exposure. Just try increasing exposure by 20%. Your stacks will look better but if you still see some remaining Raining Noise increase the exposure a little more.

How can this happen? The Brain's recommended exposure is driven by two factors:
1. The accuracy of Sensor Analysis.
2. Sky Background measurement.

I began using The Brain in SharpCap version 3.1 way back in 2018 (I think it was). I had a really clear night at Bortle 5. That night I imaged M51 and M81. The Brain recommended a 4.7-second exposure for M81, and a 9.4-second exposure for M51. The result was that M81 was riddled with Raining Noise. I had to discard it. M51 had just a little but it became one of my finest images.

Why the disparity? I've concluded that the Sky Background measurement was in error by improperly setting the selection box around an area of supposedly dark space. I may have included a couple of faint stars. This had the effect of suggesting that I had more light pollution and therefore The Brain lowered the recommended exposure.

There are, in fact, other ways to defeat Raining Noise. Dithering should be your last choice:
1. Increase the exposure.
2. Ensure accurate sky background measurements.
3. Consider rerunning Sensor Analysis.
4. Dither.

I place Dithering at the bottom of the list because it just adds complication to your set up.

EDIT: I remember that night in 2018 well. It was frigid. I imaged M51 first then I moved on to M81. I was so cold, I was shivering that it affected my vision! I remember measuring Sky Background. I remember thinking to myself "I'm dying here, let's just do it." Most likely I made a mistake by including stars. This type of thing can happen to you. If you start to see Raining Noise as you LiveStack, abort, increase the exposure or rerun Sky Background measurement.

Brian

[tiziano]

Hi Robin

I tried to use 'dither only guiding' but it gave the error that the mount (Nexstar Celestron)
does not support 'Pulse Guiding'.

Hi Oopfan

I will follow your advice in the coming evenings.

Thank you very much for your answers,
and sorry for my english,It Is terribile

Ciao to all

[admin]

Hi,

that's a shame - basically your mount's ASCOM driver does not support the pulse guiding controls needed

There may be a workaround... Some Celestron Nexstar mounts have an autoguider port - if you have this and an ST4 autoguider port on your camera, then you can connect the two with a cable and choose 'On Camera ST4' as your mount option in SharpCap. I think that should let SharpCap trigger the guiding movements (although you will lose access to GOTO, co-ordinate readout and plate solve/sync in SharpCap)

cheers,

Robin

[jsadoulet]

Hello Robin
I hope you are good !
I have the suspicion of a bug in the dithering processs without guiding. Even if the option "dither only on RA" is not ticked, my frames seem to be dithered always in the same direction ....
Maybe I am wrong. But...
My mount is an EQ6 Skywatcher linked to the computer via Synscan hand controler and ASCOM
I am always at the most recent version of SharpCap.

Could you please check or tell me ?

Thank you
Jean-Paul

[admin]

Hi Jean-Paul,

the best thing to do is to see if you can find the SharpCap log from that session and we can check the values recorded in it for the dithering movements - there will be lines in the log like

Code: Select all

Random dither of (<some numbers>) arcsec; rates are {<pulse guide rates>}, pulse guide times are (<some other numbers>)s"

One of these will show up for every dither, and the numbers should be randomly distributed with a mean of zero (ie we should see roughly equal numbers of + and - values). If that is true then we need to check the code that turns those numbers into commands for the mount, or perhaps how the mount moves in response to the commands, if we see all +ve or all -ve numbers then we need to check the code calculating the random dither movements.

cheers,

Robin

[jsadoulet]

Hello Robin

Thank you for the tip. I didn't have the idea to look at SharpCap Log. Pretty detailed.
I asked Excel to draw the dethering path (here attached).It seems to be OK, even quite impressive.

The process I have:
1/ Start Live stacking
2/ At 3 minutes, dither
3/ At 5 minutes, save the stack
and back to 1/

What is the best process ?
Thank you for your answer

Jean-Paul

[admin]

Hi Jean-Paul,

yes, that graph looks fairly reasonable in terms of random movements. Of course there is another twist here - those values show the amounts of dithering that SharpCap requested, but what we don't see is how the mount responded to those movements. Maybe the mount responds more quickly to a +RA pulse guide than a -RA pulse guide, so even if the requested times were roughly equal in both directions, there could be a drift in position. Equally, there could be a drift in position (due to polar alignment errors or tracking rate errors) overlaid on top of the dithering, which could mean that overall there is a drift in one direction with small random nudges on top.

It might be worth testing with a bigger dither step (you can set this up in the SharpCap guiding settings). That would mean longer guiding pulses and the sort of effects mentioned above would be less important relative to the actual dither movements.

You could also look in the existing log if you have one containing many short stacks as you describe above for lines like

Code: Select all

New stack first frame processed, mount co-ordinates are <Co-ordinates>

If all you are doing are your steps 1,2,3 above, then the only reason the co-ordinates would differ from one stack to the next would be the dithering movements - these co-ordinates should therefore have random small changes that are coming from the random dithering offsets. If we don't see that then that gives us something to investigate.

On the subject of the best procedure - I would usually suggest dithering several times during each stack rather than once - the idea is to spread the hot pixel effects around over several random pixels to make them less noticeable.

cheers,

Robin

[jsadoulet]

Hi Robin

RA and DEC are randomized properly. I will try to change my sequence next clear night at site.

My best

Jeazn-Paul