dithering without guide camera

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tiziano
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Joined: Thu Jan 10, 2019 5:42 pm

dithering without guide camera

#1

Post by 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
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admin
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Re: dithering without guide camera

#2

Post by 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
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oopfan
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Re: dithering without guide camera

#3

Post by 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
Posts: 23
Joined: Thu Jan 10, 2019 5:42 pm

Re: dithering without guide camera

#4

Post by 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
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Re: dithering without guide camera

#5

Post by 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
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