Baader Ha 3.5nm or 6.5nm Filter

[nephilim]

Hi,
As the title suggests, I'm looking at getting one of the above CMOS optimised Ha filters but unsure which one. The question I'm really asking is will I see a big enough difference between the 3.5 & 6.5 to justify spending £230 (3.5nm) over £130 (6.5nm)?

At the moment I'm imaging with the Samyang 135ED f2 lens (and ASI533 Camera) & although the recommended use is between f10 & f3.5 for these Ha filters, I'm happy enough stopping down to f4 plus I'll be buying the WO ZS 81 APO for this winter @f6.9 so these filters will suit both my systems.

Over the next couple of months I'll be looking at also getting both the SII & OIII to complete my narrowband filter set. Obviously the ££ difference adds up when buying all 3 (£306 more if buying the 3.5nm) but I'm happy to spend the extra if there's going to be a noticeable difference in between the two.

I'm very much a novice at the moment & not really clued up on the difference between them results wise. All I can think of is that you need shorter exposure times with 3.5nm over 6.5nm to obtain the same results although I'm probably wrong with this. All advice will be very welcome.

TIA

Steve

[tooth_dr]

There are a few members following this thread, you might get into/images in there or a better response?

 

[nephilim]

1 minute ago, tooth_dr said:

There are a few members following this thread, you might get into/images in there or a better response?

 

@tooth_dr Great that looks ideal, thank you very much.

[Mike73]

I went for the 6.5 Ha but tbh that was purely down to budget (or lack of it) but I will be following this thread with interest. 🙂

[Pompey Monkey]

I'd advise based on your light pollution conditions:

If you work from a good dark site, then the 6.5 nm versions will do you fine.

However, if you are suburban/city based, then the 3.5 nm will reduce the effects of light pollution a lot more, improving contrast and making your processing much easier.

I'm in a Bortle 7 (6 on an exceptional night) location, and when I changed to 3 nm from 7 nm nb filters, the difference was very apparent.

Having said all that, nb imaging does require long exposures (10 minutes+) and you need to be sure that your EQ5 mount can consistently do that, or you will be chucking out lots of subs...

[nephilim]

51 minutes ago, Pompey Monkey said:

I'd advise based on your light pollution conditions:

If you work from a good dark site, then the 6.5 nm versions will do you fine.

However, if you are suburban/city based, then the 3.5 nm will reduce the effects of light pollution a lot more, improving contrast and making your processing much easier.

I'm in a Bortle 7 (6 on an exceptional night) location, and when I changed to 3 nm from 7 nm nb filters, the difference was very apparent.

Having said all that, nb imaging does require long exposures (10 minutes+) and you need to be sure that your EQ5 mount can consistently do that, or you will be chucking out lots of subs...

Thanks for the reply 😊, luckily I'm Bottle 3-4 so that's not a problem. I also need to change my signature gear as that's well out of date. I've now got the Explore Scientific EXOS II PMC8, ZWO asi 533 with Samyang 135ED f2 & guiding with SW Evoguide 50ED & ZWO asi 120mm-s guide camera, all controlled withe the ASIair Pro, so I think I'll be fine with 10min subs. 

Regards 

Steve

[Pompey Monkey]

7 minutes ago, nephilim said:

Thanks for the reply 😊, luckily I'm Bottle 3-4 so that's not a problem. I also need to change my signature gear as that's well out of date. I've now got the Explore Scientific EXOS II PMC8, ZWO asi 533 with Samyang 135ED f2 & guiding with SW Evoguide 50ED & ZWO asi 120mm-s guide camera, all controlled withe the ASIair Pro, so I think I'll be fine with 10min subs. 

Regards 

Steve

I also forgot to say that the 3 nm Ha also blocks a lot more of the light pollution from the Moon, meaning that you can image for a larger part of the month, whatever Bortle skies you have!

The OIII and SII filters are not so good at blocking the moonlight.

 

[nephilim]

Just now, Pompey Monkey said:

I also forgot to say that the 3 nm Ha also blocks a lot more of the light pollution from the Moon, meaning that you can image for a larger part of the month, whatever Bortle skies you have!

The OIII and SII filters are not so good at blocking the moonlight.

 

That's good to know thank you. From what you mention regarding LP & my lack of it I'm leaning towards the 6.5nm I think although I'll do a little more research as I won't really be needing them for a couple more months when we have longer dark skies 🙂

[Ags]

The 3.5 nm filter will reduce broadband sources vs the 6.5 nm. Broadband sources include the Moon, light pollution and stars, so the narrower the Ha filter the smaller the stars are in the final image. Both filter options let through the same Ha signal. So the same amount of light comes through from the nebula, but the 3.5 nm is less likely to be swamped by non-nebula light.

[nephilim]

3 minutes ago, Ags said:

The 3.5 nm filter will reduce broadband sources vs the 6.5 nm. Broadband sources include the Moon, light pollution and stars, so the narrower the Ha filter the smaller the stars are in the final image. Both filter options let through the same Ha signal. So the same amount of light comes through from the nebula, but the 3.5 nm is less likely to be swamped by non-nebula light.

Thanks for the reply. That's good to know 🙂

[tooth_dr]

I agree that the 3.5nm Ha is the one to go for, as you can image around the moon more, well worth the extra £ given that most nights that are clear seem to have the moon around.  Actually when the moon isnt around and it is clear, I tend to gather broadband data (and occasionally Oiii data) anyway and not NB in general!  I live in a decent/low Bortle 4 location too.

Edited August 3, 2021 by tooth_dr

[nephilim]

1 hour ago, tooth_dr said:

I agree that the 3.5nm Ha is the one to go for, as you can image around the moon more, well worth the extra £ given that most nights that are clear seem to have the moon around.  Actually when the moon isnt around and it is clear, I tend to gather broadband data (and occasionally Oiii data) anyway and not NB in general!  I live in a decent/low Bortle 4 location too.

@tooth_dr   I was thinking about just going for the 6.5nm but now I've thought a little more & taken into consideration that your right & most clear nights do tend to have a Moon up there shining bright, 3.5nm is the way to go.

The old saying of buy 'cheap, buy twice' would be very relevant as I just know that the first clear night with a full Moon, I'd be kicking myself for not spending the extra ££ on the 3.5nm & considering just how much I've spent over the last 12 months, a few hundred pounds is a drop in the ocean really.

Steve