Jump to content

Methodical way to determine what filters are necessary?


Recommended Posts

Hi All,

So now that I know my rig can technically image I think now its down to signal to noise and the use of filters - to maximise my target signal versus the camera reading the stuff I dont want.

Even thought I'm technically Bortle 5, fundamentally I am not getting the fainter detail that I want to get, even with longer exposures of 10mins with a 16" aperture and reasonable tracking.

I have an older KAF-8300 colour ccd and while I'd like a more sensitive and lower noise large cmos, I'm still on a budget and its turning out to be still impressive for where I am at.

So I think the next question is how much filtering do I need to pay for to let the signal through versus the sky glow.

There looks to be everything from cheaper broadband LPF, CLS UHC then gradually moving through tri, dual and single narrowband filters of ever reducing bandpass width and increasing expense.

I have only tried nothing and with an IR cut filter - so I'm letting everything in currently.

Presumably it is simply that the more you spend the fainter can be detected.

Yep I'm mainly talking emission nebula with the last statement as opposed to broadband targets.

I appreciate in theory there's the usual question of what are the street lights, however the modern reality is that most now will be LED and it will be difficult to determine what there actual range of frequencies will be.

So the main question - is there a methodical approach to determining what we can get away with versus light pollution, as opposed to either having a punt at what may work or going for the most expensive which isnt sadly an option?

p.s. yes I am using cal frames, although not flats yet.

Obviously buying them all and returning the ones that dont work isnt really an option sadly.

Also it looks like theres two requirements filters for emission nebula and filters for broadband targets.

Any advise would be appreciated thanks before I start this next journey.

All I can think currently is go for an L-Enhance for starters.

This is my first and only decent image currently with just simple stretches (not the one a friend did in pixinsight) - just awaiting clouds to go for next...

If I'm missing anything else please let me know, greatly appreciated thanks very much.


Link to comment
Share on other sites

One aspect I do wonder about - whether its major or minor at this stage is if my tracking isn't accurate enough then I'm  not concentrating the fine detail enough - effectively blurring it...

I get 1 arcsec total reported by PHD2, however I'm not using an OAG therefore I appreciate all I can actually go off currently is the above image.

What do you think based on above image, do you think I should be able to get finer detail and therefore this question is really about filtering?

Sorry this was actually the image I intended to put up - not that I've got many to put up - only these two...


Link to comment
Share on other sites

What is the guide camera duration Nick? I have mine at 2.5 seconds and get sub 1 arcsec and probably 0.5 when seeing is good. I'm in Bortle 7. PHD2 is very good at compensating from what I have found.


There are filters that can filter LED spectrum streetlights. I have one that I occasionally use on my colour CMOS called the IDAS D2. I think that has been superseded now by a newer version. It came as a 2" filter. It is a full spectrum filter that lets through RGB but blocks LP from several wavelengths. No good for narrow band on a colour camera but great for full colour images. Also good for reflection nebulae.


I've not tried any of the LExtreme/LEnhance kind of filters. But I do have the Altair Astro Tri Band filter that lets though Ha, Hb and OIII wavelengths. Altair also do a range of newer multi band filters which are narrower that the original. So not knowing your budget I guess AA or similar filter will get you on the emission nebulae. Ultimately when you can get to buying a CMOS then make sure you buy a filter that will do both your current CCD and the potential future purchase. I personally would avoid clip in filters that only fit your DSLR. Those filters won't work on dedicated astro cameras so are a waste of money. I bet you could get some mileage (no light year pun intended 🙂 ) out of your current ASI178MM especially with the focal length you have on the beast. You have lots to experiment with there. For us all in the UK cloud cover is ever present though 😞  


I tend to just do mono captures nowadays with narrowband filters although I do have LRGB in the same filter wheel too but have to keep exposures short on the latter because of the LP.



Link to comment
Share on other sites

Thanks Terry, yep when seeing better phd2 can drop to 0.7 which is crazy but not sure if believe it, although at 1.8m focal length it shows even smallest wobble. I'm usually at 1sec but have tried increase and sometimes even decreased in better seeing. I don't use the DSLR anymore, mainly getting used to the cooled qhy9c but now that I've got better stars I may have a chance and will see what the 178mm can do as you suggest although it is pretty small. I've seen the idas filters so will have a better read. Budget around £150 for emission nebula filter but part of me just wants to know if a cheaper lpf filter would do everything in my bortle5 - i have a couple of options to try but thought it was worth a question as good clear skies are too few and now that it's starting to work im desperate to get imaging. Thanks very much.

ps I have a friends lovely altair 26c camera and his altair tri filter but he'll want it back soon and no clear nights since borrowed it. So either I try the tri filter with my qhy9c or his 26c. Id love to know what scope can really do, but also need to know what my camera can and cannot do probably more so...just need two clear nights...

Edited by Demon
Link to comment
Share on other sites

  • 2 weeks later...

So this is where I am at the moment after more reading and comments thanks (for one shot cooled colour camera, although I am also going to play with little mono camera as suggested when time allows).

I'm going to put emission and planetary nebula to the side also as basically my understanding is the more you pay and the narrower the bands the more faint detail Ha and OIII you can pull out and we're typically less interested in the other colours - so thats sorted and I just need to pay the money.

When it comes to galaxies its a whole different story - I'm bortle 5 so dont have to be as aggressive as those in bortle 8 as I should already get a better signal to noise ratio, however my thinking is that dimmer galaxies in bortle 5 is effectively the same as brighter galaxies in bortle 8. So if I get an agressive filter I can likely reach dimmer galaxies where I am. So I think I'm going to start out fairly aggressively with a UHC but letting a little colour in. This should make a significant difference in SNR on dimmer galaxies I think...All this is also making me realise I need to flock the hell out of it and block all potentially like leaks so as not to compound the problem (Cuivs vids making even more sense now)...

Edited by Demon
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...