North America Complex with the Samyang 135mm

[JimFox]

That's probably not the proper way to phrase it, but the 135mm is really wide and I just did a GoTo NGC7000 and let the camera pick up the surrounding area. So perhaps I could have even framed it well, but all of this is just kind of some experimentation with the Samyang 135mm turned into a mini telescope.

One thing I have noticed with it being so wide is the background can really get a bit wonky and uneven. I try to keep my processing simple, so even though Graxpert is the rage, I often will still just use ABE or DBE when using my Askar Fra500mm. In this case neither did that good, a lot of brightness still on the edges. So I ran Graxpert on top of the ABE or DBE. That was better, but still not great. I then tried Graxpert first, it did a good job on taming the stars down, but it also didn't take care of the brighter background on the left side of the image. So in the end I probably broke every rule in the Astro Processing Rule Book.... but I ran Graxpert, then ran ABE on that result, and then ran DBE on top of that result. Finally the background was pretty even. 2 of the corners were a bit bright, but I just took care of that later in Photoshop.

I have also noticed with my testing of the 135mm Samyang on SH2-240 that the background comes out pretty uneven and some crazy background colors. Perhaps it's just the nature of the beast with it being so wide. And maybe the better route is to just stick with the Askar Fra500 with a reducer which takes it to 360mm and just do mosaics instead? But that would require so many more nights, which have lately been hard for me to come by. So it's nice getting it done all in one image. I will just have to work on my workflow some more I guess.

A couple of things to note. I set the Samyang 135mm at f5.6. You will notice diffraction spikes on the brightest stars, that's a result of the blades used on the Samyang. Thanks to @Bill Richards for sharing that. So there is no way not to have that when using this lens, unless perhaps if I shot it at f2.0.

Also, this from Monday night out in Joshua Tree, the moon was 63% so a bit bright, but Monday night was the last of the clear skies for a week or so, so I ran out there. Of course there was clear skies out in Joshua Tree over the weekend, but I had already made plans for my birthday and my sisters birthday for us to be up at Pismo Beach, so I wasn't able to get out to Joshua Tree then. Anyway, my goal was to shoot the Spaghetti nebula all night, well, once it rose high enough above the horizon to start it. So I picked NG7000 to do while I was waiting for the Spaghetti nebula to get high enough. I decided to get close to 2 hours, not really enough, but enough time that I could do something with the result. Also, after that I saw M42 was above the horizon, so I have been wanting to image that with the Horsehead all in one image for quite a while, so then I spent 2 hours imaging that.... so I ended up with only 5 hours on the Spaghetti nebula, but I wanted to satisfy my curiosity on the others, as I can get the Spaghetti again later in 2 weeks.

20 Lights @ 300 secs with Optolong LQuad filter
30 Flats
30 Dark Flats
30 Darks

ASI2600mc Pro - Astro Camera
Samyang 135mm - Scope @ f5.6
ZWO AM5 - Mount
ZWO EAF
ASIAir Plus - Astro Controller
Processed in Pixinsight
Graxpert
BlurXterminator
NoiseXterminator
StarXterminator
NarrowbandNormalization
Finished in Photoshop

All comments are welcome,

Jim

PS. This is totally uncropped. I do see where a tighter crop might be better. But I wanted to share just what the 135mm takes in uncropped first.

 

[Bill Richards]

A couple of things to note. I set the Samyang 135mm at f5.6. You will notice diffraction spikes on the brightest stars, that's a result of the blades used on the Samyang. Thanks to @Bill Richards for sharing that. So there is no way not to have that when using this lens, unless perhaps if I shot it at f2.0.

Get youself a few step-down rings and you won't have those diffractions spike. This $13 ring will reduce your aperture by 51% (so, F2 to F2.8) - that should eliminate most of the distortion from the outer perimeter of the lens. I doubt you really need (or want) to go to F4 or F5/6. That just requires more exposure time.

 

[JimFox]

Get youself a few step-down rings and you won't have those diffractions spike. This $13 ring will reduce your aperture by 51% (so, F2 to F2.8) - that should eliminate most of the distortion from the outer perimeter of the lens. I doubt you really need (or want) to go to F4 or F5/6. That just requires more exposure time.

Hey Bill, thanks!

I am sorry, but I am lost as to how the step down ring would change anything. Where does it get mounted? If it's in the image train it's going to affect my back focus and I won't be able to focus.

I had used it last time at f4, I turned it to f5.6 this time. I don't want to go faster as I don't have any filters fast enough to work at f2.0 or f2.8. In my testing I don't want to use this lens at f2.0 for sure, it's got horrible artifacts then. They clear up about f4.0.

Thanks!

 

[Bill Richards]

Hi Jim,

These adapters thread into the face of the lens where a filter would go. There are intended to enable you to use a filter that is smaller than the lens supports. So they do not affect your backfocus at all. They have the effect of reducing your aperture using a perfect circle, so no straight edges that produce diffraction spikes.

If you can work with f3.35 instead of f4, then a 77mm-to-46mm step-down ring would work. Here is a complete set of adapter rings for a lens with a 77mm filter thread which includes a 77mm-to-46mm adapter. I couldn't find anything smaller than 46mm so f3.35 is probably the best you can do with that lens using this approach.

 

[Mike Lewis]

Jim,

Well I would say all the fussing with the background gradients has not hurt the image any, and I have resorted to such things myself a time of two if it seemed like I just couldn't get a decent gradient free result otherwise. This is a fun image, as that really large FOV makes for an interesting field, and always nice to see how some of these nebulas relate to each other in the sky.

Bill's suggestion of step down rings is the way to go. It will give you the exact same effect of stopping down the lens IQ-wise. but will prevent any diffraction spikes, which are usually too busy to look good when using a lens with multiple blades (pretty much any modern lens...) I agree too, you should start off with something that gets you close to f/2.8 as that should give you decent corner performance while still keeping the advantage of light gathering. I suppose it could affect the performance of any narrowband filtering though, so if you are shooting with those perhaps closer to f/4 step down ring is best.

This is a fun image, cannot tell at small size how the stars are in the corners, but from this it looks like that Samyang lens is a keeper.

ML

 

[JimFox]

Hi Jim,

These adapters thread into the face of the lens where a filter would go. There are intended to enable you to use a filter that is smaller than the lens supports. So they do not affect your backfocus at all. They have the effect of reducing your aperture using a perfect circle, so no straight edges that produce diffraction spikes.

If you can work with f3.35 instead of f4, then a 77mm-to-46mm step-down ring would work. Here is a complete set of adapter rings for a lens with a 77mm filter thread which includes a 77mm-to-46mm adapter. I couldn't find anything smaller than 46mm so f3.35 is probably the best you can do with that lens using this approach.

Thanks so much Bill for taking the time to explain that. Now I get it. I will definitely look that up.

 

[JimFox]

Jim,

Well I would say all the fussing with the background gradients has not hurt the image any, and I have resorted to such things myself a time of two if it seemed like I just couldn't get a decent gradient free result otherwise. This is a fun image, as that really large FOV makes for an interesting field, and always nice to see how some of these nebulas relate to each other in the sky.

Bill's suggestion of step down rings is the way to go. It will give you the exact same effect of stopping down the lens IQ-wise. but will prevent any diffraction spikes, which are usually too busy to look good when using a lens with multiple blades (pretty much any modern lens...) I agree too, you should start off with something that gets you close to f/2.8 as that should give you decent corner performance while still keeping the advantage of light gathering. I suppose it could affect the performance of any narrowband filtering though, so if you are shooting with those perhaps closer to f/4 step down ring is best.

This is a fun image, cannot tell at small size how the stars are in the corners, but from this it looks like that Samyang lens is a keeper.

ML

Thanks Mike. I figure I won't get out with the Astro rig for another 10 days or so, that will give me time to get the rings Bill suggested and i Can try that.

I actually totally skipped over looking at the stars in the corner on this. I will do that for the next one I process. Not sure how I totally glossed over that.