bobro

2 hours ago, Peco4321 said:

Here it is.

MasterFlat_ISO800.tif

Great to see you getting flats to work now - really makes a big difference to your images!

I wonder if your scope is a litttle out of collimation? A stretched version of the flat should be fairly symmetrical in brightness, whereas yours is offset. This can be seen in coma in the stars in the darker regions of the first image without flats. For comparison here are stretched flats for our scopes. No big deal though...

Hi Peter,

You did much better than my attempt on the Veil last night - reflections inside my scope due to the Moon I think. Perhaps a cowl will help. Clouds too!

If you're not already doing this, before saving the image in DSS set the Saturation shift (Saturation tab) to about 20% before aligning the RGB histogram peaks (RGB/K Levels tab).

Bob

29 minutes ago, Nigel G said:

I have always had per channel background cali set to no, and read to keep it that way with DSLR's so not sure about that

Probably depends on how an image is subsequently processed e.g. I normally use Kappa Sigma clipping in DSS which (I understand) needs a consistent background level in order to operate correctly. Hopefully someone will be able to explain a bit more about Background Calibration......

Probably a good idea to ensure DSS stacking has 'Per Channel Background Calibration' set to 'Yes' to ensure a consistent sky background level before stacking.

4 hours ago, Art Gecko said:

the Western Veil Nebula

Just my sort of image! Sometimes images can seem to be a bit over processed (easy to do) but this one has a lovely natural colour progression to it that is easy on the eye. Lovely and well done!

51 minutes ago, Stub Mandrel said:

I'll be trying to break the EQ3 sub-length record soon

Another Stub sub challenge to us all?  

Here's Mr Pelican looking a bit less blue - tried out (in GIMP) decomposing an image to separate RGB layers for the first time, allowing the blue haze to be adjusted out. Quite a useful feature.

Peter's flat shows as taken at ISO3200 and 1/4096 sec exposure. I guess the speed is as fast as the camera will go. Perhaps the shutter is a bit sticky at this fast speed. In any case it looks a bit overexposed - a good plan is to have the histogram around the mid point.

Flats don't need to be taken at the same ISO as images (I previously thought flats needed to be taken at the same ISO as images but was corrected by someone with more knowledge and did some tests that showed flats worked irrespective of the ISO used). So I suggest reducing the shutter speed and ISO if necessary in order to obtain flats with roughly a mid point on the histogram.

Thanks guys for the images of the Crescent Nebula. It's certainly not the easiest subject! If I try it I think a test with a good long exposure is called for - perhaps 10-12 minutes and preferably with a darker sky.

1 hour ago, Stub Mandrel said:

but here's a 50% Ha lum with a standard DSLR image

Neil - could you post the standard DSLR image for comparison? Thanks. Bob

1 hour ago, mikey2000 said:

Here's my attempt at the Crescent Nebula NGC 6888 in Cygnus.

I like the sharpness and clarity of the image Mikey. Can you tell me what subs were taken? (It helps me evaluate the image/target and also decide if I should have a go at it.)

Thanks

Bob

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.

1 hour ago, Art Gecko said:

magnitude is a measurement of the total light from the object as if it were compacted into a star like point... whereas surface brightness is exactly what it says on the tin.

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.

4 hours ago, Stub Mandrel said:

First results with using Ha

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.

24 minutes ago, Art Gecko said:

You said you need to test to find the f# of your scope, but how has it changed from when you bought it?

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

33 minutes ago, Art Gecko said:

Interested in how you've altered the focal ratio of your scope Bob?

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

Well if I can't find the target with the scope - put a wide angle lens on!   21 subs @ 120 secs with 135mm lens. Unfortunately didn't notice the camera display was on - some light from this registered on the sensor. It did show me how much brighter the image is with the lens at f4 when compared to my scope, so I think I'll do a comparative test to find out the real f# of the scope.