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  1. #1
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    Default I'm at it again - Trifid & Lagoon



    Hi,

    Have attached two images, The Trifid and Lagoon nebulas. Both images were taken with my LX200 10" at f6.3 polar mount, unguided. Both are binned 2x2 and are 120 second exposures. I tried 4 minutes, but since I am not guiding yet, 4 minutes was completely unacceptable and showed I need to refine alignment. These shots were done after PEC training using PHD with my DSI III as shown in a previous post.

    Does anybody have an idea how to significantly reduce bloating in my bright stars while still getting nebular detail? Camera is a DSI III pro. Once I'm happy with tracking I'll start with tri-color imaging.

    An interesting sidenote is that by the time I started these shots, a high cirrus cloud layer had moved in. If I had popped my head out the observatory, I would have closed up and gone to bed, but with only a dozen stars visible to my eye, these are what I got. Add to that my 42.2N latitude and looking south is the glow of the Chicago western suburbs and I'm pleased with the results (all things considered). My wife wants to move to the SW US and I confess that both for my flying and astronomy hobbies, the idea has it's appeal.

    Dave
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  3. #2
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    Default

    Have look at this Reduce Star Bloat

    Clear Skies

    Pete
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    Default

    Try taking more and shorter exposures, lots of them, and stacking. Here's an image of the Lagoon that I took a couple weeks ago with my DSI II Pro monochrome. This is from about 14 frames, recorded as .bmp with Envision, darks subtracted while captured, then stacked in DSS and processed a tiny bit in Gimp.
    The stars could be sharper -- I really don't know what I'm doing yet --, but there's a good amount of detail in the nebula, and this came from just getting more photons, I think. This was in the Megrez 90, on the CGEM, unguided.

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    Default

    al, one thing that may help you is to set Envision to save as FITS files. Unless I misread, I think you are saving in bmp. I have read different opinions about this, but I am pretty confident that if your imaging device records/saves in either raw or FITS, you will have more data to play with. After you have processed it you can save in tiff, jpg, or bmp (whichever is applicable for posting purposes).

    Skyhawk: I'm surprised that I don't see any vignetting considering that you are using an f/6.3 focal reducer. I hate imaging in f/10 so I often use the focal reducer and it seems to always leave me with vignetting. Flats should take care of this, but I seem to be shooting those somewhat improperly.

    I think both of you took some nice images, regardless.
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    Great link Pete...
    Thanks
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    Default

    Thanks, blaise, I read this too late to try it last night, did last night's saving in .tif instead. Frustrating results. Will try .fit asap.

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    Default vignetting

    Blaise,

    Thanks for yours and others thoughts.

    I think that vignetting from use of a focal reducer is very much a consequence of it's placemant in the optical path. One reason I bought the Moonlite focuser was because it holds the FR internally and in what they refer to as the "sweet spot" for it.

    Years and years ago, what have become focal reducers were called telenegative correctors. They can be placed in various positions (back then in front of the optical path to convert a "normal" lens to a "wide angle" for 35mm cameras) in the optical path (limited by mechanics) and end up with slightly different properties depending on location within the path and distance from other elements and focal plane.

    The actual image size cast by optical systems of the typical 8-12 inch SCT should, be quite sufficient with a FR to cover the small chip in a DSI 1,2 or 3.
    Without a FR they can cover 90% or more of a full frame 35mm camera which is 24X36mm.

    Dave
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    Dave, I am aware of 3 reasons for star bloat that occur during image capture:

    (1) Trailing, which is evident in your image of M20. Obviously the remedy is either better alignment, guiding, or shorter subs.

    (2) Focus being off, which may be the case in your M8 - even the smaller stars are pretty big circles, rather than points of light. How are you focusing?

    (3) Too long of exposure, resulting in overexposure of the brighter stars. As mentioned above, shorter subs will help. M8 and M20 are very bright objects, so you can easily shorten your sub times. For instance, this M8 I took was made of 30sec subs.

    You can also introduce bloating in post-processing by stretching the stars too much.
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    Default

    Dave:

    Thanks for the info. I never really considered (or I should say realized) that the entire mechanism could be replaced where a focal reducer could be "installed" within the focuser. I went to moonlight's page and indeed, there it was : )

    It's amazing that vignetting can be eliminated by doing such a thing. The difference in placement between the way I currently do it (screwing it directly to the rear cell in comparison to within the focusing unit itself) and what moonlight offers must be very small, yet the difference in outcome is evident.
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