NGC6888 Crescent Nebula by 120mc-s: Page 2

NGC6888 Crescent Nebula by 120mc-s

firstc8

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firstc8

I read more on narrowband, according to some reports, fast scope needs wide narrowband filter, Hyperstar for example should use 35nm bandpass.

When I stacked reducers the F ratio was around 3.3, but the Ha was standard 6nm I think. Could that have caused more color shift?

riklaunim

firstC8 wrote:
I read more on narrowband, according to some reports, fast scope needs wide narrowband filter, Hyperstar for example should use 35nm bandpass.

Not true to be exact, and using 35nm narrowband filter will be waste of money. What's true is that very very fast f/ratio hits the filter at an angle that make it work less efficiently. That's why for example Baader offers filters optimized for f/2, while standard version will rather work ok at f/4 around and up.

can be as narrow as possible

H-alpha ~6-7nm in most cases (to pass also emission)

can be narrow but it's not as important as for

My narrowband HST palette M1 looks like so (420mm focal length):

firstc8

riklaunim wrote:
firstC8 wrote:I read more on narrowband, according to some reports, fast scope needs wide narrowband filter, Hyperstar for example should use 35nm bandpass.

Not true to be exact, and using 35nm narrowband filter will be waste of money. What's true is that very very fast f/ratio hits the filter at an angle that make it work less efficiently. That's why for example Baader offers filters optimized for f/2, while standard version will rather work ok at f/4 around and up.

can be as narrow as possible

H-alpha ~6-7nm in most cases (to pass also emission)

can be narrow but it's not as important as for

My narrowband HST palette M1 looks like so (420mm focal length):

Very nice, that may be what I need to do to combat light pollution, eventually.

Do you think I should give F6.3 reducer a try? At such long FL of 1200 mm will have to wait for a rare exceptional seeing condition, or find a dark site. Guide scope may not be sufficient either, but I don't know if an OAG will work with F6.3?

firstc8

This time I tried a more balanced color play.

riklaunim

I use C8 and C11 at f/6.4 a bit. It's not easy, but for some objects it can work (but used camera had bigger pixels ;))

C11 f/6.4, DSI III Pro, Ha/O3/O3:

GSO 6" RC with 0.75x reducer-corrector, CLS CCD, Atik 314L+

But easiest DS imaging I had was with a APO refractor (TSAPO65Q in this case) - highly corrected field of view, small telescope (stable and wind-proof), quite short focal length (seeing not a problem), and no moving mirror like in SCT.

betacapricorni

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riklaunim

was captured later than H-alpha, and there were few stars that changed brightness noticeably. There are some variable stars in that region.

betacapricorni

Ah, ok thank you so much

Inviato dal mio GT-I9301I utilizzando Tapatalk

firstc8

Tried to remove the generic 0.5 reducer only use the F6.3, FL was too much for the mini guider to handle long exposures. Don't know if an OAG would help.

I am concerned not able to find a guide star, from my mini guider scope the closest suitable guide star will likely be outside of the OAG pickoff.

On the other hand, l just realized my focuser and the filter wheel together had pushed back focus passed 5" (sensor to the F6.3 reducer), so next time will remove the focuser and try shorter back focus to see if the 0.5 reducer curvature can be improved.

Since I am using the small 1/3 sencor, only need the center area to be in good focus. Fingers crossed.

firstc8

Tried stacking F6.3 and the generic 0.5 reducer again, this time close to each other (really stacked one on the other) and kept reducer 63mm (Celestron suggested 127mm for F6.3/2) from the sensor. The issue of curvature was much improved. However the images were washed out by full moon nearby.

riklaunim

The stars are still far from pin-point. They are elongated - like if the image moved during capture.

firstc8

riklaunim wrote:
The stars are still far from pin-point. They are elongated - like if the image moved during capture.

I will try to isolate the issue. There are several possible causes. The AVX mount not able to handle all the weight. I now have the C8, dew shield, small cam with motorized filter wheel, F6.3, F.5, mini guide scope with dew shield, guide cam, a small counter weight on the scope, and all the wires hanging everywhere.

Or the motorized filter wheel not able to provide precise focal plane across the filters, or the filters themselves not up to the quality standard (the mono camera and filter wheel are bought cheaply), used to learn mono and narrowband imaging. Or it is simply the generic 0.5 reducer (it has truly a very small flat area, or not al all, even the 1/3 sensor has severe vignetting after image stretch)...

Will use the one shot cam next time duplicate steps on M1 for comparison.

Mono and narrowband can do beautiful things, but hardware needs to be upgraded for it to be worth the effort.

firstc8

Looks to me I need an off axis guider. Not what I had wished for but might be the only cure.

riklaunim

OAG will help only if the guiding error is due to mirror flop. If it's to aggressive or imperfect guiding, balance then it won't. And it's much harder to use OAG than external guidescope.

firstc8

riklaunim wrote:
OAG will help only if the guiding error is due to mirror flop. If it's to aggressive or imperfect guiding, balance then it won't. And it's much harder to use OAG than external guidescope.

Happy New Year!

I narrowed it down to flex, which I think is the same you are saying?

The reason was, I noticed as time progresses over the night, the star movement gets faster, and stars became more elongated.

riklaunim

This can also be uneven mass placement, like heavy guidescope on the side and not on top of the telescope :)

2016 isn't here yet, but it's coming :

firstc8

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firstc8

Ok I think I have figured out two issues. There may be more.

One, my mini scope has a field of view that was too much larger than my scope, or the FLs were too far apart. The guider was able to provide a general tracking but not able to prevent minor movement so the stars have different shapes between long exposures.

I think I have solved it by adding a 3x Barlow to the mini scope, and stretched it to the very limit to achieve focus. Now I have similar FOVs between my guidescope and the imaging scope.

It worked because now all my stars stay in the same shapes between 300s exposures.

But the stars are still not round, which let me to find out the second issue, my C8 was out of collimation.

Unfortunately I had to wrap things up as the clouds came in again. Will have to collimate my C8 later.

Of course the miniscope flex is a great concern with the 3x Barlow and 224 weighing in, once the C8 is collimated will be able to find out if it is going to be the limiting factor. But now at least I know why.

maurizio

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