Ultrastar C Limitations

[HiloDon]

Hi All,

I wanted to start this thread to share some of my experiences with the SX Ultrastar C used with Paul Shears' excellent Starlight Live software.  I have been using the Lodestar X2m and X2c for quite a while, and it opened up a whole new world for me.  I took a chance and tried the Ultrastar C to get better image resolution and color.  I am very fortunate to have great skies here in Hawaii, and even more fortunate to have access to a C14 Edge with Hyperstar at probably the best sky in the world for astronomy.

I have been very happy with the results I achieved with the C14 w/HS as well as my Evo 6 w/HS.  Results with my standard Meade 8" SCT at my home observatory have been mixed, and good results of dimmer DSO's are difficult to say the least.  I want to share some of these results with the community to get some feedback.  Hopefully, this information will help others achieve better results with their setups, and help some in their decision to purchase this camera.

First, I will start with an example of the Flame Nebula.  The first is one taken with the C14 w/HS and the second with the M8 at about F4.5.  Going any lower in focal ratio will cause unacceptable vignetting and corner coma, even with my Optic .33x reducer.

Not too bad in my opinion, but the noise appears to be more pronounced in the M8 image.

Now lets's look at M101.  To me this is a difficult object.  First, the C14 wHS, then the M8.

Here the Ultrastar C's deficiency becomes more apparent.

Now for a real test, the Horsehead Nebula.

First, the C14 w/HS, the the M8.  I did two with the M8 at different focal ratios.  Note the vignetting and coma with the F3.5 setup.

I tried everything to get a better image, but the noise is just overpowering.  I did try more stacks, but it really didn't get much better after five.  Perhaps a hundred stacks would be a little better.

My conclusion is that unless fast optics and clear skies are available, the Ultrastar C will be difficult to use for dimmer objects.  If the goal is to just see them, I think the Lodestar would be better suited for viewing and may actually provide better images.   Here's one I took of the HH with the Lodestar and ES152 AR.

Look forward to seeing your thoughts on this.

Thanks,

Don

 

[nightvision]

Don, That's a great reality check for those of us considering these cameras, looks like for most of us in poor sky areas (and with modest scopes) it makes more sense to go the Lodesatar route.  Would binning reduce the apparent noise in the M8 Ultrastar images?

 

Tony.

[HiloDon]

Hi Tony,

That's why I posted this.  Another route you could consider is going mono and using filters.  Some are doing that now that Paul has released the spectral imaging feature in SLL.  Results from the mono Ultrastar by others look very good even at slower focal ratios and poorer skies.

The Lodestar is still a winner in tough conditions.  I believe binning the 825 sensor in Ultrastar C will lose the color and result in resolution not much better than the Lodestar, but I have not tried it yet.  It may improve the noise, and would certainly be faster.

Thanks for your feedback.

Don

[aparker]

Don,

Very nice images.  I'm always impressed with what you can do with that Hyperstar C14.  I am not surprised at the superiority of the results over the Meade - you have nearly four times the aperture at roughly the same focal length (700 mm for the C14 vs 900 mm for the Meade at f4.5), so you would expect the C14 to be greatly superior.  I did notice in your image captions that most of the C14 images used shorter subs and more of them, which should result in less apparent noise.  Also, you may have needed to stretch the 8" scope's images more aggressively to bring out objects like the HH, which will also produce an apparently "noisier" image.

For those considering getting one of these cameras, a really informative comparison would be between a color Ultrastar and a mono Ultrastar on the same scope, same targets, etc.  I'm not sure if anyone here has access to both cameras to do such a test.  I would be happy to do it if someone has a color Ultrastar to lend ;-)  I don't see myself going down the OSC road anytime soon.  I feel like I have years of work ahead just trying to get good at capturing monochrome images and using filters.

From a purely analytical standpoint, you would expect much less sensitivity for some objects, and at least modestly reduced sensitivity for all objects, from a OSC camera.  The worst case is a pure Ha emission nebula like the HH or Rosette.  Here you are essentially throwing away 3/4 of the pixels since the GGB cells of the Bayer matrix pass zero Ha light.  On top of that, the R cell filter does not have perfect transmission, so perhaps you are getting 0.25 x 0.9 or 22.5% the sensitivity of a mono Ultrastar.  Even in the best case scenario, which would be a blackbody emitter that peaks in the range of the G filters (say a G0 star), you might get two pixels (GG) passing 50% of the incident light, while R&B pass 25%.  So the average is 37.5%, again decremented for imperfect transmission for any of the filters.

Again, I applaud you for starting this conversation.  It's going to be very helpful for peolpe thinking about getting into EAA to have some realistic expectations set as they think about what equipment to use.  Color images are very nice in many ways, but much harder to achieve than mono.  And, as the astrophotographers always say, OSC may be the cheapest route to color imaging, but it is also the slowest and most technically challenging to get a really good result with.

 

Alex

 

[RobertI]

Hi Don, this is an informative thread, thanks. Given what I have seen of the Ultrastar colour to date, I concluded that the most likely confguration for me would probably be my C8 with hyperstar, so it would be good to see some results from your C6 with hyperstar from your home observatory of the same objects as above (got a feeling you may have already posted these in a previous thread).

thanks

rob

[HiloDon]

Thanks, Alex and Rob.

I think I can respond to both of you in the same post.

I realize that the C14 w/HS is the ultimate setup, but probably elusive for most of us.  But, I suspected that a big factor in the success I was getting with it was the fast focal ratio, not necessarily the aperture. That's why I bought the Evo 6 and a Hyperstar.  I think the images below support my theory.  Even with a small aperture, the Ultrastar performs well.  That's why I concluded that the speed is a main factor.  The other factor is sky quality.  The C14 and C6 images were taken at the Mauna Kea VIS located 9000 ft above sea level.  The M8 images were taken at my home observatory at 600 ft.  Although seeing is very good and dark here at my home, it's hard to equal the VIS sky.  Since I can't bring the C14 here or the M8 to the VIS, I will run some comparison tests with the C6 w/HS as Rob suggested.

Alex, your description of the OSC is on target.  I am considering getting a mono Ultrastar for the reasons you stated.  Base on work by others, I suspect the mono to perform much better at slower speeds and poorer skies.  I have taken some NB Ha images with my wide field 50mm guide scope and Ultrastar and got some acceptable images, but the mono would be much better.  With Paul's SLL, I can run the signal through the red channel and get the same red color.  The visitors just love color.  I prefer the mono.

On the stacking, Alex, I don't find much, if any, change after five stacks.  In some cases there are more, but it's just that I forget to shut it off if I'm talking to visitors as it's stacking.

Thanks again for the constructive feedback.

Don

Note:  The last image of Eta Carina is redone from a stacked fits file.  The exposure was 5 x 30s.

[RobertI]

Thanks Don.

[Spacedout]

Thanks Don, great post and it's just reaffirmed my decision to buy the Hyperstar.  

 

Best wishes

Karl

[HiloDon]

Hi Karl,

I didn't know that you decided to buy a Hyperstar.  If you haven't ordered yet, make sure you get the counterweight with it.  I don't think there's an extra charge.  It mounts onto your rear cell. HS will work great with your C11 and Infinity.  Look forward to seeing some images.

Don

[Spacedout]

Thanks Don,

 

Not pulled the trigger yet but I have spoken to an astro supplier for info.  They mentioned the weight too after asking me hat scope I was using.  I think tomorrow is D day 

[HiloDon]

Karl,

I think if you order it for an Alt-az mount, you get the weight.  You don't for a GEM, but it's the same cost.

Don