Astrophotography: A beginning.

[Plamadude30k]

man if I could lug a 120 lb rig to Hawaii, I could nearly afford it right now!! But ain’t gonna happen!! (I want the f/2 rig; I’m certain; money better spent.)

Please continue your digression, I was just getting into it!…

Go for the f/2 rig. My air mattress will still be waiting when you save up enough again to get out here. That goes for all y’all out there, too. Just let me know well in advance if you want to come visit.

Okay, as for flats, darks, and biases, here’s the info:
Flats: Exposures of a ‘flat’ field with a uniform brightness across the field using exposure times similar to that of the data. One trick is to point the telescope at a flat, evenly illuminated wall or board close to the telescope (so that it’s out of focus). Another trick is to use the sky either just after sunset or just before sunrise when it’s uniform in brightness, but you can’t really see stars. Sometimes, using the telescope, you can still see stars, so to make a good flat, you might need to take a few images of several different locations (with different star patterns) and average them (or, really, do a median combine).

Darks: This is the most important calibration frame for DSLRs. As long as they aren’t cooled, the sensors see a lot of dark current. Basically what happens is that the temperature is high enough to ‘boil off’ some electrons in the sensor, which makes it look like you’ve detected photons when you haven’t. To correct for this, take images at approximately the same exposure time as your data with the lens cap on and do a median combine (for DSLR purposes, averaging is just fine-on astronomical sensors, median combining is to get rid of cosmic rays).

Biases: This one probably isn’t useful for DSLR astrophotography-other noise sources (see above) way overpower this. Essentially, you use biases to calibrate out the base level of electron charge in each pixel. This only matters if you’re doing extreme precision or extremely low flux objects. Bias frames are taken buy keeping the lens cap on and taking photos with the minimum possible shutter speed.

[fluffybunny]

So rav, the last time I looked at f2 rigs for celestron (aka “Fastar” their marketing name) they involved a using camera body the would not add to the center obstruction. Are you talking about mounting your DSLR body where the secondary mirror is on an SCT?

[orionid]

rav – I have a pdf of one calculated for my 5.1, I just can’t get to that one right now. the old newt in the garage is a flea market find from two years back and has an 18″ primary with a 21 inch tube. the mirrors are dirty as crap and it needs colimated. I don’t know what the focal length is yet since i haven’t had it out. the scope alone is nearly 200 pounds. the mount is an old ac motor driven equatorial with two different knobs for coarse and fine speed adjust.

[fluffybunny]

rav – I have a pdf of one calculated for my 5.1, I just can’t get to that one right now. the old newt in the garage is a flea market find from two years back and has an 18″ primary with a 21 inch tube. the mirrors are dirty as crap and it needs colimated. I don’t know what the focal length is yet since i haven’t had it out. the scope alone is nearly 200 pounds. the mount is an old ac motor driven equatorial with two different knobs for coarse and fine speed adjust.

That sounds like an awesome find. 18″ is a fairly unusual size for mass produced mirrors. Coulter made some 17’s and Meade made 16’s. It might be a one off or better yet something from Obsession and the like.

[chupathingie]

Biases: This one probably isn’t useful for DSLR astrophotography-other noise sources (see above) way overpower this. Essentially, you use biases to calibrate out the base level of electron charge in each pixel. This only matters if you’re doing extreme precision or extremely low flux objects. Bias frames are taken buy keeping the lens cap on and taking photos with the minimum possible shutter speed.

Seconds on that. Althought there are many out there using CCD/peltier devices that this is still useful for.

As for darks, take 15-20 and average them. Hot pixels will stay constant, and you’ll get a very good average of the noise on the rest of the chip to subtract off of each light sub before stacking.

Lights: The more the merrier. While it is true that each additional light you take adds a smaller portion to the final image (and nudges the noise floor downward slightly), these tiny incremental changes to the S/N ratio become very significant when trying to stretch the contrast down near the noise floor to bring out faint details and wisps of nebula. For instance, in Rav‘s image made with 3 lights the noise floor is going to become apparent very quickly when the levels are stretched to bring out the fainter wisps in M42. With more subs (40+), not only will the noise floor be lower, but it will also be smoother and will show up more as a brightening of the background as opposed to hard static. At this point, simply setting the black point of the image has the effect of dumping much of that noise into the trash while letting you tease out fainter details close to the bottom of the histogram. There is always a lot more data in an image than it appears SOOC, so even the tiniest advantage you can gain in reducing the noise floor allows you to recover more of that data.

[Plamadude30k]

Lights: The more the merrier.

Truth. When I’m doing photometry, I usually take a few hundred flats (or lights, I think the terms are interchangeable). I had a research colleague who would insist on taking well over a thousand for each night of observation, but she was a little crazy. There IS a point where taking more flats wastes more time than it’s worth, but if you’re willing to waste some time, it won’t hurt.

[ravnostic]

fluffy that’s the basic rig, yes. I’ll lose about another 4 square inches of light gathering (beyond where the 2ndary sits anyway), but I’ll capture stuff in incredibly short times. I’ve seen uber-phuqing-fantastically detailed shots with frames at 15 seconds each.

[fluffybunny]

fluffy that’s the basic rig, yes. I’ll lose about another 4 square inches of light gathering (beyond where the 2ndary sits anyway), but I’ll capture stuff in incredibly short times. I’ve seen uber-phuqing-fantastically detailed shots with frames at 15 seconds each.

That’s a good idea. You have an 11″ right? I probably could not get away with that much center obstruction on my 8″ but the potential might be worth
risking the time/money to try. I have a DSI-II that I could fall back to if my 7D did not work.

/hmmmmmm,….

[chupathingie]

I have a DSI-II that I could fall back to if my 7D did not work.

That was my first thought with an 8″, the DSI makes for a much smaller obstruction. Part of me wishes the Hyperstar folks would offer an off-axis option. It would likely increase the image train to ~f3, but the camera could then be mounted to the side of the OTA Newtonian-fashion, eliminating the obstruction issue for all but the size of the secondary mirror.

[fluffybunny]

I have a DSI-II that I could fall back to if my 7D did not work.

That was my first thought with an 8″, the DSI makes for a much smaller obstruction. Part of me wishes the Hyperstar folks would offer an off-axis option. It would likely increase the image train to ~f3, but the camera could then be mounted to the side of the OTA Newtonian-fashion, eliminating the obstruction issue for all but the size of the secondary mirror.

Kind of a newt-schmidt hybrid, I like your thinking. I guess you would need some glass in there to keep the image circle in the neighborhood of sensor size for the DSLR and a flat secondary. Simple ring mount for the camera. Doesn’t sound too hard.

[chupathingie]

Okie dokie… I think I’m ready to post a couple. These are 100% crops of a stack composed of 22 lights and 18 darks, 2 min exposures at 1600 ISO on a 5DII thru a 70-200mm f2.8L @200/2.8. I did a few things wrong; 1st was leaving the UV filter in place (don’t do this, you will wind up with prominent lens flares), 2nd was wasting over an hour of shooting time troubleshooting by leaving the long exposure noise reduction on (still don’t know how/why it was on, but there you have it) and 3rd was to shoot only a series of 2 minute subs on a bright nebula… next time I’ll make time to shoot 15/30/60/120 second subs to recover data for the blown-out core.

Processing done strictly in linux with free tools from the repos. The heavy lifting is actually done with Imagemagick, believe it or not. I downloaded Hugin’s pano tools to align the frames, and Krita for final level adjustments. No noise reduction applied besides dark subtraction and an average stack. Aside from the final levels adjustments, all processing was done via the command line. More subs would smooth things out a LOT in the fainter fringes, my levels are stretched to the ragged edge to pull out the darker nebulosities in the outer reaches.

[ravnostic]

LOVE the Horsehead shot! Dammit, clouds are to be moving into the valley tomorrow night, my planned observation night! But the forecast looks a WEE bit better than last nights. I’m debating whether to even go–the rig is aweful heavy to lug around for cloudy skies.

I want to try piggybacking my 300mm on the scope for shots like these, though.

[orionid]

Holy hells, man!

[lokisbong]

I agree with orionid here! Holy hells is right. those are both very cool.

[chupathingie]

well thanky! 🙂 Both of those are crops from the planned widefield, but I left my UV filter on in error and the bright belt stars left very visible flares reflected in several places. The original FOV covered just north of Orion’s belt to just south of his sword. I was happy to find that 22 subs actually cleaned things up to the point that a full-rez crop didn’t look ugly at all; so I wound up with 2 images from the set. Anyone wondering about the benefits of a larger number of lights need only compare a single exposure to the averaged final. The difference is immediately obvious. The single image shows obvious static, while the averaged stack is silky smooth. I bumped up the black point and ran the gamma up above 2 before the noise started to become objectionable and backed down to a trade-off point between wispies and noise.

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