General Discussion on Filters for Deep Sky Imaging

[ollypenrice]

PS Another feature of the Astrodon data is that you can process it very aggressively, viz:

This stretch raises eyebrows but both Tom O'Donoghue and I use this Curve. It will bring up the stars very strongly with 7nm data but you can get away with it in the Astrodon. A conventional log stretch will give you a softer look but a) I like the hard stretch and b) it works far better when used to enhance the red layer in an LRGB image and that's how I nearly always use it. The very high contrast is less washed out by the red channel.

Olly

[teoria_del_big_bang]

Wow, thata an incredible stretch but very effective and agree with you entirely about the resulting image.

Steve

[vlaiv]

7 minutes ago, ollypenrice said:

I'm not sure about this. I would have expected it to be the other way round, though I haven't done a comparison. What is widely agreed upon, however, is that the 3nm is significantly more moon-proof than the 7nm. I certainly find that, as do many others.

I think you are right about doing the actual measurement - that is the best way to go about it, but here is my reasoning.

Regardless of the level of LP, difference between 7nm and 3nm filters remains a fixed ratio - 7nm filter will let in roughly x2.33 more light pollution.

In LP skies it will let in x2.33 more LP and in dark skies and very low LP it will still let x233 more LP thru.

In high LP skies - both 7nm LP and 3nm LP will be high compared to shot noise and significant contribution to overall SNR - so you can't stretch signal very much and achieve good contrast.

As you go to darker skies - there will be signal strength for which 7nm LP is still significant so you can't stretch that signal and get good contrast, but for 3nm - and it's x2.33 less LP - it is starting to be less significant than shot noise - and you can further stretch it.

Now that I'm writing this - I realize that both LP strength and target strength play a part here - so it depends both on where you image and what you image (how faint it is and what is your aimed total exposure).

[kirkster501]

20 hours ago, teoria_del_big_bang said:

Maybe even get out to Ollies place which I promised myself I would do one day.

 

Seriously mate, you'll get a lot more benefit learning from Ollie than you will buying the filters.

[teoria_del_big_bang]

1 minute ago, kirkster501 said:

Seriously mate, you'll get a lot more benefit learning from Ollie than you will buying the filters.

Oh I realized that a while ago 🙂 

[Craney]

20 hours ago, teoria_del_big_bang said:

Maybe even get out to Ollies place which I promised myself I would do one day.

......just take a little time to think about the choice of hire car....

[teoria_del_big_bang]

9 minutes ago, Craney said:

......just take a little time to think about the choice of hire car....

Is this advice from experience - not totally sure what you mean 😕

[Craney]

58 minutes ago, teoria_del_big_bang said:

Is this advice from experience - not totally sure what you mean 😕

I do not know Ollie, but he sounds like he likes a 'proper' car and can get the most out of it.

One day, fingers crossed.... I'll book a month at his place and turn up in one of these.....

[teoria_del_big_bang]

1 minute ago, Craney said:

I do not know Ollie, but he sounds like he likes a 'proper' car and can get the most out of it.

One day, fingers crossed.... I'll book a month at his place and turn up in one of these.....

Oh I see.
I take it you will not be taking your own gear with you then otherwise you are going to have to get one hell of a "grab and go" to fit it all in that.

But turning up in  that would be something 🙂 

Steve

[Craney]

Whilst we are dreaming,  Epsilon 180 in the boot....just........ 

Salma Hayek might need to vacate the passenger seat to get the full set of AstroDon's, Chromas and Baaders in ......  

[teoria_del_big_bang]

1 minute ago, Craney said:

Salma Hayek might need to vacate the passenger seat to get the full set of AstroDon's, Chromas and Baaders in ......  

Blimey those AstroDon's must be good

[City9Town0]

Better than Salma... For some that could well be the million dollar question!😍

Maybe she'll walk!

[ollypenrice]

1 hour ago, Craney said:

......just take a little time to think about the choice of hire car....

😄 Discount for rear wheel drive...

🤣lly

[teoria_del_big_bang]

Just now, ollypenrice said:

😄 Discount for rear wheel drive...

🤣lly

I will remember that, just hope travel is back on soon otherwise it will be  all electric 🙂 

[ollypenrice]

26 minutes ago, Craney said:

I do not know Ollie, but he sounds like he likes a 'proper' car and can get the most out of it.

One day, fingers crossed.... I'll book a month at his place and turn up in one of these.....

BIG discount for a Shelby Cobra. Free if I can borrow it for the day. If it's a genune one, not a replica, I'll pay you to come!

🤣lly

[MartinB]

Reading this thread I am very much reminded of the world of hifi.  My 30+ year old Thorens record deck is giving up the ghost so I have been researching a new deck.  All the reviews and forum posts talk about this blowing away that.  The Rega P6 is much better than the old RP 6 but the P8 is another great leap forward and the P10 is in a different class again.  I can't get to listen to any of these decks at the mo but I know I will struggle to tell the difference.  However, I also know that if I get the P6 I will be forever wondering whether I should have gone for the P8, and if I got that  the P10 would be nagging my waking thoughts!. I am sure there are differences but do those differences matter?  This depends to some extent on personality, pragmatism and also, coming back to filters, imaging aspirations.  So it is very important to take a step back and ask yourself what you are trying to achieve.

Lets start with halos, these really realy upset some people, they do me to some extent.  People will spend a fortune to free themselves of halos whilst at the same time buying very expensive Tak souped up reflectors which produce star spikes and not bat an eyelid!  I think Baader did go through a bad spell some years ago and I suspect Olly was a victim of this.  I think things are better now and I would never,ever describe my Baader OIIIs as "junk" .  I have 2 and they are both fine ilters, but not perfect.

I initially used Astronomik filters and moved to 7/8nm Baaders and have been pretty pleased with them but when people started raving about 3nm Astrodons I was interested.  I wanted to see for myself and @ollypenrice very kindly sent me a couple of Ha subs taken with the same scope (a tak FSQ 85 if I remember correctly) one using a Baader 7nm and the other an Astrodon 3nm.  It was obvious which was which by looking at the stars which were, of course, dimmer with the 3nm.  It would be very strange if they weren't.  I can't remember what the target was now but on stretching I just couldn't make out the difference in the nebulosity, maybe my eyes are bad.  In the same way I probably wouldn't be able to make out instability in the treble or lack of definition in the base comparing a Rega P8 with a P10.  Subsequently I have seen a post giving a similar comparison and I could see an improvement in contrast!

In light polluted Chesterfield using a QSI 532 with a Tak FSQ or MN190 I am not above the read noise with 30 min subs using the Ha 7nm filter, this would be more pronounced still with a 3nm filter.  This is an increasing consideration with darker skies for anyone hoping to get the most out of a 3nm Ha filter.  However, low read noise CMOS cameras are well place to take advantage of very narrow band pass.

I now have a ZWO 294 in my observatory and have taken the plunge and bought a full set of Chromas.  The NB are 5nm because I am often imaging at F3.7 (3nm can be too narrow with fast systems).  They aren't parfocal!!  They don't produce halos.  Subjectively they seem to perform very well, just like my Baaders.  They cost me a lot more money than the Baaders.  The daft thing is, I don't regret buying them, it has something to do with wanting any shortfalls in may imaging being down to me rather than some weak link in the chain.

So, if you still need to scratch the itch try getting hold of a pair of subs taken at a similar time, in the same conditions with the same optics, one using 7nm and one 3nm.  Do your own process (A simple DPP type stretch in curves is good) and compare (ignoring the smaller stars!).

 

[ollypenrice]

2 minutes ago, MartinB said:

(ignoring the smaller stars!).

 

Starnet has rather changed the game so far as star control is concerned, it must be said.

I'm not a spend for spend's sake person, not at all. I'm perfectly happy with Baader LRGB, for instance. But, having used both the Baader 7 and the AD3 I con only say that the AD3 is spectacular.

Olly

[tooth_dr]

52 minutes ago, Craney said:

Whilst we are dreaming,  Epsilon 180 in the boot

I’m happy to go along too 😂 I’ll even share the boot with the scope