Hyperstar NGC6888 from Blacklands

[RobertM]

This is my first real stab at a Hyperstar image from a DSC where almost everything went right (well almost).  First thing to learn is what ISO to use with what exposure length for each image and I think I got this wrong here.  Because of the new mount not wanting to dither exposures (despite guiding ok) I decided to keep exposures short at 30 seconds.  Trouble is you need a lot of these and it takes forever to stack the buggers afterwards.

The second thing is to check that the flats did actually get saved !  I woke up at about 8am and looked around for something to take flats against, what I needed was something large and white - Micks Van !  The side made a perfect flat field so I duely took 10 images but for some reason they didn't save - when I got home I couldn't see them anywhere !  Le petite problem ! I did manage to flatten the image using a synthetic flat frame, even so there are a couple of warts so It's not perfect.  One thing I'm very happy with though is the star shapes over the whole image.

It doesn't compare with what Mark is doing but I still enjoyed processing it none the less.

Comment most welcome.

Details of capture:

Optics: Celestron C11 Hyperstar3
Focal length: 570mm
Focal Ratio: f/2
Camera: Canon 1000D
Filter: IDAS LPS P2
Scale: 2.1 arcsec/pixel
Subs: 113 x 30s ISO 800 (56.5 mins total)
Location: Blacklands Farm, Kent
Date: 04/06/2011
Processed in Pixinsight 1.7



Link to full size uncropped: http://farm3.static.flickr.com/2457/5817675690_5131bcc3ba_o.jpg

TFL Robert

[mickw]

That's looking very good Robert with a lot of detail and I like the widefield

Would lower iso and longer subs improve things or achieve the same but with greater sensitivity ? 

Do you really need that many subs as each 30 sec is equivalent to say 5 min at a slower fr ?

You were lucky to find a clean bit on the van 

[Rocket Pooch]

Crap I've moved?

[MarkS]

Crap I've moved?

???

[Rocket Pooch]

Location: Blacklands Farm, Kent

[mickw]

That was supposed to be last week 

[RobertM]

Ooops ! 

Thanks Mick.

The reason not to go for lower iso (otherwise I'd agree) for dark skies is that the sensor read noise goes up significantly in these Canon cameras with higher ISO.  At ISO 800 it's around 6e whereas at ISO 100 it's about 21e (source sensorgen.info).  In a heavily light polluted area the higher read noise gets swamped by the photon shot noise so isn't so much of an issue but at a dark sky site it would make a difference.  Though it's possible , given enough subs, that high levels of read noise can be overcome in stacking.  The lower the read noise at DSC the more faint stuff can be pulled in, just look at the Atik 314L that Chris/Fay have had for a good example of a low read noise camera and the difference it can make.  I will try a slightly lower ISO of 200 and much longer subs at the next deep sky opportunity as the dynamic range goes up significantly but the read noise doesn't.  Good question though and it does require some thought.

You were lucky to find a clean bit on the van 

I washed it - didn't you notice

[Fay]

Very nice Robert, like the larger version. Do you think it wants just a little bit more colour?

[RobertM]

Hmmm, I think I know what you mean.

[JohnP]

Robert pretty amazing for 30secs - you must be pleased. I guess longer subs would be needed in order to background nebulosity showing.... Nicely processed as well...

[MarkS]

Robert,

That's a great demonstration of the power of the Hyperstar!  Very nice image.  The stars all have a good shape right out to the corners (except maybe the bottom left?).  You can't beat good optics!

I'm assuming this Canon is modified?

I'm bound to compare it to my own image taken with the Tak: http://www.markshelley.co.uk/Astronomy/2010/ngc6888_09072010.jpg

Mine is 2.5 hours at F2.8  yours is 1 hour at F2.  I can't see much difference between them which is exactly what we'd expect since we've both collected more or less the same number of photons.  The only thing I would say is that you must have a lot of star colours in your image just waiting to be liberated by some good processing!

That Hyperstar is certainly a force to be reckoned with - with those fast and corrected optics it's a stonking photon collector!!

Mark

[JohnP]

Mark yours had a load more of the background neb showing but again this is probably down to processing like the star colour.

Both great images,  John.

[Rocket Pooch]

If you want a comparison I have an RGB Image somewhere of NGC6888, its less than 2.5 hours of data with the QSI and the ED80, I noticed on these images there's a lot more noise in the images and the star shapes are not perfect, in Marks image the bight stars look very pushed they have an inner ring on some of them.

I'll dig out the data and process it later this weekend.

Chris

[Rocket Pooch]

Hi,

Ok this was some data from last year under 2 hours of RGB and about 2 hours of Ha, I parked this image because I was fed up with processing, you can see I have not removed darks properly from the image.  I'll get another couple of hours data on it this year to bring out the dust lanes then process it properly, remove the darks and also desaturate the bright stars and bring out the dust.

I've put this up because its just under 4 hours of data.

Chris


NGC6888-RGBHa by chrissuddell, on Flickr

[MarkS]

All in all, an interesting set of comparisons ...