M11 / NGC 6705, “Wild Duck” Cluster (Part II)

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rjbokleman
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M11 / NGC 6705, “Wild Duck” Cluster (Part II)

Unread post by rjbokleman »

Trying to figure out what's going on between these two images and these two scopes, so any help would be appreciated.

In both cases the target was M11 "Wild Duck" cluster for comparison.

First up is an image with my Sky-Watcher 100ED Pro Refractor. Details are in the image, but taken with a Canon T5i w/no focal reducer although I've recently acquired the SW .85x. Only 30 minutes of exposures here. [This is the same image I posted a few weeks ago, just for convenience in comparing the two.]

Image

Second is an image with an Celestron Omni 150 XLT Newtonian. Same Canon T5i, but using a Baader MPCC Mark III for Coma. [This one, I've never posted and is still in my Astrobin Staging area.]

Image

Again, the details are in the image, but this was nearly an hour of exposures at different ISO's stacked using the same set of tools and processing.

The main differences in capture was the SW100ED using BackyardEOS (BYE) and the Omni was using SGP. However, the Omni also used BYE's focusing routine as I don't like (or still need to adjust to) SGP's manual (no motor driven focusers here) focussing routine.

Neither image has Flats or Bias. They both have Darks. The only other difference was the moon phase being nearly full when the Omni's images were taken.

When comparing them notice the vignetting in the Omni. Awful. The SW100ED had half the exposure time and yet the image is nice, sharp, and clear.

I was told today, that to fix the Omni 150 XLT photo's of their vignetting, all I needed to do was take some Flats and apply them. Is that true or is there something else going on here?

Appreciate any helpful insight into this as I'm not sure the investment in the Baader MPCC Mark III was worth it...
Ron B.
T5i/700D, ASI1600MM-C, ASI120MM, ASI174MM, XAGYL 7x36mm FW
Astronomik Deep-Sky(RGB), CLS, Ha, OIII, SII
SV60EDS 60mm f/5.5 APO
AT65EDQ 65mm f/6.5 ED Quadruplet
SW ProED 100mm f/9.0 Doublet APO
C8 EdgeHD, AT130EDT
AVX, iEQ45 Pro

http://www.astrobin.com/users/rjbokleman/
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Apollo XX
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Re: M11 / NGC 6705, “Wild Duck” Cluster (Part II)

Unread post by Apollo XX »

Hey Ron,

Cool comparison! I've also heard that flats can fix vignetting, so you might want to try that. The single biggest difference I'm noticing is the significantly brighter sky background in the Omni image. Part of that almost certainly has to do with the presence of the moon, and some of it may be attributable to the inherent characteristics of the reflector design, ie; reduced image contrast due to central obstruction. I'd bet that getting rid of the moon would make a huge difference.
"The purpose of life is the investigation of the Sun, the Moon, and the heavens." - Anaxagoras
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AndyG
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Re: M11 / NGC 6705, “Wild Duck” Cluster (Part II)

Unread post by AndyG »

Hi Ron,

I agree with Mike that the full moon is definitely an issue. That is what is creating the bright background and highlighting the vignetting.

The vignetting itself is relatively minor and easily dealt with by applying some flats. I recommend constructing an inexpensive flat box; there are lots of plans for them online, and I can show you mine if you are interested.

Regarding the sharpness difference, there may not be a simple answer, it may be a combination of several factors.

1. Atmospheric seeing conditions. This varies dramatically from night to night and even over the course of a night. Skies that look perfect to the naked eye could be unsuitable for imaging. I often discard a full night's subs because of poor seeing.
2. Focus. The faster scope will have a more stringent critical focus zone and may be harder to focus. For non-automated focusing I like to use a Bahtinov mask. (and poor seeing conditions make it hard to get accurate focus)
3. Optics: Collimation, coma corrector-to-chip spacing, these need to be perfect to get tight rounds stars all across a large DSLR chip
4. Guiding. The larger, heavier scope will be more taxing on the mount and may be harder to guide. (and bad seeing makes guiding difficult)
5. Stacking. A few bad subs (bad focus or bad seeing) can throw off the final stacked result. Make sure to grade the subs and throw out the bad subs before stacking.

Lots to think about and lots of things that can go wrong. This stuff is really hard!
Andy
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rjbokleman
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Re: M11 / NGC 6705, “Wild Duck” Cluster (Part II)

Unread post by rjbokleman »

Fantastic gentlemen!

I did find this article doing a quick search on a Flat Frame Light Box: http://interstellarstargazer.com/2013/0 ... ent-panel/

The Newt does have much less contrast (as rumor has it), as proven by the images presented, and for all the reasons cited including the full moon that night. The bottom line too, is this scope isn't an astrograph, so *perhaps* I'm just wasting my time trying to make it one?

I was mainly concerned that using the Baader MPCC was the root cause and given it's price would consider returning it. At this point, I think it depends upon whether I want to continue to try and use this scope for imaging or not or cut my losses and stick with my refractor(s).

In fact, I haven't shot M11 with my little SV60EDS yet for additional comparison. Need to do that soon to round out the experiment. :lol:

The learning continues... :shock:
Ron B.
T5i/700D, ASI1600MM-C, ASI120MM, ASI174MM, XAGYL 7x36mm FW
Astronomik Deep-Sky(RGB), CLS, Ha, OIII, SII
SV60EDS 60mm f/5.5 APO
AT65EDQ 65mm f/6.5 ED Quadruplet
SW ProED 100mm f/9.0 Doublet APO
C8 EdgeHD, AT130EDT
AVX, iEQ45 Pro

http://www.astrobin.com/users/rjbokleman/
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rjbokleman
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Re: M11 / NGC 6705, “Wild Duck” Cluster (Part II)

Unread post by rjbokleman »

Andy,

BTW, couldn't agree more with this point:
4. Guiding. The larger, heavier scope will be more taxing on the mount and may be harder to guide. (and bad seeing makes guiding difficult)
I noticed that I had to put my 11lb weight on the mount all the way out to the end to get close to balanced with the Omni. This is due to all the equipment and the fact that I'm using 11" dovetails on top and bottom to remove any flex in the tube. In fact, I think I could use an extra 2-3lbs or so to get it just right. All of this is only adding to the challenge.

The SW100ED doesn't need the 11lb weight out at the end as the camera (being at the opposite end) is helping offset the balance. Of course, the little SV60EDS weighs almost nothing by comparison to the other two and in fact to balance it I have to push it forward so far that the focuser is almost touching the base of the mount.

The bad news about the refractor is that getting the camera located in the exact same degree or rotation over 2-3 nights can be a challenge where the Newt, its easy to align it with the focuser base each time.

Ah...the trials and tribulations!
Ron B.
T5i/700D, ASI1600MM-C, ASI120MM, ASI174MM, XAGYL 7x36mm FW
Astronomik Deep-Sky(RGB), CLS, Ha, OIII, SII
SV60EDS 60mm f/5.5 APO
AT65EDQ 65mm f/6.5 ED Quadruplet
SW ProED 100mm f/9.0 Doublet APO
C8 EdgeHD, AT130EDT
AVX, iEQ45 Pro

http://www.astrobin.com/users/rjbokleman/
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AndyG
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Re: M11 / NGC 6705, “Wild Duck” Cluster (Part II)

Unread post by AndyG »

rjbokleman wrote:
The bad news about the refractor is that getting the camera located in the exact same degree or rotation over 2-3 nights can be a challenge
Sequence Generator Pro has a Manual Rotator function that will use plate solving and prompt you how far you need to rotate and which direction to match your reference frame.
Andy
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Pete
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Re: M11 / NGC 6705, “Wild Duck” Cluster (Part II)

Unread post by Pete »

Ron, flats will clean that up. Vignetting is almost universal when imaging.

I combine set of 15 simple Tee shirt flats taken at twilight. And also integrate the appropriate darks into the flats. Then, when imaging, I do reductions with bias, darks and corrected flats (which are all at the same temperature). Actually there are 15 bias, 15 darks, 15 flats, and 15 darks for the flats. Use median combine for everything but the flats themselves, where you should do an average combine.

Pete
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rjbokleman
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Re: M11 / NGC 6705, “Wild Duck” Cluster (Part II)

Unread post by rjbokleman »

Sequence Generator Pro has a Manual Rotator function that will use plate solving and prompt you how far you need to rotate and which direction to match your reference frame.
Nice, hadn't discovered that feature yet. Should be very helpful.
Ron B.
T5i/700D, ASI1600MM-C, ASI120MM, ASI174MM, XAGYL 7x36mm FW
Astronomik Deep-Sky(RGB), CLS, Ha, OIII, SII
SV60EDS 60mm f/5.5 APO
AT65EDQ 65mm f/6.5 ED Quadruplet
SW ProED 100mm f/9.0 Doublet APO
C8 EdgeHD, AT130EDT
AVX, iEQ45 Pro

http://www.astrobin.com/users/rjbokleman/
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rjbokleman
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Re: M11 / NGC 6705, “Wild Duck” Cluster (Part II)

Unread post by rjbokleman »

Pete wrote:Ron, flats will clean that up. Vignetting is almost universal when imaging.

I combine set of 15 simple Tee shirt flats taken at twilight....
Pete,

I did some reading during lunch today about that very approach. Using T-Shirts, with a bungie cord wrapped and doubled up over the objective and pointed toward a bright sky. Just keep them nice and 'Flat' - pardon the pun. Seems simple enough. The wife is gonna love this. I get teased sometimes for running around the house in a white T-Shirt all the time, so I can just hear yet another joke coming about my T-Shirts. :lol:
Ron B.
T5i/700D, ASI1600MM-C, ASI120MM, ASI174MM, XAGYL 7x36mm FW
Astronomik Deep-Sky(RGB), CLS, Ha, OIII, SII
SV60EDS 60mm f/5.5 APO
AT65EDQ 65mm f/6.5 ED Quadruplet
SW ProED 100mm f/9.0 Doublet APO
C8 EdgeHD, AT130EDT
AVX, iEQ45 Pro

http://www.astrobin.com/users/rjbokleman/
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Pete
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Re: M11 / NGC 6705, “Wild Duck” Cluster (Part II)

Unread post by Pete »

I've actually stretched 2 layers of white Tee shirt over a rectangular wooden frame that I hang over the nose of the scope while doing flats. I store this physical flat "frame" in a plastic baggie in the observatory for use every month or so.

Pete
Pete P.
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