LRGB image with Hyperstar@F2.66: NGC7023 Iris Nebula

[Ikonnikov]

After trying some narrowband with my C8 Hyperstar plus aperture mask (making it F2.66 ) I've had a go at luminance images to combine with RGB data from an OSC camera for NGC7023. My hope was that sharper luminance at the slower F ratio plus mono camera would make up for the lower resolution of OSC images (taken @F2.1).

I started off using a luminance filter for both cameras but I overestimated my sky quality  and the SNR of images improved when I reverted to an IDAS LPS P2 filter. I've ended up combining subs from both (mono/L 4min with L-filter, 7min with LPS P2 and OSC/RGB 2 min and 5 min L-filter, 7 min with LPS filter) for the final images plus some short 15 s exposures for star colour/ cores. Total integration time was ~7 h for RGB and ~4 h for L (weather got in the way). 

Processing in PixInsight included DBE, linear TGV denoise, HDR transform, LHE Curves transform and SCNR. I used mild deconvolution on the L to bring out more detail in the nebula core then tried to rescue stars by blending with an image without decon, I also reduced star sizes in OSC images a little before LRGB combination due to more bloat in them at F2.1.  Final image: http://www.astrobin.com/full/125823/0/ or with a little more saturation/stretching http://www.astrobin.com/full/125823/C/

In the future I think I'm going to try an even smaller mask for L images (aiming for maybe F2.8) to see how much more I can reduce star FWHM and maybe take OSC images @F2.66 so the discrepancy isn't too great between them.

As ever,  any comments/suggestions would be most appreciated!

Paul

[lensman57]

That looks nice Paul. I am not a fan of HyperStar but this one is well put together and processed. Great image.

A.G

[Syed Saif Alam]

Blown away

[ollypenrice]

The dusty part of this object, which you have certainly captured, is not something often seen from LP sites - so very well done. I prefer the more saturated one and feel that it would take even more saturation if done craftily. In PI I would have no idea but I do know some PS dodges for doing this.

Very good image, anyway.

Olly

[Ikonnikov]

Cheers for the positive comments folks.

The dusty part of this object, which you have certainly captured, is not something often seen from LP sites - so very well done. I prefer the more saturated one and feel that it would take even more saturation if done craftily. In PI I would have no idea but I do know some PS dodges for doing this.

Very good image, anyway.

Olly

Thanks Olly. Regarding my sky quality, although it wasn't quite as good as I thought it might be for LRGB it's still by no means bad here (now I've moved some way from Leeds); there's clear countryside out the back leading to moorland so I probably shouldn't whinge too much. It's just that the street lights (and the rest of the smallish town) out front cause some orange glow but it does seem to be well dealt with by the IDAS filter.

I agree the image definitely needs a bit more zing in the colour department; I'm not a PS owner though so would unfortunately be unable to act on any of your undoubtedly very helpful advice for the moment. I guess I'll have to play around with PI and trawl the forums some more to see if I can improve things without overcooking it...

Paul 

[sharkmelley]

That's a beautiful image and it demonstrates the power of the Hyperstar.  Well done!

However, I'm really confused with your talk about masking.  Are you deliberately reducing the effective diameter of the aperture?  So you are slowing the acquition rate of photons which compromises the whole raison d'etre of a Hyperstar?  But why? 

I don't really understand how this might affect FHWM, although reducing the F-ratio might make focusing less critical.

On the negative side, it could also be that your mask is adding all the multiple diffraction spikes to the bright stars.

Mark

[ollypenrice]

That's a beautiful image and it demonstrates the power of the Hyperstar.  Well done!

However, I'm really confused with your talk about masking.  Are you deliberately reducing the effective diameter of the aperture?  So you are slowing the acquition rate of photons which compromises the whole raison d'etre of a Hyperstar?  But why? 

I don't really understand how this might affect FHWM, although reducing the F-ratio might make focusing less critical.

On the negative side, it could also be that your mask is adding all the multiple diffraction spikes to the bright stars.

Mark

There's been a thread on this recently but I can't remember where! The upshot is that the Hyperstar is unrealistically fast with a very shallow focal depth and by stopping it down a little it is easier to hold within focus and is probably not working the lens edges so hard. It was a good thread. I'll be trying the Rowe Ackeramann soon so am interested in all this.

Olly

[Ikonnikov]

That's a beautiful image and it demonstrates the power of the Hyperstar.  Well done!

However, I'm really confused with your talk about masking.  Are you deliberately reducing the effective diameter of the aperture?  So you are slowing the acquition rate of photons which compromises the whole raison d'etre of a Hyperstar?  But why? 

I don't really understand how this might affect FHWM, although reducing the F-ratio might make focusing less critical.

On the negative side, it could also be that your mask is adding all the multiple diffraction spikes to the bright stars.

Mark

Thanks Mark. I don't understand why the aperture mask works but it does (open to any explanations)! The best star half-flux diameter I could get in Nebulosity with the Baader 7nm Ha filter without the mask (F2.1) with good seeing is about 1.5-1.6 pixels  but with the 160mm diameter mask I made  (reducing  to F2.66) it is consistently  1.1-1.2 pixels, i.e. in the order of 25% less and it certainly also shows on the sharpness of extended objects too. The effect of slightly increased exposure times on star FWHM values seems to be small in comparison.

As you mention this increase in F ratio also results in (~75%) increase in the size of the critical focus zone meaning easier focusing and less effect of temperature on focus drift. Also at F ratios slower than F2 or so there's also the benefit of increased transmission though narrowband filters, so at least for NB imaging the increase in exposure time needed is less than expected.

It certainly pained initially to lose 35-40mm of aperture but the results so far seem to show it's worth it for the increase in image sharpness. Anyway, I figure that if you stay above ~145mm diameter for the C8 (ie. below F3) you still have a set-up much faster than most other imaging systems. And you can simply take off the mask or use a smaller one if you want to increase sensitivity at the price of sharpness (e.g. for very faint objects, RGB to complement higher res luminance or video astronomy). And of course since focal length is independent of the aperture size all images are at the same scale for combination.

I suspect there may be little need for masking on larger HS models  with longer focal lengths (i.e. C11 and above). Indeed I've seen superb images captured on these systems without one (e.g. http://www.astrobin.com/82757/) but I borrowed the idea from Mike Hawtin who masks down his C6 HS  to very good effect (http://www.insideastronomy.com/index.php?/topic/821-hyperstar-mirror-lock-trial/).

Regarding spikes on bright stars it may be that the mask increases these slightly although they were still present without using it in some of my older images (e.g. see http://stargazerslounge.com/topic/208473-experiences-with-hyperstar-imaging-my-first-4-months-sorry-for-the-monster-post/) and its edge is pretty clean; there will also likely be some caused by my cable holder which although curved isn't perfectly smooth.

Paul

[pmesquita]

Probably the best image I've ever seen with Hyperstar. And I kow the hyperstar system...my 1st imaging year was with hypoerstar!

congratulations, it's beautiful!!!

cheers

paulo

[MaartenR]

After trying some narrowband with my C8 Hyperstar plus aperture mask (making it F2.66 ) I've had a go at luminance images to combine with RGB data from an OSC camera for NGC7023. My hope was that sharper luminance at the slower F ratio plus mono camera would make up for the lower resolution of OSC images (taken @F2.1).

I started off using a luminance filter for both cameras but I overestimated my sky quality  and the SNR of images improved when I reverted to an IDAS LPS P2 filter. I've ended up combining subs from both (mono/L 4min with L-filter, 7min with LPS P2 and OSC/RGB 2 min and 5 min L-filter, 7 min with LPS filter) for the final images plus some short 15 s exposures for star colour/ cores. Total integration time was ~7 h for RGB and ~4 h for L (weather got in the way). 

Processing in PixInsight included DBE, linear TGV denoise, HDR transform, LHE Curves transform and SCNR. I used mild deconvolution on the L to bring out more detail in the nebula core then tried to rescue stars by blending with an image without decon, I also reduced star sizes in OSC images a little before LRGB combination due to more bloat in them at F2.1.  Final image: http://www.astrobin.com/full/125823/0/ or with a little more saturation/stretching http://www.astrobin.com/full/125823/C/

In the future I think I'm going to try an even smaller mask for L images (aiming for maybe F2.8) to see how much more I can reduce star FWHM and maybe take OSC images @F2.66 so the discrepancy isn't too great between them.

As ever,  any comments/suggestions would be most appreciated!

Paul

I have a question about your setup, which I can't seem to find on the internet. Do you use a filter holder in front of your Hyperstar or do you have the filter installed on the bottom of the Hyperstar?

I have a QHY10 color ccd camera with very little room between the Hyperstar and camera so it would be nice to install the filter on the bottom, but I'm not sure the IDAS D1 filter support this.

Thanks!

[sharkmelley]

A 2" filter screws directly into the Hyperstar camera adapters.  The IDAS D1 works well on the Hyperstar.

Mark