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what went wrong?


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Usually I image DSOs with a 700mm frac and get good results. But recently I have had my 2000mm SCT on the mount to do the moon. So rather than remove it, I decided to try imaging part of the Rosette with it (realising I would not have the FOV to get the whole target in).

But the results are disappointing. I don't use guiding, these are just 20sec subs. I used NINA's direct dither function (which dithers just straight to the mount). The image scale with this camera ASI2600MC Pro)  at this FL was way oversampled at 0.39 arcsec/px so I used 4x4 binning in NINA to get a more reasonable 1.55 arcsec/px

So what went wrong? Not only do the stars look blurry (despite autofocus working well), but there is a TON of walking noise (which should have been dealt with by dithering every three frames). It's only half an hour of integration time, but still...

 

IC 410.jpg

Edited by StuartT
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I think the rule of thumb is to dither around 10 pixels in the main camera - if you haven't altered the dither settings and you have binned 4x4 your main camera pixels will effectively be 4x larger, so you may not have dithered enough. To be honest it looks pretty good to me at that focal length without any guiding. 

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As stated above. Given you are not guiding this is a pretty good result. At this sort of FL you are bound to get some drift even at the pixel scale you are using.

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1 hour ago, iantaylor2uk said:

I think the rule of thumb is to dither around 10 pixels in the main camera - if you haven't altered the dither settings and you have binned 4x4 your main camera pixels will effectively be 4x larger, so you may not have dithered enough. To be honest it looks pretty good to me at that focal length without any guiding. 

 

1 hour ago, Clarkey said:

As stated above. Given you are not guiding this is a pretty good result. At this sort of FL you are bound to get some drift even at the pixel scale you are using.

Ok. Many thanks both. I must admit, I didn't think about having to increase dither amount because of binning, but it makes sense. Only thing is I don't know if there is a setting for that in NINA. (or at least, I haven't found it)

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11 minutes ago, StuartT said:

Ok. Many thanks both. I must admit, I didn't think about having to increase dither amount because of binning, but it makes sense. Only thing is I don't know if there is a setting for that in NINA. (or at least, I haven't found it)

image.thumb.png.53fa0d0aa694b79d7d195e8db2cf9404.png

Steve

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41 minutes ago, teoria_del_big_bang said:

image.thumb.png.53fa0d0aa694b79d7d195e8db2cf9404.png

Steve

Thank you!

Is there a reason to have dither in RA only set to ON? I would assume that this might 'create' walking noise. Wouldn't it be better in both RA and Dec to mix things around more?

 

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4 minutes ago, StuartT said:

Thank you!

Is there a reason to have dither in RA only set to ON? I would assume that this might 'create' walking noise. Wouldn't it be better in both RA and Dec to mix things around more?

Pass on that one.
I actually use PHD2 and when that is selected the dither in RA only is off, I just switched over to direct guider to see if the option of changing the amount of pixels in dither was still there (which it is), but when I selected direct guider it seems to default to be on, no idea why - unless I accidently clicked on it when using the snipping tool to get the screen shot ?????

When I had PHD2 selected:
image.thumb.png.57a7b3b19712c9111b85d0bb061aa466.png

When I selected Direct Guider:

image.thumb.png.3242116f4275fb7d8bad99e70991abcc.png

If I select it to off then next time it remains off, so no idea why it defaulted to on the fist time, unless like I said I accidently clicked it.
But I would agree with you that it should normally be selected off.

Steve

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12 minutes ago, teoria_del_big_bang said:

Pass on that one.
I actually use PHD2 and when that is selected the dither in RA only is off, I just switched over to direct guider to see if the option of changing the amount of pixels in dither was still there (which it is), but when I selected direct guider it seems to default to be on, no idea why - unless I accidently clicked on it when using the snipping tool to get the screen shot ?????

When I had PHD2 selected:
image.thumb.png.57a7b3b19712c9111b85d0bb061aa466.png

When I selected Direct Guider:

image.thumb.png.3242116f4275fb7d8bad99e70991abcc.png

If I select it to off then next time it remains off, so no idea why it defaulted to on the fist time, unless like I said I accidently clicked it.
But I would agree with you that it should normally be selected off.

Steve

Thanks Steve

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The choice to only dither in RA is to help those folks with mounts that perhaps don’t play nicely when the guide star in Dec is thrown (with a dither set to both axis) - perhaps taking an age to recover. Especially those that have the PHD quide setting changed from ‘auto’ to North or South only guide commands. That helps if you end up in a constant backlash battle and then the mount is jumping from one side to the other…

Edited by TakMan
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2 minutes ago, TakMan said:

The choice to only dither in RA is to help those folks with mounts that perhaps don’t play nicely when the guide star in Dec is thrown - perhaps taking an age to recover. Especially those that have the PHD quite setting changed from ‘auto’ to North or South only guide commands. Helps if you end up in constant backlash and then the mount is jumping from one side to the other…,

So not really applicable if dithering but not guiding I presume ?

Steve

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That’s a very good point Steve!

If you don’t have a guide lock position to arbitrarily shift / dither…. was dithering taking place in the first place…?!

Edited by TakMan
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56 minutes ago, TakMan said:

That’s a very good point Steve!

If you don’t have a guide lock position to arbitrarily shift / dither…. was dithering taking place in the first place…?!

Yes. NINA has a function called Direct Guide. So (presumably) it just sends a command to the mount to randomly vary from the RA and Dec of the target (which is stored in the sequence). My subs do look like they were dithered, because I quickly browsed through them in FITS viewer and they jiggled about 

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I get similar bloating of stars with its smaller brother (ASI533) with my 780mm focal length refractor. Results in ~1 arcesecord per pixel at 1x1 binning but stars do not improve if I perform 2x2 resampling. 

I've only just started using NINA, but from the ASI Air I used to use, you would need to change dither amount based on the telescope use. I'd like to think that NINA is clever enough to work this out if you state your focal length and camera pixel size. 

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