Orion's nebula

[Space Oddities]

Finally, clear skies! Since November, I swear, there must have been 3 nights without any cloud...  

So despite the cold, yesterday, I went to the city park to photograph Orion. My last attempt was more than a year ago, with just a tripod, so I was eager to photograph it with an EQ mount and a longer focal length.

Here is a first version!

Acquisition details:

• Fujifilm X-T1 (unmodded)
• Canon 300mm f/4L + 1.4x teleconverter at about f/7
• IDAS LPS D1 (Bortle 7/8)
• iOptron Skyguider Pro
• About 20 minutes of data (39x30 seconds)
• 42 darks, 37 bias, no flat (yet)
• ISO 800
• Post-processing in Photoshop and DeepSkyStacker

 

I used the LPS filter for the first time, so colors might be a bit off... I'm still not an expert in RGB alignment!

Also, I left my remote control at home, so all I could use was the in-camera intervalometer, which is limited to 30 seconds exposures. Damn, I knew I left something... I should make a checklist!

I also had troubles with my Skyguider mount... again. It's back from repair, because it was wiggly, and it seemed fine until last night. But it seems to be wiggly again, and about 50% of my subs showed trailing and had to be thrown away... I think it's because of the vibrations of the ground. At 8pm, despite the cold, there was lots of dogs running and playing around my tripod... At least they didn't bite the tripod's legs... But I'm still concerned about that wiggly mount. Perhaps I should ask for a replacement...  

Anyway, despite these issues, I find the result nice! And definitely better than my last attempt, so it looks like I'm learning a thing or two!  

Feedback appreciated!  

 

[Sunshine]

Wonderful shot of M42!

[Space Oddities]

Thanks Sunshine! I still feel it could be a lot better, and it's exciting!  

[thomasv]

Nice. And re-assuring, as I've just bought a 58mm IDAS LPS D1 to go in front of my canon 300mm f4. Clear aperture of the filter is about 53mm, so will end up with f5.7, which I thought would be a bit too slow but you say you are using a teleconverter, so imaging at f7. So, you've taken this from inner city Munich? I'm in a Bortle 5/6 zone, so bit less LP too.

 

[Space Oddities]

Yes, and my aperture calculation was a bit approximative, since I used the thread size, 58mm. With a 53mm clear aperture (thanks!), at 420mm, that would be closer to f/8 than f/7! But it did a good job anyway, and M42 is quite bright. I think I could have used a higher ISO too.

Here is more information on the sky:

Coordinates: 48° 09′ 48″ N 11° 35′ 38″ E
SQM: 19.25 mag./arc sec2
Brightness: 2.15 mcd/m2
Artif. bright.: 1980 μcd/m2
Ratio: 11.6
Bortle: class 6
Elevation: 504 meters

[dannybgoode]

I like that a lot. There’s a real sense of depth and structure to the nebulosity that is often missing from M42 images. 

Yes, very nice indeed. 

[Space Oddities]

Thanks a lot, Danny!

[FaDG]

Hi Pierre, nice shot from such a portable mount!

You wrote: " also had troubles with my Skyguider mount... again. It's back from repair, because it was wiggly, and it seemed fine until last night. But it seems to be wiggly again, and about 50% of my subs showed trailing and had to be thrown away... I think it's because of the vibrations of the ground. ".

Yet I was wondering: shooting unguided (I assume, as not mentioned in your setup list) @ 420mm focal length, could it just be plain Periodic Error from your mount instead of vibrations? Have you ever checked before the max exposure time at such focal length?

My experience with the Star Adventurer (at least mine) is that I'd never get untrailed shots unguided at that FL, even rather short ones. It's not accurate enough. Guided it's another story, though.

Fabio

[Space Oddities]

Hey Fabio,

Thanks! Indeed, I'm not using any guiding. I'm not sure it's worth it with such a mount.

I haven't thought that could be an issue, to be honest. I know there are limitations, but I'm not sure that's the case here, as I've used the 300mm lens (without the TC) and longer exposures without having to throw away so many photos. 

Below is a comparison of 4 consecutive shots of 30 seconds each. The first one turns out OK, but the next three are weird. Could that be PE?  

Also, I've noticed that the nebula isn't always centered in the frame. I never experienced this before. It's not slightly shifting over time to one side, but it's kind of making jumps. I made a GIF with most of my JPEG to show this (about 40 minutes of data):

[FaDG]

To me the drift seems exactly in the RA direction, and furthermore you can notice that there is a brighter spot which marks the time where PA is lower (hence the star stays put around the same pixel) and then starts drifting in RA+ or RA- until it stops in a different position. I imagine there could be a slight dec drift too if the PA wasn't spot on, but the main effect is in RA.

You could try stacking the pictures WITHOUT registering and check if this holds true for the overall set: I expect that if you really nailed Polar Alignment you'll see the movement basically back and forth along the RA line, else you could have  a slow up/down drift in Dec with sinusoids along RA.

My suggestion is anyway that you perform a measurement of the Periodic Error, either using a CCD camera (like the ASI) or just by taking one 10 minutes (or more) photo with your PA way off: in this way you'll induce drift and will be able to measure the amount of PE pixels. This will tell you how long you could expose at different Focal Lengths. Anyway, maybe using the 300 without 1.4 extender you'd have a higher rate of keepers and brighter pics, thanks to the faster focal ratio.

In the end, when working unguided, it's the tracker setting your max focal length and exposure time couple! Personally I'd choose a combination giving me at least 75% keepers. Also remember that here you're working at the celestial equator, where the PE shows at its worst. When you image at higher declinations you have a bit of margin.

Keep on experimenting.

Fabio

[Space Oddities]

Thanks a lot Fabio! I was familiar with the concept of PE, but I needed to understand what it actually is. After reading your message, I found this interesting and well written article that explains what PE is. They have this animation that clearly shows what's on my pictures:

Also, I rechecked the pictures made with the 300mm, and confirmed in Stellarium: my target (M31) was much higher in the sky at that time. Which explains (+ the shorter focal length) why I had almost 100% keepers, from what I recall, despite the longer exposures.

So that's a good lesson for me!  I didn't think PE would be more visible at lower altitudes, but that's quite logical after all.

I'm not sure it's worth investing in a guiding system though, especially with a SkyGuider. Technically, I could, as I have the camera and the scope, but I like having a very portable kit, since I have no car and no garden  Everything has to fit in a backpack! Also, I've reads many times that for this kind of mount, the benefit is limited.

Thanks for your help!  

[FaDG]

I fully agree that a guiding setup goes against the philosophy of such a portable setup, and the lack of Dec guiding should also be taken into account, so perfect polar alignment and balance become a must. 

You could use a full moon night to characterise the PE curve of your tracker, and use this knowledge to drive your choice of exposure in order to ensure a high percentage of keepers. 

Happy that i could help. 

Fabio