The Crescent Nebula (NGC 6888) is an emission nebula in the constellation Cygnus. It's about 5000 light-years away from Earth. The blue areas show the location of oxygen (O-III) gas, while the other areas of the Nebula and the surrounding clouds show the location of hydrogen (H-II/H-alpha) gas.
From science.nasa.gov: The Crescent Nebula is about 25 light-years across blown by winds from its central, bright, massive star. Its central star is shedding its outer envelope in a strong stellar wind, ejecting the equivalent of the Sun's mass every 10,000 years!
This was my first image with the Celestron EdgeHD 8 after spending a week and a half or so ironing out all the wrinkles, learning the scope, and getting it set up for imaging. I've imaged Crescent Nebula before, but nowhere near this long a focal length. The last time I imaged this Nebula was at 360mm fL with my AT60ED refractor, and I was very curious how much more detail could be shown.
I think the next time I try this target I'll try using a monochrome camera and individual S,H,O filters. (I have yet to try my filter wheel and ASI2600MM on the EdgeHD 8.)
Celestron EdgeHD 8 with 0.7x Reducer (fL = 1422mm)
ASI2600MC Pro (OSC camera)
ZWO AM5 mount with guiding via OAG and ASI174MM Mini
Light Frames [Total Exposure 4:35] used Antlia ALP-T Dual (Ha, OIII) Narrowband filter -- 300s x 55 frames
All light frames calibrated with dark, flat, and bias frames
Pre- and post-processed in PixInsight
Additional post-processing in Photoshop for exposure adjustments and generation of jpeg
The Crescent Nebula (NGC 6888) is an emission nebula in the constellation Cygnus. It's about 5000 light-years away from Earth. The blue areas show the location of oxygen (O-III) gas, while the other areas of the Nebula and the surrounding clouds show the location of hydrogen (H-II/H-alpha) gas.
From science.nasa.gov: The Crescent Nebula is about 25 light-years across blown by winds from its central, bright, massive star. Its central star is shedding its outer envelope in a strong stellar wind, ejecting the equivalent of the Sun's mass every 10,000 years!
This was my first image with the Celestron EdgeHD 8 after spending a week and a half or so ironing out all the wrinkles, learning the scope, and getting it set up for imaging. I've imaged Crescent Nebula before, but nowhere near this long a focal length. The last time I imaged this Nebula was at 360mm fL with my AT60ED refractor, and I was very curious how much more detail could be shown.
I think the next time I try this target I'll try using a monochrome camera and individual S,H,O filters. (I have yet to try my filter wheel and ASI2600MM on the EdgeHD 8.)
Celestron EdgeHD 8 with 0.7x Reducer (fL = 1422mm)
ASI2600MC Pro (OSC camera)
ZWO AM5 mount with guiding via OAG and ASI174MM Mini
Light Frames [Total Exposure 4:35] used Antlia ALP-T Dual (Ha, OIII) Narrowband filter -- 300s x 55 frames
All light frames calibrated with dark, flat, and bias frames
Pre- and post-processed in PixInsight
Additional post-processing in Photoshop for exposure adjustments and generation of jpeg
Greg, a wonderful capture. Colors are great and nice details as well as round stars....wahoo!!!
Terri
Thanks, Terri! Sorry, don't know why but I didn't get to reply to this until today.
It came out a little 'crunchy', but I wanted to see what I could see in terms of detail. I tried to reduce the stars just a bit, but I still have the little spikey things coming from one side of the larger stars from the dew heater ring on the correct plate (I think). Not quite sure if I can process those out without going one by one, but they're more visible when I do a BlurXterminator reduction on them, so I eliminated that in the processing.
Thanks again for taking a look!
Greg
Thanks, Terri! Sorry, don't know why but I didn't get to reply to this until today.
It came out a little 'crunchy', but I wanted to see what I could see in terms of detail. I tried to reduce the stars just a bit, but I still have the little spikey things coming from one side of the larger stars from the dew heater ring on the correct plate (I think). Not quite sure if I can process those out without going one by one, but they're more visible when I do a BlurXterminator reduction on them, so I eliminated that in the processing.
Thanks again for taking a look!
Greg
Hey Greg, not counting response turn around (smile)…. Glad to see you using the forum. On the stars, do you know that the spikes are due to the ring heater? I generally turn it off for planetary imaging as I talked to Celestron support when I was having collimating struggles and when they found out I was using the ring heater they told me it is incompatible with planetary imaging.. to which I said… “what ??”. Funny they sell the thing and it causes issues. In any case, I have one on my 11” as well, but keep it turned down. Next time I do any deep sky with it I’ll have to see if that’s a result.
Terri
@terri I'm actually not 100% sure that the spikes are due to the heater, Terri. I haven't had a dry enough night to test without using it. I think that will be the definitive test to see if the spikes are due to the heater, collimation (although the secondary shadow sure looks centered me to in defocused stars), or something else. One other thing I'm going to try is to loosen the retaining screws just a bit for the dew heater ring/corrector plate and see if that helps at all. I don't think I overtightened them, but you never know. (Do they make micro torque wrenches? 😀 )
Wow, that's odd: don't use it for planetary imaging?
It really does a fantastic job at preventing dew, but how they can solve the differential heating problem seems like a tall order.
I'll keep you posted!
Greg
Yes, nice image. I too like your choices in the color processing.
The "spikes" certainly are not objectionable in the image.
Yes, they are probably due to warm air currents and if not from the dew heater, could be from the primary. If the air temperature was dropping fairly fast the primary may never have cooled down enough. Some SCT's come with fans to ventilate the inside for this reason. My 12.5" Dob had a terrible heat plume off the primary until added 3 little fans to the vent holes in the back plate. One way to check for this is to look at and maybe stack a few images from the beginning and a few from end of the run and see whether or not both sets have spikes. And check to see if the position of the spikes changed as the scope rotated relative to the ground.
As you surmise it could be astigmatism from a pinched corrector plate. An easy test is to view a bright star. Slightly defocusing, if I remember correctly, will make the spike easier to see. If it is caused by a heat plume, moving slightly to either side of focus the spike should shift the spike from one side of the star to the other. You may also be able to see the plume silhouetted against the image of the mirror if you defocus a lot if the plume is strong enough. Also, if you say look at star in the East and then the West and wait a little while, the spike should rotate around the star relative to telescope and the eyepiece view. If it is astigmatism, moving to either side of focus will cause the spike to rotate 90 degrees. It should also show on both sides of the star which makes this unlikely to be astigmatism. If is a plume due to the dew heater, if you also use a dew shield, the shield may make it worse.
I looked more closely and see the spikes in the upper right are on the opposite side of the stars compared to the stars in the lower left. I don't know what that is telling us.
Paul
@pwalker Thanks for the thoughts and the suggestions, Paul! Wish I could get some more decent nights to test all these things out. It's frustrating to have so many things to try and not have the skies to try them!
And as you said, the position of the stars makes a difference in the spikes so it could be two (or more!) problems compounding the effect of the dew heater ring. It's just going to take more fiddling.
Thanks again!
Greg
