Iris Nebula - Tak Mewlon 180
Nothing since September … bad weather, cloudy, storms, winds, humidity … I didn’t get the chance to shoot anything.
I have this weird habit to cope with bad weather in buying new accessories … From the last shots in Sardegna, I had this halo thing on Deneb and I was wondering what I could do about it except tilting my camera to have the halo centered. What about avoiding the halo ?
I found a filter with a feature “anti-halo”. I bought it … and now I have four filters. Changing manually a filter during the night is a bit of challenge. What about a filter wheel ? I like blue so much that I’m a fan of the manufacturer PegasusAstro ( a greek so an European company …it changes from China or USA). Let’s go for this wheel with 7 slots for 2inches filters, at some point I’ll get the RGB filters and a monochrome camera.
Oh, and the power supply was also a new challenge, with a lot more equipment to power … Pegasus manufactured a saddle powerbox that matches their mount Nyx101 :
Now I can connect all equipments to this saddle and it should bring a new easier cable management ( so I thought)
Ok, I have a lot of things there, and Iris Nebula was definitively an object I wanted at all cost to get on January. The size of it pushed me to think about using for the first time the Mewlon 180c. I originally acquired it for planets with a Barlow Televue 2x (that I didn’t try yet neither …), but I felt that the focal length with my current camera would be a perfect match for this nebula.
The Takahashi Mewlon is a Dall-Kirkham Cassegrain, with an amazing quality of mirror as I could read about it in some tests. Compared to the Schmidt Cassegrain (my Celestron C9.25), there is no Schmidt lense and it gets a artefact called coma at the borders of your pictures. They are offering a flattener that corrects this artefact, and it also reduces the focal length to 1760mm instead of 2200mm … Iris Nebula will be then very matching the size of the camera sensor …
I spent hours and hours to finalize this setup, as there is no much room to get the back-focus accurate (I feel this post is already too much technical, so I’ll skip the long explanation about back-focus … it is a precise distance between your last lense to the camera sensor … it must be very precise). I needed some tests before shooting this magnificent object.
Last thing before the test : as the focal length of a Cassegrain is pretty long, the guiding system I got (a 135mm focal scope) is not good. For this setup I need an OAG (Off-Axis Guide). I got one from PegasusAstro, and it is part of the optical chain.
Ok, let’s test all this. I realize that I’m really putting myself into weird and complicated situations all the time … I’m a big overthinker, and I like challenges. Let’s say that this setup is basically a huge challenge… I got my first light of the Takahashi but my guiding system didn’t work … I really struggled with that … I feel that the mirror of the OAG is not catching anything … I need to work on it for my next shooting session. From this, I decided to shoot pictures with a 40 seconds exposure, skipping guiding. I also don’t have any Autofocuser for the Takahashi, so I need to focus manually. But I got a great surprise : I don’t even need a Bathinov mask because of the diffraction of the spider holding the secondary mirror. Have a look at the picture of the post : you can see at least two stars with diffraction spikes ! It is a natural diffraction, not a post-treatment artefact. With these spikes, if I can get them centered on the stars, I have my focus. I can also say that the knob focuser is very precise and accurate ! Two good things to start !
The filter wheel is well managed by N.I.N.A. (the software I use to shoot). Third good surprise !
And that’s it, I let the setup shooting for hours, checking from time to time if it is still centered on the nebula (I had to correct it several times) and I got 550+ lights pictures …
The result was average … I got a strong vignetting, maybe caused by the light pollution at my place (that was supposed to be a testing session so I didn’t go to my usual dark spot) and a bizarre background noise … maybe also because of light pollution but I’m not sure. A couple of days later, I took flats, biases and darks calibration pictures to try to eliminate these two issues : it worked well !
The stacking phase was amazing : almost 8 hours of stacking process on my personal Macbook 2015 … I started to sort bad shots manually, getting around 240 pictures and the stacking process kept around 180 shots … not that much light in total … I’m currently working on a personal project and I felt it was an opportunity to use it for this stack : I put all 550+ shots and the calibration frames and I got a final stack of 480 shots so 5 hours 20 total exposure ! And here’s the result of my testing session. I can probably do better with a guiding system so I’ll work on it.
Exifs : Takahashi Mewlon 180c + ReducerFlattener 0.8x /PegasusAstro Nyx-101 + PlayerOne Poseidon C Pro cooled at 0°C / PlayerOne Luminance Anti-Halo filter / Filterwheel Indigo PegasusAstro /- Guiding System OAG Pegasus with PlayerOne Ceres C
550 lights 40 seconds each - 480 selected for stacking (5 hours 20 min exposure for final shot)
50 flats, 50 biases and 50 darks
Stacked with Pixinsight (WBPP)- Post-Treatment with Pixinsight with very minimum processes.
