m96

[alacant]

Hi everyone.

The only clear night we're likely to get all week so of course it just had to be poor seeing -around 1.5"- and so no way to get in close. Back to short telescopes and galaxies looking like out of focus blobs. 

Thanks for looking and as ever, any suggestions for improvements most gratefully received.

700d + nt150s; 2 1/2 hours @ ISO800

[tooth_dr]

Very nice Alacant.  Suggestions - looks a little yellow perhaps, but plenty of the detail in it, oh and you shouln't be posting in the beginners section for imaging.

[Demonperformer]

Nice wide field shot considering the conditions. 

[ollypenrice]

That might be one to add to on a better night. There's quite a bit of good signal in there.

Olly

[coatesg]

Looks good - 1.5" (FWHM?) doesn't sound that bad to me...! I reckon on getting 2" seeing - any less and I'm technically undersampling...

[alacant]

48 minutes ago, coatesg said:

1.5" (FWHM?)

Hi.  It's what it says here. Dunno what it means, but I think of it as a measure of how a guide star bounces around. It seems to match what we are likely to get in PHD2. The lower it gets, the longer -focal length-  we can go. E.g. it's not worth going for 1200mm unless it's gonna go below unity. Cheers.

[ollypenrice]

FWHM describes the full width at half maximum brightness of the bell curve for the star's brightness. That's the diagram on the left. The one on the right shows what happens if you use a saturated star to take an FWHM reading. (Not relevant here.) The 'half max' width is measured simply because the outer boundary of the star is so hard to define.

Both seeing and cloud will increase an FWHM value but for different reasons. In poor seeing a star may be sometimes small on the chip but will also be moving around, producing a smeared oversize image over time. For guiding it is best to use long subs in poor seeing so that the star produces an image of its average position, from which a guiding centroid will be derived by the software. (It'll find the middle of the blob and guide on that.) This is better than having the mount chase the changing apparent position of the star.

The same applies to FWHM focus in poor seeing. Longer subs stop the FWHM value from jumping around meaninglessly.

Only with good FWHM values is it worth shooting high res data. Mono users can, though, use nights of poor seeing to shoot colour. The Luminance from a stable night can restore the lost resolution. You might be able to use the same technique with an OSC camera by making a synthetic luminance from the good night's subs and using all of them for the colour.

Moisture in the atmosphere simply increases the star size by dispersing the incident light. The seeing itself can often be excellent on nights of thin high haze.

Different rigs will produce different FWHM values on the same night. At 3.5"PP we can frequently get under 1.00, 0.85 being the record. On our 0.9"PP rig the FWHM is far more variable with a best (rarely seen) of 1.2.

Olly

 

[Gerry Casa Christiana]

8 minutes ago, ollypenrice said:

The seeing itself can often be excellent on nights of thin high haze.

Wow really? Why does the thin haze help? Does it average out seeing?

[alacant]

3 hours ago, tooth_dr said:

looks a little yellow perhaps

Yeah. Colour! Had another bash with the boss looking over my shoulder:

[ollypenrice]

1 hour ago, Gerry Casa Christiana said:

Wow really? Why does the thin haze help? Does it average out seeing?

I doubt it. I think it has more to do with the general atmospheric conditions which prevail at the time. The great planetary imager Damian Peach chose Barbados for his ground breaking imaging runs. The transparency was not particularly good but the seeing was as stable as he could find - and Barbados is not a high altitude location.

Olly

[Gerry Casa Christiana]

28 minutes ago, ollypenrice said:

I doubt it. I think it has more to do with the general atmospheric conditions which prevail at the time. The great planetary imager Damian Peach chose Barbados for his ground breaking imaging runs. The transparency was not particularly good but the seeing was as stable as he could find - and Barbados is not a high altitude location.

Olly

Very interesting. 

[des anderson]

Great wide field shots. Des