Full spectrum camera vs h-a filter on scope

[vertigo262]

i should probably post the picture

3 x 10 min

[Thalestris24]

Hi

No you don't want or need IR. All the light you need is in the visible part of the spectrum. To get a sharp image you need to cut off UV and IR. If you're having trouble understanding then I'd recommend looking up which wavelengths of light make up UV, visible and IR then see which wavelengths are emitted by nebulae. Hopefully, all will become clear when you compare those wavelengths to those which the full-spectrum camera sensor is sensitive to

Louise

[vertigo262]

oh wait! what your saying is a should use a uv ir cut filter with my full spectrum. sorry was too much info going through my brain all at once

[Thalestris24]

oh wait! what your saying is a should use a uv ir cut filter with my full spectrum. sorry was too much info going through my brain all at once

Yep! Without it you will get star bloat and blurring - like in your image! As I mentioned above, there's no real reason to have a full-spectrum mod unless you want to filter out visible light and detect IR (or UV) but that's a bit specialised. However, with the full-spectrum mod you need a good UV/IR cut filter for normal AP.  Nebulae emit light in the visible part of the spectrum but stars also radiate UV and IR.

Louise

[vertigo262]

ok then. at least that solves the star bloat which was a concern of mine when looking at other peoples images.

so if i wanted to do ha,

then all i need to do is mod a camera by  

removing the ir filter and the bebayering, but dont convert to full spectrum. unless i do, then use a uv ir cut filter with the ha filter

[Thalestris24]

ok then. at least that solves the star bloat which was a concern of mine when looking at other peoples images.

so if i wanted to do ha,

then all i need to do is mod a camera by  

removing the ir filter and the bebayering, but dont convert to full spectrum. unless i do, then use a uv ir cut filter with the ha filter

Yep! Though there's little advantage to using an Ha filter with a color camera.

[vertigo262]

yes, as you can see from my ha test I posted. Very poor results compared to everything I've seen from others.

lesson learned

I am now going back and forth on the color vs mono. the quality seems much better but what it takes to get them is a lot of work which I am wondering if it's worth it. as well as what it takes just for one image. But I've seen some pretty spectacular LRGB's

For now I will focus on ha and getting things right

[redmoo]

ok then. at least that solves the star bloat which was a concern of mine when looking at other peoples images.

so if i wanted to do ha,

then all i need to do is mod a camera by  

removing the ir filter and the bebayering, but dont convert to full spectrum. unless i do, then use a uv ir cut filter with the ha filter

Just to add more confusion, you don't need an IR/UV cut filter with a Ha Filter. The Ha filter will stop the IR and UV from going through anyway. 

If I were you, what I would do for now is get 2 filters. The first being a decent light pollution filter that also blocks UV and IR (I'm pretty sure the Astronomik CLS CCD one does this, but someone who knows more might be able to confirm) and a Hydrogen Alpha filter. 

With these 2 filters, you can use your DSLR to get colour shots of nebula, then do a H-Alpha run afterwards and combine the 2 sets of data. 

I think this is what you are trying to achieve. Also, if you are going to debayer a 7d camera, that is one brave, brave move, and I would strongly suggest against it. 

I take it you know about taking flats and darks to go along with your images? They make a world of difference, especially when you have lots of dust bunnies etc!

[vertigo262]

actually, I am leaving my 7D alone to take color, and debating on buying an old cheap slr to debayer. and it wouldn't be me, it would be someone who knows how to do it. But I am just trying to figure it all out to get a good shot. and not be an enormous price tag, as we all know how expensive this can get

But I'm looking at the amount of noise in my first 10 minute exposure, and that is bothering me as well

[redmoo]

What iso are you running at? That has a huge effect on the amount of noise that you get. I think the general consensus is that 800 or 1600 is about right.

Also, at 10 minutes, what do you unprocessed subs look like? Are they completely washed out or ~50% on the histogram? If I were to guess, I would say that you are getting overexposed shots, and hence the very obvious amounts of dust bunnies. Instead of 10 minute shots, do 5 mins, but twice as many! 

Also, flats and darks make a huge difference. 

[vertigo262]

here is a sub, it's ISO 800, although the one I did at 1600 with 2 min exposures seems better less noise and shows up better after stretching

but I see people doing hour long exposures, they don't seem to over expose.

I didn't have time to do the calibration frames that night, but it still seems extremely noisey after stretching

[vertigo262]

wrong button, here is the image since I can't figure out how to edit on this form

[redmoo]

Looks pretty well cooked to me! I think you want your histogram to be at ~50% when you are done. I am at work at the moment, so cant stick this into photoshop, but looks like it is above that.

Also, when you see people listing 1 hour exposures, I think they are referring to total exposure time, as in 6x10 mins or 12x5 mins etc. 

One other thing though, this is all done without a modded camera as of yet? You are going to struggle to get anything out of the horsehead nebula as it is pretty much all Ha emissions (which an unmodded camera will block a lot of).

If you get a clear night, try a different target. Perhaps M42 is a good starting point, with some long and some short exposures. I spend my first 10 imaging runs on that same target, as I was still learning what to do and what not to do, so I knew that things were getting better (or worse!).

If you don't like the idea of M42, you can do M81/M82 as well. That doesn't rely on Ha at all, so would be a better test of your imaging setup.

Just a few ideas! 

[Thalestris24]

Hi

The trouble with using an Ha filter with a colour camera is that only 1 in 4 pixels truly respond to Ha/red light. This means a commensurate drop in resolution. With an uncooled camera there will be a lot of noise from long exposures. A debayered camera will be ok for acquiring Ha data but will still be noisy if it's not also cooled.

Louise

[vertigo262]

Redmoo,

the mean in photoshop is 229.30, way above 50

Louise,

I was thinking about the noise, not only with Ha, but with color as well. I think the ha on a mono camera will be at least a full resolution of data. But I'd assume the same noise due to what the sensor is. Unfortunatly, the ccd's are extremely expensive, and I've only seen them up to 10 mp as well. 

which brings up the debates on cooled SLR's vs CCD. so this I will be researching to try to find a cheap way of doing it with the least noise as possible. If this is even possible I do not know. But I'm sure many have been where i am right now on this issue

I have seen some OSC with spectacular output. so if me merging an Ha with RGB is the direction I should go, I am not sure. But then from all my reading LRGB really has no comparison.

But either way, a cooled sensor seems to be the solution for everything. 

[Thalestris24]

Hi

I guess most of the red nebulosity detected by a modded dslr will be from Ha. Using an Ha filter will cut down on lp but reduce resolution. I suffer from bad lp too and use lp filters. I have a debayered 1100d/Rebel T3 in my sig. Unfortunately it's been out for repair since last July. If I ever get it back I plan on trying it with narrowband filters. However, I would expect 10min exposures, say, to be very noisy without cooling. I might try and put together some sort of cooling enclosure for it which may, or may not, help. It's a shame mono ccd's are so expensive!

Louise

[riklaunim]

I use 3 minute exposures even for narrowband (but that's not for a DSLR), so you can keep exposure down but collect more frames. The only requirement is to have signal from the photographed object higher than the shot noise. For a mono-only image a very narrow nebula/UHC filter could be better (cut down LP, but pass H-alpha and [O III]).

And don't be afraid of resolution problem - DS imaging is usually way below telescope resolving limit (that's what planetary imaging must take into account). As for mono camera prices - they aren't that high (when compared to new DSLR prices).

[vertigo262]

Louise,

Get that camera fixed and get back out there! The prob is, if I do 1 rgb + 1 ha, I need 2 cameras. otherwise, I play with the big boys and do LRGB. I'm used to shooting 18 mp, so a 10 mp ccd kind of hurts as well since that's the largest I've seen. plus any larger would be very expensive.

riklainim,

The one thing I like about SLR's is I use Backyard EOS. that is also holding me back on CCD's even though I don't want to make the investment. But, is their something comparable to BYE for ccd's, because I really like all it's features, and ability for framing, and polar alignment through liveview 

[Thalestris24]

Louise,

Get that camera fixed and get back out there! The prob is, if I do 1 rgb + 1 ha, I need 2 cameras. otherwise, I play with the big boys and do LRGB. I'm used to shooting 18 mp, so a 10 mp ccd kind of hurts as well since that's the largest I've seen. plus any larger would be very expensive.

riklainim,

The one thing I like about SLR's is I use Backyard EOS. that is also holding me back on CCD's even though I don't want to make the investment. But, is their something comparable to BYE for ccd's, because I really like all it's features, and ability for framing, and polar alignment through liveview 

Hi

I'm waiting for the supplier to fix the debayered camera. I have a full-spectrum one as well plus a qhy8l ccd. I use APT for dslr and ccd image capture. I don't see why you'd need 2 cameras if you wanted to use an Ha filter - probably you'd just need two imaging sessions. Or you could use a filter wheel. The problem with dslrs is that it's difficult to do that with field flatteners or coma correctors because of the back focus. It's maybe easier with a frac+flattener as you can get ff's with longer than normal back focus requirements. I've not seen a coma corrector which does that.

Louise

[Thalestris24]

ps here's my thread with an Ha image I took with the 150pds/qhy8l last summer. I simply attached the Ha filter to the front of the coma corrector. Worked ok but not very convenient and was a pig to focus because of the long focus adjustment exposures needed.

http://stargazerslounge.com/topic/223903-first-light-with-ha-7nm-with-qhy8l/

Louise

[vertigo262]

the reason I'd need 2 cameras is my 7D is color and I want to leave it that way. I want to merge one color rgb and replace the red channel with ha. that is why, so I do my normal shoot, then a ha shoot. then change out the red channel. so I'd need one color and one mono. unless I went hargb and used a color wheel. but a lot more work

[vertigo262]

those ha's are extremely impressive, was that 1200 subs for the california nebula? at what exposure?

[Thalestris24]

those ha's are extremely impressive, was that 1200 subs for the california nebula? at what exposure?

Hi, yeah they are both single 1200s subs with the qhy8l ccd which has a good response in the red part of the spectrum. Because of lp I can't do such long exposures normally, even with a lp filter, so they are the only nebulas I've ever picked up - I've not done any Ha since. Still, they give an idea of what can be done with an osc + Ha filter though mono would be much better.

Louise

[vertigo262]

What is the exposure settings per sub on those images? they are spectacular

[Thalestris24]

What is the exposure settings per sub on those images? they are spectacular

Hi

Um, I've said twice - they are each single subs - exposure for each was 1 x 1200 seconds (1 x 20 minutes). I don't think they are particularly good but do show some of the nebulas.

Louise