Galaxy NGC55 in Sculptor

Another galaxy, pretty close to NGC253 that I posted recently, this is galaxy NGC55.

IMAGE LINK: https://flic.kr/p/znoX​mh

Nice shot. Can you give details on how you took it?

G'day. Not sure how much detail you're after. Taken with my 506mm focal length refractor (TMB92SS) on goto GEM mount (Takahashi EM200). Was probably something like 20x 8min luminance shots, and 10x 2min RGB shots all calibrated, stacked, and combined. Camera is a special cooled mono CCD camera (QSI583ws) with LRGB filters (Astrodon).

troypiggo wrote in post #17760325
G'day. Not sure how much detail you're after. Taken with my 506mm focal length refractor (TMB92SS) on goto GEM mount (Takahashi EM200). Was probably something like 20x 8min luminance shots, and 10x 2min RGB shots all calibrated, stacked, and combined. Camera is a special cooled mono CCD camera (QSI583ws) with LRGB filters (Astrodon).

yup, that's all the details we all wanted to know

its good to include a note on gear used, exposure times etc when posting pic's
gives people an idea of what was done to achieve the image, helps others with same or similar gear what to expect from their setup etc

cheers
Dave

Wow 8 min exposures. Makes me jealous with my Ioptron skytracker. Nice work Mate.

FEChariot wrote in post #17763617
Wow 8 min exposures. Makes me jealous with my Ioptron skytracker. Nice work Mate.

Reply to comments by FEChariot:

I don't think you need to be jealous of Troy's 8 minute exposures. If you align your iOptron Sky Tracker precisely, you should easily be able to take 8 or 10 minute exposures, too. I haven't had any problems at all with my iOptron Sky Tracker failing to track. I think the iOptron Sky Tracker can hold it's own against the competition!

After reading your post, I decided to see what, if any, limitations there are to long exposures with my Sky Tracker. (Months ago, I had taken 9 minute exposures with my home-made Scotch Mount, and the Sky Tracker tracks better.) I went out tonight and took some long exposures of Jupiter. I was careful to align it, which didn't take long at all. I had to experiment to find the right ISO and aperture, but finally got this one at 604.7 seconds, f:22, and an ISO of 100. I just checked the time, opened the shutter, and went into the house for about 10 minutes, went out and closed the shutter, and went back in while the camera took another blank exposure for the Noise Reduction. These images are from the same exposure. The first is full frame, and the second one is a 100% crop, and doesn't show any elongation of the stars.

So, set it all up in some really dark location (not like my back yard in Dallas where I took these) and take some LONG exposures. That tracker might just surprise you.

(Sorry - my lens isn't listed, so I set it to the 40mm Micro. Has anyone noticed that there aren't ANY prime lenses listed for Nikon, the list just skips from wide-angle to zoom. How can anyone add camera and lenses to the list??? I have a lot of cameras and lenses that aren't listed.)

Don't mean to hijack the thread guys, but how do you protect against dew? 10 minutes at night would surely mean some dampness has to set in on the glass.

alphamalex wrote in post #17973012
Don't mean to hijack the thread guys, but how do you protect against dew? 10 minutes at night would surely mean some dampness has to set in on the glass.

I've heard of dew shields that help, but a dew strap/heating element will do a better job. I Googled "dew shield" to find out what all's out there, and there are a lot of sites, and forums that discuss it, including a few links to places you can get a battery-powered dew strap. Try Google and see if you can find something that will suit your needs, and suggestions about how to apply them.

Roy A. Rust wrote in post #17955490
Reply to comments by FEChariot:

I don't think you need to be jealous of Troy's 8 minute exposures. If you align your iOptron Sky Tracker precisely, you should easily be able to take 8 or 10 minute exposures, too. I haven't had any problems at all with my iOptron Sky Tracker failing to track. I think the iOptron Sky Tracker can hold it's own against the competition!

After reading your post, I decided to see what, if any, limitations there are to long exposures with my Sky Tracker. (Months ago, I had taken 9 minute exposures with my home-made Scotch Mount, and the Sky Tracker tracks better.) I went out tonight and took some long exposures of Jupiter. I was careful to align it, which didn't take long at all. I had to experiment to find the right ISO and aperture, but finally got this one at 604.7 seconds, f:22, and an ISO of 100. I just checked the time, opened the shutter, and went into the house for about 10 minutes, went out and closed the shutter, and went back in while the camera took another blank exposure for the Noise Reduction. These images are from the same exposure. The first is full frame, and the second one is a 100% crop, and doesn't show any elongation of the stars.

So, set it all up in some really dark location (not like my back yard in Dallas where I took these) and take some LONG exposures. That tracker might just surprise you.

(Sorry - my lens isn't listed, so I set it to the 40mm Micro. Has anyone noticed that there aren't ANY prime lenses listed for Nikon, the list just skips from wide-angle to zoom. How can anyone add camera and lenses to the list??? I have a lot of cameras and lenses that aren't listed.)

Hosted photo: posted by Roy A. Rust in
./showthread.php?p=179​55490&i=i73598627
forum: Astronomy & Celestial

Hosted photo: posted by Roy A. Rust in
./showthread.php?p=179​55490&i=i71174441
forum: Astronomy & Celestial

God, how the hell did you get a 10 minute exposure? After about 5 I start getting some star trailing even with a really good alignment.

Do you align right on Polaris, or check to see which direction to offset the alignment? Polaris is almost exactly 40' from the Celestial Pole. I check on Stellarium to see the direction of the offset, and then use the scope iOptron provides to align the tracker 40 minutes away from Polaris in the direction I find on Stellarium. The first time I did that, I had horrible star trails, until I realized the alignment scope inverts the image. Since then, I just set the scope in what appears to be the opposite direction I find on Stellarium.

iOptron and others provide an 'App', for $2.50 or so, to load on an iPad or other portable device, that shows the offset direction. I don't have an iPad or iPhone, so I just check Stellarium. Works so far!

Roy A. Rust wrote in post #17973402
Do you align right on Polaris, or check to see which direction to offset the alignment? Polaris is almost exactly 40' from the Celestial Pole. I check on Stellarium to see the direction of the offset, and then use the scope iOptron provides to align the tracker 40 minutes away from Polaris in the direction I find on Stellarium. The first time I did that, I had horrible star trails, until I realized the alignment scope inverts the image. Since then, I just set the scope in what appears to be the opposite direction I find on Stellarium.

iOptron and others provide an 'App', for $2.50 or so, to load on an iPad or other portable device, that shows the offset direction. I don't have an iPad or iPhone, so I just check Stellarium. Works so far!

I use a polar finder app for the offset location of polaris and do my best to get it spot on. It does change a little bit if I move the camera after alignment for final adjustments before the shots are taken, but not too much. I'll have to try the opposite offset since the app I use has the same offset alignment as stellarium.

The angle of view (focal length of the lens) has a lot to do with this. It's easy to take very long exposure with a short focal length. As the focal length gets longer, it gets much more difficult to track accurately.

The Sculptor Galaxy (NGC 253) is just slightly narrower (in the long direction) than the width of the full moon.

Interestingly I was able to get a 10 minute exposure with no star trailing with my 24-120mm lens at 50mm, but with the 11-16mm lens or 24mm lens I start getting star trails around 5 minutes. I did get 5 minute exposures with the 50mm lens no problem, didn't have time to test longer exposures as dawn was rapidly approaching, next time out I'll going to give that a try for 10 minutes.

My question is at what point is it not worth it take longer exposures? I wonder if my time would be better served taking, say, several 5 minute exposures instead of only a few 10 minute exposures. From what I understand, to get the most out of something like a sigma-clipping median average you need a good 15+ exposures. I'm not going to be able to get 15+ 10 minute exposures on a night, but 15+ 5 minute exposures should be doable. That way I can image more panels/objects in one night.

Or maybe I should just buy another tracker, lol.

pdxbenedetti wrote in post #17974406
Interestingly I was able to get a 10 minute exposure with no star trailing with my 24-120mm lens at 50mm, but with the 11-16mm lens or 24mm lens I start getting star trails around 5 minutes. I did get 5 minute exposures with the 50mm lens no problem, didn't have time to test longer exposures as dawn was rapidly approaching, next time out I'll going to give that a try for 10 minutes.

My question is at what point is it not worth it take longer exposures? I wonder if my time would be better served taking, say, several 5 minute exposures instead of only a few 10 minute exposures. From what I understand, to get the most out of something like a sigma-clipping median average you need a good 15+ exposures. I'm not going to be able to get 15+ 10 minute exposures on a night, but 15+ 5 minute exposures should be doable. That way I can image more panels/objects in one night.

Or maybe I should just buy another tracker, lol.

I've done 8 minute exposures with a 135mm lens with no star trailing.. but that was a mistake (I had set the aperture to f/11 and that wasn't my intention. I quickly realized my mistake and used a much lower f-stop and hence a much shorter exposure time. In other words... there's probably not much reason to shoot such long exposure times unless for some reason your aperture is limited to high values.

Also... the discrepancy between the two lenses may well just be the accuracy of your polar alignment (and not really the lenses).

Both my Losmandy StarLapse (tracking head for a photo tripod) as well as my Losmandy G11 telescope mount (when I use my apochromatic refractor to do imaging) have a really nice polar alignment scope that nails the positions of two stars in Ursa minor. There's a point to align Polaris (technically about 2/3rds of a degree off true north) and Delta Ursa Minoris (the next star in the handle). With both stars aligned to to the target points in the polar alignment scope's reticle the mount is has a shockingly good polar alignment. But even with that, changing lenses can alter the weight on the camera nose and cause the mount to "flex" just a tiny bit -- enough to throw off the alignment. I've noticed that most of the tracking mounts are set up so that you have to align them BEFORE you add the weight of the camera (that's unfortunate because the weight of the camera and lens can change things.)

More factors are involved than just polar alignment accuracy. While we presume the motors rotate at the same speed all night, that's not necessarily he case. Tiny error in the precision of the gears, bearings, and other mechanical flaws can cause the head to shift in declination during part of a worm cycle... or appear to speed or slow the tracking rate in part of the worm cycle (these are forms of "periodic" error) and there's even non-periodic error types (bearing that roll around at a rate different than the rate of the worm rotation would be just one example.) There are other issues such as the perfection of the orthogonality of the axes (you'd think an RA and Dec axis would be precisely 90º off each other -- perfectly orthogonal. But machining errors can mean it's really just a tiny fraction off of being a perfect 90º angle.

For all of these reasons, astro-imagers tend to use auto-guiding systems when using a telescope.

The errors I mentioned above are usually small and while noticeable at high focal lengths, they usually are not noticeable at short focal lengths. In other words you probably don't need to worry about them when using most typical camera lenses (especially anything wide-angle).

It starts to get off-topic but I have an image of M27 that I took with a Celstron C14 (3910mm focal length) scope and I couldn't get "round" stars until we replaced the dovetail (it came with an extruded aluminum dovetail rail and we had to replace it with a solid machined dovetail rail) and also encapsulate the optical tube in mounting "rings" to solve the flexure issues.

My main point is that in terms of mount tracking quality... you really only can compare two mounts if using similar focal length and weight on the mount because it's so much easier to get good tracking (and thus longer times) at short focal lengths.

I encourage people to begin the hobby with short focal lengths then work their way up because short focal lengths are more tolerant of tiny amounts of error that would be a problem if they were using longer focal lengths.

pdxbenedetti wrote in post #17973284
God, how the hell did you get a 10 minute exposure? After about 5 I start getting some star trailing even with a really good alignment.

I just thought of something else that might affect star trails on really long exposures... The rigidity of the tripod you're using. If you notice a lot of wiggle when you touch the lens to focus or change anything, your tripod isn't really rigid and might be flexing as the center of gravity changes due to the movement of the camera and lens on the mount during a long exposure - especially if your tripod has an adjustable center pole to adjust height. After I started using the iOptron tracker and the long, heavy Tamron 150-600 lens, I upgraded to a Fancier FT-717 tripod, and have noticed a LOT of difference in the amount of vibration as I focus or otherwise touch anything on the camera. With the new tripod, there's hardly any movement of the star or other object in magnified live-view as I focus.