I used to experiment with a 120mm F5 achro refractor for imaging. If the stars are not too bright in the field you can get away with it. But something like the Orion nebula with bright field stars was horrendous. With OSC anyway. A mono cam and RGB filters help a bit if you refocus between filters, but I noticed the blue channel was still bloated. Maybe with a Semi-Apo filter it would have been better. In narrow band they work fine.
Without sounding like a complete novice, can you explain what narrow band is and what's involved in it ?
Narrow band imaging uses very sharp bandpass filters to isolate various nebula emissions like Ha, SII and OIII. They are similar to nebula filters but isolate one passband at a time.
Narrow band imaging uses very sharp bandpass filters to isolate various nebula emissions like Ha, SII and OIII. They are similar to nebula filters but isolate one passband at a time.
I see. Surely that is no good with an unmodified DSLR then ? Dedicated camera and filter wheel is the way to go for that ?
I am assuming then that you stack the various bands together ? Do you have to take as many subs for each band or is your total integration split between the various filters ?
I was reading something in startools where it effectively creates new stars and you substitute them if your stars are really crook.
I was going to try using my 150 ed refractor once and combine a red and blue filter to see if that could get better stars for a substitute approach...should make them smaller at least maybe...anyways the fact is you will learn ...I did some photos a decade ago with it and no filter and apart from the violet it was rather sharp...these days you can process it out ..I think you already enjoy the chalenge of getting the best out of your gear.
I know in syartools you can mask by colour so that would help..that sort of approach.
Alex
I see. Surely that is no good with an unmodified DSLR then ? Dedicated camera and filter wheel is the way to go for that ?
I am assuming then that you stack the various bands together ? Do you have to take as many subs for each band or is your total integration split between the various filters ?
No doesn't work very well with a standard DSLR. Really needs a mono camera.
The various bands are applied to an RGB colour palette. There is a lot of flexibility and creativity here as to where you put what. A common one is to put sulfur in the red channel, hydrogen in the green channel and oxygen in the blue channel, but other combinations are possible.
I don't have narrow band filters yet for my mono as they are kinda expensive. Maybe next year.
You was also asking about the F5 Achro. Here's a muck about pic I took the other night of galaxy NGC1398 with a Skywatcher 120mm F5 achro. Nothing serious it's just 3.5 minutes per channel (RGB) and luminance. I didn't even bother to take dark frames. There was some blue fringing around the bright stars but I photoshopped most of it out. Looks like my flattener spacing need attention too.
Karlzberg has been on at me about getting Startools. I downloaded the trial last night then my computer crashed so I haven't been back to it. There's a limit to how much I'm willing to process an image. I'm happy with playing around with the data, balancing, stretching, sharpening etc but I draw the line at taking out bits I don't like and replacing them. All credit to those that create great images by using this technique, I'm just saying it's not a direction I want to go.
The information you're providing is awesome. Thank you for taking the time. I won't keep nagging you but I do have one last question with somethings you've just written. RGB I get..... I think...... I assume that's just individual subs for each channel but what is a luminance sub ?
RGB is what your DSLR takes as it has red, green and blue filters over the top of the sensor inside your camera.
It can also be individual red, green and blue filters that would be in a filter wheel.
A luminance filter is pretty much clear and allows the RGB light through but blocks the UV and IR. So luminance is used to pick up detail or to quickly detect fainter galaxies.
Colin I think you can take B/W with a dslr ...could it be used as a sort of luminance?
I dont think anything changes from something I have read but I ask nevertheless.
Alex
Your DSLR takes black and white photos which is why they need to be debayered. Debayering means that it turns the RAW black and white image into a colour image.
A luminance collects all three bands in every pixel where as the DSLR has 1/4 red, 1/4 blue and 1/2 green.
Yep, as Colin mentioned I use a filter wheel with my mono camera and shoot through separate red, green and blue filters. It's more work than one shot colour, but lets one be more creative too. Ha can be added to RGB for extra pop in nebula in other galaxies for instance.
Would turning the dslr mono and using rgb filter produce something similar?
Yep. If you were to remove the bayer matrix you would have a genuine mono camera.
Glen did this with one of his a year or two ago and built a cooling unit. He had good success with it.
It may be more expensive but it would probably be better just getting a mono sensor from ZWO or QHY. I think I remember Glen having some issues getting it all done but it was a while back now.
Who would have thought from asking people to post their favourite f/10 photos I would learn so much. Thank you all for your invaluable input. It's greatly appreciated that you've all taken the time.
The f/5 is currently having a run outside. Wow it captures so much more light..... Including moon glow so much more sensitive to all forms of light. It will be interesting to see how it goes with just light pollution in a couple of weeks time. I'm happy if it's reasonable and I only have to drop one stop on the ISO but if I've got to drop a couple, it kind of defeats the purpose of having a faster scope..... Thoughts ?
Try various iso It surprises me one can get away with 1600 but it works for me now..once its was 800 mostly ...but it will be what works with the exposure conditions permit...from seeing to mount performance...I think the faster will surprise you...try 30 seconds and work up ... higher iso may help with the colour maybe..
Just experiment.
Well tonight I set up in hope with the nikon and the 70 / 200 lens at 200 and photographed lights at a village on the distance using 6400 iso and 8 seconds...amazing.
I actually read that exact same site this morning. That coupled with the experience I've had with my last OTA has made me draw up a bit of a table for my settings. In my current skies, I'm never going to get the best out of my new scope. Having said that, I think I've worked out a balance. I'm going to run several diffenent exposure times tonight and record the out comes. From that site, ISO 800 seems to be the best SNR for my camera which is what I ran last night. The problem is, even at 22sec, the sky washes everything out. Tonight's recordings will be as ISO 800 and iwouldnt just run small sets at different exposure lengths to record the point at which sky glow is acceptable. Given that the moon is at nearly 90%, this will become my worst case scenario. In 1 1/2 weeks when we get back to 50% moon, I'll run the same test and record my findings. Finally in 2 1/2, I'll run the same test with a new moon. My old setup was slow enough that it would sit within the tolerances of my mount in these skies. The 22sec came from that was all I could rely on the smoothness of my mount for. 22sec at ISO 3200 created small sky glow but tolerable with a full moon so I never had the options I do now.
Along the way, I plan to carry out the same test in diffenent skies to create a long term table as reference. Needless to say, going f/5 has created some challenges for me but I'm loving the data analysis and the opportunity to create a good reference table for my subs. An opportunity I've never had.
so much more sensitive to all forms of light. It will be interesting to see how it goes with just light pollution in a couple of weeks time. I'm happy if it's reasonable and I only have to drop one stop on the ISO but if I've got to drop a couple, it kind of defeats the purpose of having a faster scope..... Thoughts ?
