I would disagree on not processing darks, they are important on DSLRs IMHO.
Unless the DSLR is temperature regulated, it's a hiding to nowhere...it's not possible to calibrate the lights well when the sensor temperature fluctuates so widely over the course of a set.
Dithering, on the other hand, is much more effective (with a well behaved mount) at stacking out the pattern noise...this includes thermal noise as they won't occur at the same locations in the frame with respect to the subject.
My AVX mount isn't well behaved by itself for imaging, but the MGEN made it track perfectly at 400mm focal length. I've got a variety of scopes to choose from all with very good optics - I'm curious to try imaging with the 8" Edge @ 1400mm to see if it holds up.
I took a set of darks last night anyway to see if it makes any difference - haven't re-processed yet.
The Lagoon images I took were with no moon and no filter. I'll try my IDAS LPS P2 next time to how much it helps.
I'm on the border between yellow and green zones.
Yeah the AVX should be interesting @1400mm. Both axes on mine are fairly stiff with the clutches off so you never know until you try
I've done some imaging with my Edge 8 and reducer and the Celestron OAG. With all that the focal length works out at about 1450mm, but it guides easily enough on my EQ6. I've even taken some images with my full frame with that setup and it was barely vignetted (to the eye).
I restacked the Lagoon image with darks and compared the images. The first one had a couple dozen blue and red speckles through it (had to look close) which were removed in the second image via the darks.
Night time lows are currently around 3 degrees. Typical summer low temp's are around 15 degrees. How much worse should I expect the 600D noise to be in summer?
Noise doubles about every 6C increase in temperature, consequently you need a lot more subs to reduce it. If you're handy, knock up a little alloy box big enough to house it and stick a Peltier cooler on the side. There are several designs around, but check out Gary Honis' website for a good read.
If you're going to use it solely for astro it might be worth considering fitting a cold finger and removing the IR filter. The possibilities are endless!
If you're going to use it solely for astro it might be worth considering fitting a cold finger and removing the IR filter. The possibilities are endless!
I'm keeping things simple at the moment: using the family DSLR for astro, easy to set up and control AVX mount, simple stand alone autoguider.
For some reason though I find myself reading reviews of the SXD2, CEM60, Sony A7s, etc....... don't know what that's about.
Gotchas I've read about:
- Sony A7s removes small stars along with noise.
- Vixen SXD2 has no polar alignment routine
I like how fast the ASPA is and it did ok with 4 min exposures on my first guiding session (with 400mm focal length).
If it's easy to set up I tend to keep observing and if it's easy to set up and get PA I'll keep imaging. If it's a PITA to get ready I might just watch a movie instead
All advice welcome though and I take your point about drift alignment being better.
Tony - I tend agree with you about set up time. I have to set up from scratch everytime. It takes me almost 1.5 hrs before I am taking my first image. I don't want to add to that. I find the aspa is okay and the MGEN corrects well even at a focal length of 2300mm. I can get 2 min subs unguided at f480mm
Tony, I can be set up, polar aligned and guiding in half an hour, provided no gremlins pop up. My gear is stored near the observing spot and I have tiles on the ground to mark the position of the tripod feet.
I reckon the CGEM is due for an update anytime - hoping for a lighter mount with belt drives.
Got 10x3 minutes of eta before the clouds came in tonight. I was a little more careful with the ASPA and the stars are very round in the enlarged original file.
So the guided AVX seems quite adequate for shorter focal lengths.
Looks like I'll score a clear night on Wednesday and I'm keen to get some more imaging in.
ISO 800 seems to work well with the 600D but should I use different ISO's for different objects? Would ISO 400 be better for clusters to bring out star colours? For longer focal lengths and galaxies is there a best ISO?
With a bit of moonlight would shorter exposures be better?
Looks like I'll score a clear night on Wednesday and I'm keen to get some more imaging in.
ISO 800 seems to work well with the 600D but should I use different ISO's for different objects? Would ISO 400 be better for clusters to bring out star colours? For longer focal lengths and galaxies is there a best ISO?
With a bit of moonlight would shorter exposures be better?
Hi Tony,
I'm also using a Canon 600D (un-modified), had it for approx 15 months. I'm using it with an 8" SCT and image at F10 + F6.3. I find ISO 800 is best for the camera and I can achieve 10min subs when my guiding is good. The sweet spots with this camera appear to be: 60sec = ISO 6400; 120sec = ISO 3200; 270sec = ISO 1600; 300-600sec = ISO 800.
If I'm imaging nebula, galaxies and galaxy clusters I aim for 10min subs at ISO 800. I tried 15min at ISO 800 however there was too much noise. If I have trouble guiding I drop to 270sec at ISO 1600 (not sure why but I've found 300sec at ISO 1600 produces more noise than 270sec).
Long story short, I created some test images using the figures above for each ISO and noticed that the end result (brightness wise) was the same for each image, except noise. The ISO 800 image at 10mins was the cleanest. Mind you, using a 10min sub each time will take a significant proportion of your night and may result in lost subs due to wind etc. Hence of late I'm generally sticking with 180-270sec subs at ISO 1600.
If the moons out you may need to reduce the exposure time and/or ISO as you'll get a washout effect and you may have trouble distinguishing your target from the background. For brighter objects like the Tarantula it's possible, however you may struggle when compared to say The Helix.
Tony, a little moonlight is OK while testing, but unless you're on the opposite side of the sky even a little crescent can cause a background gradient.
ISO / gain is one of those things you'll need to experiment with to see what gives the best results on your targets. ISO1600 is another doubling of the signal and noise.
Remember the goal is to maximise signal over noise...if you inspect the histogram, aim to have the big peak about 20-25% away from the left edge, then that should mean your background sky is over the read noise.
I'm also using a Canon 600D (un-modified), had it for approx 15 months. I'm using it with an 8" SCT and image at F10 + F6.3. I find ISO 800 is best for the camera and I can achieve 10min subs when my guiding is good. The sweet spots with this camera appear to be: 60sec = ISO 6400; 120sec = ISO 3200; 270sec = ISO 1600; 300-600sec = ISO 800.
If I'm imaging nebula, galaxies and galaxy clusters I aim for 10min subs at ISO 800. I tried 15min at ISO 800 however there was too much noise. If I have trouble guiding I drop to 270sec at ISO 1600 (not sure why but I've found 300sec at ISO 1600 produces more noise than 270sec).
Long story short, I created some test images using the figures above for each ISO and noticed that the end result (brightness wise) was the same for each image, except noise. The ISO 800 image at 10mins was the cleanest. Mind you, using a 10min sub each time will take a significant proportion of your night and may result in lost subs due to wind etc. Hence of late I'm generally sticking with 180-270sec subs at ISO 1600.
If the moons out you may need to reduce the exposure time and/or ISO as you'll get a washout effect and you may have trouble distinguishing your target from the background. For brighter objects like the Tarantula it's possible, however you may struggle when compared to say The Helix.
The test with the Edge didn't work out because I was too cavalier with setup, got over-confident, so I went back to basics.
In the couple hours between sunset and moonrise last night and with cloud bands rolling through I scrounged another successful image - I paid attention to good alignments and calibrations and tracking was perfect with the AVX and Esprit 80. Even with thin/moderate cloud rolling through the MGEN never lost its guide star.
The LEDs on the MGEN are constantly going on/off, showing the guiding corrections. Is that normal for guiding with any mount, or do the guiders work harder with lesser mounts?
I can only chime in with observations about using PHD2 with my EQ6...if I balance too well, it's pulsing back and then forth in RA ever couple of seconds. If I've done a good job of polar alignment then Dec corrections are rare - maybe a couple a minute.
Iffy seeing can cause too many corrections as the guider thinks the star is moving when it may just be bloating / fluctuating.
What sort of exposure time are you using for the guider? There will most likely be a sweet spot between ignoring the above chasing and seeing noise in the guider image...try 1 or 2 second exposures. There's a risk that too frequent pulses from the guide box coupled with backlash in the mount mechanism may amount to mischief.
My sense is that as a starting point you just want round stars across the field.
