Flintstone Stargazing

January 27, 2008

The Great Orion Nebula Shootout

Filed under: astronomy, Astrophotography, Equipment, Mars, Messier Objects, Observation Journal — Tags: , , , — Ed @ 11:52 pm

Tonight was beautiful – the Moon was not going to rise until late, and it was clear and about 40°F outside. All in all, really good conditions for me. I decided that tonight I would test out my new (cheapo) 70mm refractor that I blogged about the other day. I’d done some work on my mount, so I was hoping to also see if that had worked as well as I’d hoped and I’d be able to take some long exposures.

I set up the way I normally do, leveling the scope, using the polar scope to align the mount on Polaris and then running the alignment procedure with the Autostar software. I again got the magic, “You are less than 5′ from the pole” message so my alignment was very good. I then took a good amount of time to view some different targets comparing the views in each scope. I had a look at Mars, M42-The Great Orion Nebula, The Double Cluster and all looked really, really good in both scopes. I actually started to wonder how much better my 8″ scope was so I resolved to find out. I first hooked up my DSI II camera to the 70mm refractor and aimed at the Orion Nebula. Because I was using the diagonal on the scope, the image was originally a mirror image so I fixed it in the software afterward, but that’s why the image from that scope is not oriented the same way as my other one. Here’s the result:

M42/M43-The Great Orion Nebula on 1/27/08
Taken with my 70mm refractor

This is a combination of 18 15 second images plus 10 60(!) second images. The 60 second images were spot on – no movement that I could tell. It makes me want to try some 2-3 minute exposures on some of the fainter targets – I can’t wait. Overall, it did what I believe is a very creditable job on the nebula. My only complaint (and this is a known issue with achromatic refractors) is that there is a bit of a purple halo around the brightest stars.

I next took the exact same series of images through my 8″ Schmidt-Newtonian. Here’s the image I got:

M42/M43-The Great Orion Nebula (8
M42/M43-The Great Orion Nebula on 1/27/08
Taken with my 8″ Schmidt-Newtonian

Because the focal length is longer (812mm vs 700mm 600mm), the magnification is higher on the 8″ scope than on the 70mm refractor. (Magnification is calculated by dividing the focal length by the eyepiece size. The eyepiece of the DSI II is aproximately 8mm so the magnification for the refractor would be approximately 87.5X 75X and the 8″ scope would be 101.5X) That’s why it appears more zoomed in. It also is much, MUCH brighter – there’s quite a bit more detail. So I guess that answers the question as to whether they actually show the same amount of detail. The 8″ is clearly much brighter and more powerful. I am really anxious to try the refractor on the Moon however, the larger field of view means I may be able to get the entire moon in one shot and refractors are known for providing better contrast/clarity on planets and lunar views. I also took images of Mars with both scopes that I’m processing now. I’ll probably upload those tomorrow.

Overall, a really great night. I truly enjoyed the visual observation I did tonight – both scopes performed admirably and it was nice to be reminded of how much fun visual observation can be. I also was pleasently surpised at the results of my tests. I think I’ve got a great setup to capture a number of different types of images in the future.

Update: Here’s the Mars image from the refractor. Wow. Unfortunately, I couldn’t tell until I procesed it, but the image I got from the 8″ was slightly out of focus.

Mars on 1/27/08
Mars on 1/27/08
Taken with my 70mm refractor

This image was taken using not only the 70mm refractor, but the web cam that came with it. I paid $25 total for both (and a terrible tripod and eyepieces, which I didn’t need). What a deal.



  1. That’s really quite fascinating. I didn’t realize that a refractor could give you such a nice shot. Did you process both exactly the same way? Because the contrast and darks on the refractor look a little better.

    Comment by Pierre — January 28, 2008 @ 1:53 pm

  2. I was really surprised out how good that cheapo scope did. Of course, Orion is just about the brightest DSO out there. I’m going to try some open clusters and other stuff in the future (though not tonight – it’s cloudy)

    Here’s a more complete explanation of what I did with each scope:
    I used Autostar Envisage to capture the images from each scope – it automatically subtracts temperature-adjusted dark frames and saves each out as 2 FTS images (an RGB one and a Luminence one). I saved out 24 15s frames and 10 60s frames for each scope. I then opened up the 15s and 60s images in DeepSkyStacker and I stacked the best 90% of the images (which ultimately resulted in a small, 30s, advantage to the SN-8 scope) using Average (I normally use other options, but wanted this to be simple). Once it had stacked the images, I adjusted the curves by eye – basically what I try to do on an image like that is make sure that black areas of the image are black, that in the case of Orion that, if possible, the trapezium is still visible (if only slightly) and that the nebulosity is across the broadest possible brightness range. I did this for the RGB and L images seperately and saved them as 16-bit TIFFs. I opened these up in Photoshop and worked with curves a bit more with the same goals as above. I then converted to 8-bit so I could stack the Lum layer on the RGB. After making sure they align properly, I export to a JPG.

    Obviously, there’s plenty of room for judgment, particularly in the curves manipulation. That’s probably the main reason for the difference between the two. The other main issue is that th images from the SN-8 were so much brighter that I really had to tone them down – especially in the brighter areas – to get the resulting image (though I guess that’s the same issue…).

    Ultimately, I was very pleased with both scopes – the SN-8 certainly gathers a lot of light, but the refractor also held up quite well. The only real issue I had with the refractor is the violet halos around the stars. I’m thinking of getting a minus-violet filter to get rid of those, but probably won’t invest in that for a while. I may just try to come up with some sort of image processing routine to remove those. Considering the cost difference between the two scopes I think I’m getting a pretty good value from that $25 refractor.

    Comment by Ed — January 28, 2008 @ 3:36 pm

  3. I would say that was $25 well spent.

    Comment by Phil — January 30, 2008 @ 3:09 pm

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