bobro

Better positioning this time on the Pelican Nebula. Longest exposure I've given so far to any image - 20 subs @ 480 secs. As you can see, still not a great result. The blue gaseous 'haze' got in the way of the red pelican, reducing contrast. Will need to practice processing on this one.

Ah! Thanks Art. Makes sense now as the Dumbbell is smaller than the Cocoon. I must find a good source of surface brightness values for potential targets. Must admit, I didn't expect to find the summer nights quite so interesting in terms of targets - but there are plenty around!

According to Wikipedia, the apparent magnitudes of the Dumbbell Nebula and Cocoon Nebula are very similar. I know this isn't an exact science but, even with 240 second exposures for the Cocoon (compared to 60 sec for the Dumbbell), the nebula was still quite faint. The sky was perhaps a bit less clear. Can anyone comment on this difference? Here is the Cocoon 'final' image - needs a lot more input. 23 subs @ 240 sec.

Fascinating objects these planetary nebulae. M27 Dumbbell Nebula : 35 subs @ 60secs. Good clear sky - could just make out the Milky Way - must try and capture some of it.

Another attempt at M57 - as suggested by Neil: more subs + drizzling. Plus better focus and used the centre of the scope's image without the CC in place in case the CC degraded the image. I think that's about the best I can manage at the moment. 32 subs @ 60 sec.

It's certainly made a worthwhile improvement to the earlier image. What's the plan for imaging with the Ha filter - B&W, false colour, other filters, improving earlier images? Now I'm typing this I wonder if the answer is 'yes' to everything!

I liked Peter's M57, so had to have a go too, stretching my setup perhaps a bit too far...19 subs @ 120sec.

Here is a comparison of images taken with and without the (DIY) Coma Corrector on my Meade 130EQ scope. There doesn't seem to be too much light loss (though this is subjective) due to the addition of the CC. Commercial CCs are usually 2" aperture, where my DIY CC is 1.25". I don't know if that aperture difference will make any difference in light loss - possibly the light loss is mainly down to the CC lens glass in the optical path.

I don't know if you are old enough to remember the days of separate exposure meters and interchangeable lenses, but once a light reading was taken, film sensitivity and exposure time was set in the meter, the f# was the result. It then didn't matter what lens was put on the camera - any lens set to the same f# delivered the same amount of light to the film plane (within reason) and hence the resulting exposure was correct. That's why I wanted to measure the 'true' f# on my scope setup - basically comparing it with a camera lens which doesn't have the same obstructions and light loss. Art enlightened me on the concept of T-stop, which is a more appropriate way of measuring what I was looking for. Using Excel, I previously did a basic calculation on my Newtonian scope, taking into account only the light loss due to the central obstruction - the result was about f5.6 (in non T-stop terms). My thinking about exposure and f numbers comes from many years of photography with cameras, so I tend to look at things that way. It does make me see a refractor scope as just a big camera lens and I would expect an f# measurement to turn out just as would a lens. A reflector scope doesn't seem quite as efficient - especially after my mods! I'll try and re-measure without the CC as rotatux suggested.

Thanks for looking into this Art. I hadn't come across the concept of a T-stop before - there's always something new to understand, making AP a great subject. No doubt you are aware that scopes such as the 130P-DS have the primary mirror moved a bit closer to the secondary mirror to help with DSLR focus. I've just read the 130P-DS has a slightly larger secondary mirror than the 130P to ensure light is not lost from the closer primary. With my scope the secondary is unchanged so there will presumably be some inefficiency there. I wonder what the T-stop of my scope is......

As previously mentioned, I wondered what was the actual f# of my scope, what with the obstruction due to the secondary mirror, scope shortening to achieve focus and the addition of a coma corrector. Here are the results of using the camera with a 135mm lens at f# from 5.6 to 16, comparing with the camera on the scope at a nominal 5. Same exposure time and ISO for all images.

A few things : Although the scope is nominally f5, it isn't really as the secondary mirror blocks some light (bought this way of course) 40mm was cut from the OTA to make it focus with a DSLR, likely resulting in some light loss at the secondary and also some loss up the focuser tube as the camera sensor is far up the tube The (DIY) coma corrector results in additional vignetting and some light loss

Replaced the scope by a camera lens (£18 from ebay). Note - the can is full of sand - contents weren't wasted!