Astrodon vs Baader Ha Filters

[davedownsouth]

Just a quick one to post a comparison of single 15 minute exposures of the Soul nebula taken last night (~90% Moon) with my new 3nm Astrodon Ha filter and 7nm Baader Ha filter.  Both images were stretched in PI to ~the same extent and as they were single shots, no calibrations (bias / flats / darks) were applied. 

My comments are that I prefer the Astrodon one; less background, more nebolosity (if that is a real word) and less 'harsh' stars, but I can imagine other people might not like this. 

What do other people think of these, and which one do you pefer?

Baader Shot:-

http://www.astrobin.com/222337/0/

Astrodon Shot:-

http://www.astrobin.com/222335/

[ChrisLX200]

That's about what I would expect to see, stars are more attenuated using the 3nM filter, background should be a little darker and higher contrast in the nebula (you lose the NII component of the nebula - if there is one). It also allows you to post-process the nebula while worrying less about the effects on star profiles, but you would use a star mask anyway. Stars can always be replaced with RGB ones for a better result. I have both 7nM and 3nM Ha filters in my Atik filterwheel but haven't used the 7nM for over a year.

ChrisH

[swag72]

......It also allows you to post-process the nebula while worrying less about the effects on star profiles, but you would use a star mask anyway.

Really? I've never used a star mask in my life! It must be something I am missing!!

I prefer the Astrodon image - More contrast and fain nebulosity showing through to my eye... I would say though that the Baader appears to be better focused... the Astrodon image looks a tiny bit off focus in comparison?

[sloz1664]

I agree the Astrodon does tease out the faint nebulosity.

Don't go down the route of using star masks Sara. You're images are good enough as it is, if they get any better I'm going to throw in the towel

Steve

[davedownsouth]

Really? I've never used a star mask in my life! It must be something I am missing!!

I prefer the Astrodon image - More contrast and fain nebulosity showing through to my eye... I would say though that the Baader appears to be better focused... the Astrodon image looks a tiny bit off focus in comparison?

This might be because the Astrodon shot was the fourth image taken after focusing the scope and the temperature was dropping quite a bit, probably started the four shots at 6°C and the shot shown was taken at ~2°C.  Having said that, it was much harder to focus the Astrodon filter compared to the Baader one.  I did this using a Bahtinov mask pointing at Caph, and for the Baader I could see decent spikes with a 15 second exposure, with the Astrodon I needed to leave it for 40 seconds, and even then it probably should have been longer.  Probably would have been quicker if I had swung the scope round to Capella, but it was a bit too low in the sky when I was doing this

[swag72]

This might be because the Astrodon shot was the fourth image taken after focusing the scope and the temperature was dropping quite a bit, probably started the four shots at 6°C and the shot shown was taken at ~2°C.  Having said that, it was much harder to focus the Astrodon filter compared to the Baader one.  I did this using a Bahtinov mask pointing at Caph, and for the Baader I could see decent spikes with a 15 second exposure, with the Astrodon I needed to leave it for 40 seconds, and even then it probably should have been longer.  Probably would have been quicker if I had swung the scope round to Capella, but it was a bit too low in the sky when I was doing this

That explains that then! Next on your list is an electronic focuser...... makes life a breeze!!  

[MattJenko]

Are these images both stretched the same in absolute terms, or relative terms to the same intensity? The Astrodon one looks like it has more signal, whereas my simple mind would assume less photons, not more would have made it through? I can just about understand contrast being better, but absolute signal for same exposure has me a little confused.

[Freddie]

The thing that isn't showing in these images which was actually the biggest difference between the filters for me was star size. For me, the Astrodon filters give much smaller stars than the Baader. This observation aligns with Matt's above about signal.

[Krzychoo226]

You are comparing 7nm filter with 3nm.. Results are obvious. Compare Baader 3mn with Astrodon 3nm.

[Freddie]

The OP made it clear they were 3nm Vs 7nm. They are just asking what people think about the degree of difference and which they prefer.

[davedownsouth]

You are comparing 7nm filter with 3nm.. Results are obvious. Compare Baader 3mn with Astrodon 3nm.

Sorry, but I do not think it is obvious.  To me, obvious would be the whole image intensity for the Astrodon filter being 3/7th that of the image for the Baader filter.  This is not the case.  I was surprised how much of the nebulosity remained for the Astrodon filter compared to the Baader filter. 

NB.  Show me where the 3nm Baader filter is and I might consider doing the test

Are these images both stretched the same in absolute terms, or relative terms to the same intensity? The Astrodon one looks like it has more signal, whereas my simple mind would assume less photons, not more would have made it through? I can just about understand contrast being better, but absolute signal for same exposure has me a little confused.

Not sure if I did stretch the same?  What I did was very simple, I took both linear images and pulled in the left and right hand sides in until they just touched the sides of the curves (hope this makes sense).  Hopefully this is the best way of giving them a comparable stretch  . 

Yes, I was expecting the Astrodon image to have a lot less to it aswell and do not really understand why it looks as 'strong' as it does compared to the Baader one

[Davey-T]

Presumably you'd need a longer exposure with the Astrodon to capture the same number of photons, did you measure the images before stretching ?

Personally I settled for 5nm Astrodons 

Dave

[ChrisLX200]

Presumably you'd need a longer exposure with the Astrodon to capture the same number of photons, did you measure the images before stretching ?

Personally I settled for 5nm Astrodons 

Dave

Not really, the 3nM will let through all the Ha light emission that is there and it won't be any dimmer than 7nM or 12nM for that matter, it's just that the wider band-pass filters will let also through the NII and other non-specific light too (which may or may not help image definition - or just increase background level).

ChrisH

[MartinB]

According to my eyes Astrodon appears to have had more stretch, the highlights are brighter. One thing a 3nm filter won't do is increase the absolute brightness of the nebula. I cannot see that the stars are more attenuated with the Astrodon, I'd call it the other way and that is probably a focus issue. Do you have CCD inspector, if not it would be interesting to have a look at the original fits to compare FWHM.

I'm half way interested in moving to 3nm filters but I'm still not convinced. As presented, the Astrodon image is delivering better nebulosity but I'm not sure whether this is related the variation in the stretch.

Thanks for posting Dave, very interesting

[DaveS]

I don't see why the Astrodons should let less nebulosity through. If we're only looking at Ha then assuming the bandwidth is less than the filter, then essentially all the light will get through, allowing for small differences in peak transmission.

The thing with the 3 nm Astrodons is that it allows you to separate Ha from [NII]. In post you can either assign them to different colours or combine them as {Ha,[NII]}

[MartinB]

I don't see why the Astrodons should let less nebulosity through. If we're only looking at Ha then assuming the bandwidth is less than the filter, then essentially all the light will get through, allowing for small differences in peak transmission.

 

The thing with the 3 nm Astrodons is that it allows you to separate Ha from [NII]. In post you can either assign them to different colours or combine them as {Ha,[NII]}

I agree, unless they were used with a very fast set up e.g. hyperstar which you weren't using! But they won't let more through

[ollypenrice]

I find a vastly greater difference in star size between the same filters. This is the parallel Tak setup with both operating side by side at the same time. I'll try to dig out some examples but there really is a huge difference for me.

Olly

[MartinB]

I remember you sent me some subs before Olly and I did't see a difference when I ran CCDinspector on them. I would love to have a look at some more unprocessed subs. Part of me wants to see a big difference and part of me doesn't! At the moment I'm a bit of a doubting Thomas. Once people have spent a lot of money on some kit it is in the interests of their personal happiness to see a benefit!! So I'm after something objective

[davedownsouth]

According to my eyes Astrodon appears to have had more stretch, the highlights are brighter. One thing a 3nm filter won't do is increase the absolute brightness of the nebula. I cannot see that the stars are more attenuated with the Astrodon, I'd call it the other way and that is probably a focus issue. Do you have CCD inspector, if not it would be interesting to have a look at the original fits to compare FWHM.

I'm half way interested in moving to 3nm filters but I'm still not convinced. As presented, the Astrodon image is delivering better nebulosity but I'm not sure whether this is related the variation in the stretch.

Thanks for posting Dave, very interesting

I do not have CCD inspector (never heard of it).  Not sure if this is going to work, but I am going to try a post a link to the two untouched Astrodon & Baader FIT files.  I have never tried doing this via Astrobin, but if it works I would be interested to see if someone who knows what they are doing (i.e., not me ), can perform an equal stretch / CCD inspection?.  Hope the files are available to you?

http://www.astrobin.com/rawdata/publicdatapools/64/

NB.  Please post whatever info you find, as I am going to be thinking about buying the 3nm Astrodon SII & OIII filters, but obviously do not want to if they are not going to gain me anything over the Baader's.

[MartinB]

Downloaded the images Dave.  What is your pixel size and focal length?

[davedownsouth]

Downloaded the images Dave.  What is your pixel size and focal length?

Hi Martin,

Pixel size is 4.54 x 4.54 (Atik one 6 binned 1x1)

Focal Length is 348mm (WO Star 71)

Cheers for trying this and interested to see your findings

[MartinB]

Here's the data from CCD inspector.  

                                FWHM                      aspect ratio                     background                   contrast                        

Astrodon                   5.82                                15                                 690                              14                                     

Baader                      4.34                                23                                 1322                            9.85                                  

Obviously there are lots of factors influencing these figures other than the filters.  Stars are slightly tighter with the the Baader image,  The tracking was more accurate with the astrodon image.  Background is darker with the astrodon and contrast greater.   

Hope Olly doesn't mind but I've used some unprocessed TIFFs he sent me some time ago and done a similar excercise.  The comparison is between a 3x30min astrodon TIFF and a 10x30min Baader (?) TIFF

                              FWHM                           aspect                             background                    contrast

Baader                    5.84                                11                                         209                             31.39

Astrodon                 6.91                                24                                         199                             34.62

Again slightly tighter stars with the baader. Very marginal difference in background and contrast in favour of the Astrodon

Make of this what you will!!!

By the way Dave, both your images showed a lovely flat field.

CCD inspector will also do a live analysis of your subs as they come through and show how key parameters such as FWHM are changing over time.  This can quickly alert you to focus drift.  

[ChrisLX200]

SGPro will do something very similar:

ChrisH

[davedownsouth]

Here's the data from CCD inspector.  

                                FWHM                      aspect ratio                     background                   contrast                        

Astrodon                   5.82                                15                                 690                              14                                     

Baader                      4.34                                23                                 1322                            9.85                                  

Obviously there are lots of factors influencing these figures other than the filters.  Stars are slightly tighter with the the Baader image,  The tracking was more accurate with the astrodon image.  Background is darker with the astrodon and contrast greater.   

Hope Olly doesn't mind but I've used some unprocessed TIFFs he sent me some time ago and done a similar excercise.  The comparison is between a 3x30min astrodon TIFF and a 10x30min Baader (?) TIFF

                              FWHM                           aspect                             background                    contrast

Baader                    5.84                                11                                         209                             31.39

Astrodon                 6.91                                24                                         199                             34.62

Again slightly tighter stars with the baader. Very marginal difference in background and contrast in favour of the Astrodon

Make of this what you will!!!

By the way Dave, both your images showed a lovely flat field.

CCD inspector will also do a live analysis of your subs as they come through and show how key parameters such as FWHM are changing over time.  This can quickly alert you to focus drift.  

Thanks for doing this Martin. Not sure I fully understand what all the terms mean, shall have to do a bit of reading p on CCD inspector . 

[davedownsouth]

SGPro will do something very similar:

ChrisH

I actually purchased a copy of SGPro a few weeks ago, which I have not really worked out how to use yet .  I bought it so that it would do autofocusing for me as I have a Lakeside focuser attached to me star 71.  Think it is going to take me a while to work out how to get this program working properly; individually I have got it to talk to the mount - focuser - filter wheel - camera, but not chance I can get it to do all four at once yet .  Topic for another thread some time soon I think .