NGC 7129 and NGC 7142

[Menno555]

Despite bad skies, I managed a bit more than 6 hours of data of the reflection nebula NGC 7129 and and open cluster NGC 7142.

I was totally thrown off during processing though: there were these color shades/slabs which were impossible to get rid off. I already started to blame my reducer.
Until I did study some captures online and did see that it's all part of it. The "pollution" is very heavy there
There is even so much gas and dust, that they know still very little about NGC 7142. It's known that's it's at roughly 6200 lightyears, yet the age is unknown.

The processing was a challenge but I think in the end the balance between the dark part on the right and the rest is good. I am pleased that despite the bad skies and my Bortle class, the brown/orange around the nebula is showing.
Had to crop it to this size because of the star deformation on the edges because of the reducer. I should place it at the back focus distance of the scope, but then the image train becomes too long to handle.

Bortle 6/7
Meade LX200 8" f/10 ACF OTA
Ioptron CEM25EC mount (no guiding)
Optec Lepus 0.62X Standard reducer (effective 0.73x)
Baader Neodymium SkyGlow filter
Zwo ASI071MC Pro camera

Captured with SharpCap Pro @ -10 Celsius / White balance R50 B50
14 x 600 sec / Gain 90 / Offset 10
57 x 240 sec/ Gain 90 / Offset 10
20 x darks, 50x flats en 50x darkflats

Stacked with DeepSkyStacker
The 2 sets in 2 groups and stacked together

Processed with Siril and Photoshop
Siril: Crop, Photometric Color Calibration and Histogram
Photoshop: Levels, Camera Raw Filter (blacks, color saturation, clarity, lichte noise reduction, sharpening), Neat Image Noise Reduction, size reduction.

NGC 7129 with NGC 7142

NGC7129-NGC7142sc.jpg (988.17 KiB) Viewed 1199 times

[timh]

Hi Menno,

That is a very attractive pairing and the colours - both of stars and of the nebula - have come out well. The reflection nebula is one that I have missed and looks well worth imaging.

The problem you mention about the necessity to crop the image when working at lower F numbers (by adding in the reducer in your case) is of course a very general one. Certainly the same is true of images that I obtain from my 200mm Newt at F 5.0. Even using the Baader MkIII flattener (F5.0) or the SW (0.9) reducer/ flattener (F 4.5) which both work well it is still the case that with a large enough sensor chip (i.e the AS1294 MC in my case) some coma is still evident out at the edges of the rectangular image.--and so I crop.

But a bigger problem than coma - and where I was curious to know whether you have run into the same - is with flats. As the nights cool I tend to use a dew shield to prevent dew forming on the secondary mirror. But if you then make flats with the identical set up (i.e with the dew shield on) as for imaging and use these for calibration there is a vignetting effect which is difficult to correct out when dealing with stacks of rather faint images (as for example with O3). At one point I thought I was discovering curious circle-shaped seas of O3 surrounding a number of nebulae until I realised that it was an artifact. I improved things bit by flocking the dew shield. But again the only clean solution in the end seemed to be to just crop down to the unaffected core of the image.

Tim

[Menno555]

Hi Tim

Curious on why you leave the dew shield on the scope when taking flats? I for one never did that and as far as I know it's also not required to do so.
About the flats themselves: I know that Zwo cameras favor a longer exposure time with dimmer light source. I now have a Geoptik flat field generator but I also did this with the white T-shirt methode.
I used to have a kind of vignetting too but with the methode below that's all but gone.
I leave Gain, white balance and Brightness (Offset) the same as in the capture and I only adjust the exposure time. For example with the Optolong filters, I make flats (and darkflats) of 5 seconds. So I set the exposure time on 5 seconds and then adjust the lightning (or add T-shirts) until SharpCap gives an ADU of between 18000 and 25000 and then make 50 flats.
With the other filters IR/UV Cut, Baader Neodymium Skyglow, etc) I make flats of around 3 seconds.
Maybe this helps you too.

Menno

[timh]

Thanks very much for those observations Menno,

I don't do that much that is very different from what you have described except that 1) my flat exposures are generally shorter (< 0.5s even with the NB filters in place) which makes me think that I should maybe try those longer exposures at lower light levels and 2) recently I have been taking flats with the shield on.

It seemed to make sense to have the optical train for flats exactly the same as for lights - hence having the shield - but maybe at high light there is just too much internal reflection from it even after flocking? Anyway I will have a go as you suggest and see if it improves things. O3 is the worst culprit since it tends to exhibit the weakest signal to noise.

Tim