Help Buying Narrowband Filters

[russellhq]

I'm pretty much set on getting into narrowband imaging but I wanted to make sure my thoughts add up and I've not missed anything obvious

My main reasons for narrowband are to combat light pollution, improve image quality (though increased SNR) and increase my range of targets.

From what I've read, the 'best' filters to get are the Astrodon 3nm but at their current price, they're well out my range. So for budget reasons, the Baader range seem the logical next one down. And for my sensor size, I'll need the 2" filters. I don't plan on using a filter wheel at this stage.

The most common filters used seem to be Ha, OIII & SII (in that order).

Narrowband works best with long exposures, 20mins seems about normal per sub.

Using a OSC camera, Ha and SII will only be picked up on 1 out of 4 pixels, whereas OIII will be picked up on 3 out of 4 pixels. This doesn't concern me too much since this is similar to how the OSC works normally, i.e. only 1 in 4 pixels picks up red and the images produced are still satisfactory. (I won't be buying a mono cam any time soon, just don't have the funds).

So, for now I think i'll go for the Ha and OIII filters, and then maybe next month get the SII. I've seen some nice bi-colour images from Ha OIII (Sara's spring immediately to mind!)

[Steve Ward]

I thought that narrowband filtering was used in conjunction with mono cameras , not sure you'll gain a lot using with OSC .... ?

[russellhq]

Hi Steve, I don't see why not. I couldn't find anything to suggest that a OSC camera would not work with narrowband filters. In fact to the contratry, this chap in Finland has made some excellent NB images with a OSC camera:

http://astroanarchy.zenfolio.com/

This album is just with his OSC camera and NB filters: http://astroanarchy.zenfolio.com/p830355939

[MeyGray3833]

Well, you're right, it is possible with OSC and it seems as though you have thought about it a bit. Ha is very good under light polluted skies, however; I would be cautious with OIII under heavy moonlight or light pollution, as gradients are still possible.

I use the Baader 7nm in Ha, SII and OIII and I find that my images are not as smooth as with the Astronomik 12nm series, especially in SII and OII, where there is less signal and you may have to expose for longer to get good signal to noise ratio.

Also, your location will have a lot to do with how good your images are going to be. If you are going to do narrow band from a light polluted area, you will need to experiment with your exposure times etc. I would try to borrow an OIII filter first and see what sort of data you can get before committing to the purchase of 3 quite expensive bits of kit. Also a filter wheel would make life easier, automated preferably, saves having to continually take new flats at every filter change and if you forget to take flats you may have to ditch your subs.

[russellhq]

...I use the Baader 7nm in Ha, SII and OIII and I find that my images are not as smooth as with the Astronomik 12nm series, especially in SII and OII, where there is less signal and you may have to expose for longer to get good signal to noise ratio...

I would not have expected the 12nm to perform better than the 8.5nm. With a wider band pass, more noise should be present and therefore a lower SNR.

A filter wheel is something I see as a luxury at the moment. I've got a nice set-up for taking flats, so it's not that much of a hindrance.

[lensman57]

I would not have expected the 12nm to perform better than the 8.5nm. With a wider band pass, more noise should be present and therefore a lower SNR.

A filter wheel is something I see as a luxury at the moment. I've got a nice set-up for taking flats, so it's not that much of a hindrance.

Hi,

One thing you got to be aware of is the OIII filter is extremely demanding of the scope . As most scopes even some triplets are at their weakest with blue, expect a lot of star bloat and halos in the captures.

A.G

[russellhq]

Thanks A.G.

I've read a bit about star bloat with OIII but did not read anyone talk about the demands of the scope before, (unless I forgot, I read a lot) so that's good to know.

I did read the experience of one astronomer who suffered from OIII bloat/halos and blamed the filters, eventually he bought the Astrodon filter and that cured his problem, but this was contradicted by the experience of another astronomer:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6153034/page/0/view/collapsed/sb/5/o/all/fpart/all/vc/1

Thanks,

Russell

[lensman57]

Thanks A.G.

I've read a bit about star bloat with OIII but did not read anyone talk about the demands of the scope before, (unless I forgot, I read a lot) so that's good to know.

I did read the experience of one astronomer who suffered from OIII bloat/halos and blamed the filters, eventually he bought the Astrodon filter and that cured his problem, but this was contradicted by the experience of another astronomer:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6153034/page/0/view/collapsed/sb/5/o/all/fpart/all/vc/1

Thanks,

Russell

Good Morning Russell,

Add me to the list of bloated ones, I have the Baader Oiii CCD and I get horrible bloat, in the end I had to resort to correction in software, which is cheating really. Most Oiii filters bloat with an Achro but the Achro it self must be well corrected otherwise it will add woderfull colours to your capture. I have done a couple of Bicolour captures so far and one way of getting rid od the bloated star was for me tremove the stars from the finished colour image and then use the Ha capture to paste the stars back in. This one worked as you can imagine there will be very little if anycolour left in the stars. You can also do an RGB capture with your QHand then do an Ha capture and then add the Ha to the red channel of the RGB enhance detail but this is really an LRGB in a way as you already know. I don't have a Newt but as these have very little CA issues they could fair better. The ED80 is remarkably well corrected in the blue for a doublet, I may have to start using mine again I think.

Regards,

A.G

[russellhq]

Thanks A.G.

I found this site which has transmision curves for a lot of OIII filters.

http://www.astroamateur.de/filter/oiii.html

Could it be that IR is leaking though and causing bloat? I see you stated you have the CCD version, but maybe it's worth checking?

[lensman57]

Thanks A.G.

I found this site which has transmision curves for a lot of OIII filters.

http://www.astroamateur.de/filter/oiii.html

Could it be that IR is leaking though and causing bloat? I see you stated you have the CCD version, but maybe it's worth checking?

I have thought about this too, in the next session the Baader IR will go in front of the FF/FR, that is if this god awful weather allows any imaging in the near future.I also think that a leaky IR is more likely to cause the magenta rings around the stars as it blends with the CA, but I am no expert and others mhave better explanations.

Regards,

A.G

[russellhq]

I have thought about this too, in the next session the Baader IR will go in front of the FF/FR, that is if this god awful weather allows any imaging in the near future.I also think that a leaky IR is more likely to cause the magenta rings around the stars as it blends with the CA, but I am no expert and others mhave better explanations.

Regards,

A.G

Tell me about it! I've not had the scope out since the start of November

I don't think the IR would cause magenta rings. Since you're mapping the OIII filter result to Blue, all signal captured would contribute to Blue regardless of wavelength.

[lensman57]

Tell me about it! I've not had the scope out since the start of November

I don't think the IR would cause magenta rings. Since you're mapping the OIII filter result to Blue, all signal captured would contribute to Blue regardless of wavelength.

You are correct Russell But I think that I was thinking about RGB capture. So many problems to worry about. I also have to  make an apology to anyone who reads my posts as my HP lap top has develpoed the Jumping cursor problem, it deletes leters  at will and starts new ones all over the place, so my posts look strange at times if I don't try and correct them half a dozen times in each post, at times I just give up and close the reply, some people might be grateful for this though, LOL.

Regards,

A.G

[Davey-T]

Here's a nice example of Badder 0111 blue bloat taken with 100mm Doublet.

Dave

[russellhq]

I found this example to be closest to the same set-up I have and with Baader filters:

http://stargazerslounge.com/topic/169667-ic1848-all-souled-out-in-three-flavours/

[davew]

I found this example to be closest to the same set-up I have and with Baader filters:

http://stargazerslounge.com/topic/169667-ic1848-all-souled-out-in-three-flavours/

Soul_2012-HST.jpg

I don't think that's at all like your equipment. He's used a mono 285 chip. I could be wrong.

I've just seen an image that is closer to your chip but uses a modded DSLR - http://stargazerslounge.com/topic/204107-horeshead-nebula-ha/#entry2161932

That shows the power of an Ha filter through the Bayer Matrix very well. There are many others.

Dave.

[russellhq]

I don't think that's at all like your equipment. He's used a mono 285 chip. I could be wrong.

I've just seen an image that is closer to your chip but uses a modded DSLR - http://stargazerslounge.com/topic/204107-horeshead-nebula-ha/#entry2161932

That shows the power of an Ha filter through the Bayer Matrix very well. There are many others.

Dave.

Thanks Dave, but I was focusing on halos/bloat when using a Baader OIII filter. The image I selected had the same optics I have and filters I'm interested in buying: ED80 with 0.85x reducer and Baader filter.

It does have a difference camera, but I'm not sure why a different camera would affect halos/bloat? I can understand though that a mono camera would produce a finer image.

[davew]

I understand now

When I started out on the narrow band trail I looked at as many images as I could. I looked at reflections, or the lack of, in as many different types of gear as I could. Then I set off to find out where reflections came from. The majority of the opinions seems to err towards the filters bouncing the light internally. Then there are the chips, chip cover slips and not too many talking about the scopes.

After a brief discussion with Ian King ( Don't read into it that he doesn't rate the other filters, we also discussed band pass ) I decided on Astrodon 5nm filters. At the time Astronomic were having nightmares and Baader were too wide.

Doesn't help you much does it ? I have never suffered from big bright circles round my stars with the Astrodon. Bit of flare on certain nights but hey.....

Dave.

there was a discussion once as to why stars bloated through narrow band filters. It would appear the way round it was to use a narrower pass band on the OIII or process it out.. It's not a fault, it's a fact of life.

[russellhq]

Thanks Dave, it's good to hear others experiences. But at £320 for a 36mm OIII filter, I could get a 7nm Ha, 8.5nm OIII and 8nm SII Bader filter set

[davew]

Don't tell anyone else but if I was in your position I'd buy a Baader Ha and see how I got on. If great then I'd get the Baader OIII. I wouldn't rush to buy an SII. Got one and hardly use it !! That was a clever way of spending £ 250. Not

Make sure what ever size you buy that you can use it in the future. I wouldn't get an unmounted 36mm or 31mm unless I had a very good reason. Be thinking of the mono camera you may buy and what wheel it will use.

Dave.

[russellhq]

Thanks Dave, I have thought about the convenience of the wheel and possible upgrades in the future but the price just seems too much.

2" Filter wheel : £400-£500

Ha & OIII 2" Baader Filters: £280

Total £680 to £780

TS Optic Filter draw + spare drawer: £125 (http://www.teleskop-express.de/shop/product_info.php/info/p5741_Filterschublade-inkl--1-Einschub-fuer-ungefa-ten-36-mm--Filter---kurzbauend.html)

Ha & OIII 36mm Baader Filters: £220

Total £345

So, quite a difference! I could spend the extra £60 and get the 2" versions while keeping the drawer.

In reality, I would only use 1 filter, maybe 2 max on any one evening imaging. With the drawer, I would swap them out, re-focus and carry out. I must say though, I've not read anybodies experience with the draw system so not sure if it's a good idea...