Chroma 3nm NB filters have landed...

[Steve 1962]

The postman arrived with a nice little parcel from Bern at MA this morning.

These are an early Christmas present to myself (this covers Christmas's until at least 2025 apparently - now that SWMBO has seen what the fuss is all about!   "They were HOW MUCH?" ).

Anyway they're Ha, Oiii and Sii 3nm filters specially optimised by Chroma for my f3.6 reduced Tak FSQ106. Bern is going to send the transmission curves - which I'll share when they arrive.

On first inspection, I've very impressed with the quality of the filters and their mounts..but the acid test is how they perform under the night sky - so watch this space.

[PhotoGav]

Makes up for the rubbish weather and will give plenty to play with once the dark skies return. Surely it can't be long now... I'll be watching for your report with interest.

[Martin-Devon]

Hi Steve - I'll also be following your progress here with interest - at some stage I want to upgrade to 2" filters and the Astrodon's are just ridiculously expensive. If your 1.25" Chroma 3nM filters work well, then I'll certainly consider trying out the 2" versions.

If you want to do a head-to-head of Chroma 3nM versus Astrodon 3nM, all 1.25" filters, you are very welcome to borrow my set of Astrodon's for a night or two - would give us both valuable comparisons.

Martin

[MarsG76]

Nice... those will give you some nice detail in NB nebulae...

[swag72]

That would be a very interesting and useful comparison indeed Martin 

[Steve 1962]

 If you want to do a head-to-head of Chroma 3nM versus Astrodon 3nM, all 1.25" filters, you are very welcome to borrow my set of Astrodon's for a night or two - would give us both valuable comparisons.

Martin - my first plan is to do a side by side comparison with my existing Baader filters. An Astrodon / Chroma test sounds like a good excuse for some social astro imaging with a nice whisk(e)y!

By the way - here are the transmission plots from Chroma for these  filters:

[ChrisLX200]

My Chroma Ha 3nM got first light tonight, sadly not under the best of conditions. Plus my camera was misting up due to the fact I failed to switch the CCD chamber window heater on :-(  I did get some useful data before the clouds rolled in though.  Going to have to wait until the moon has gone before I bother trying the OIII and SII filters.

ChrisH

[Steve 1962]

Not sure if this is a fair or valid thing to do given that there's a factor of more than three difference in price between the two  - but I'm just running a "first light" comparison between my new Chroma 3nm Ha filter and my "old" Baader 7nm filter and I thought people might be interested in seeing the difference given that Chroma filters are fairly new on the UK market.

Here's a screenshot of  ten minute subs taken one after the other through my Tak 106 @ f3.6 with the two filters this evening. All I've done is a couple of identical PI histogram stretches on both subs.

Just looking at the images, the main things that strike me are:

The lack of a halo around Alnatak with the Chroma filters.

The background looks a lot darker with the Chroma. ( about 60% of the Baader background according to my rough sampling)

The Chroma stars are about 70% of the  size of the Baader stars when zoomed in.

For those who understand these things - the image statistics from PI are:

CHROMA 3nm

            K

count (%)   99.76525

count (px)  6030860

mean        2.3292e-001

median      2.0882e-001

avgDev      1.0991e-001

MAD         1.0126e-001

minimum     1.5106e-003

maximum     9.9992e-001

BAADER 7nm

            K

count (%)   99.95471

count (px)  6042313

mean        3.7175e-001

median      3.5247e-001

avgDev      9.3091e-002

MAD         8.4836e-002

minimum     2.3239e-002

maximum     9.9998e-001

CCD Inspector shows the FWHM of the Chroma image to be 3.02" v 2.79" for the Baader (? - I'm trying to think that one out) and the backgrounds to be 516 v 666.

To me the Chroma image looks crisper and more detailed with lower noise.

One thing that I found amazing is that the focus is more than 200 steps of focuser travel (nearly 5% of the total travel) difference. 

Am I pleased with my purchase? Well - Mrs L is not convinced at all that the improvement is worth "How much!!??"          but I'm very pleased with just the absence of the halo alone - the other pluses are all extras in my book. 

I'm going to see if I can borrow an Astodon 3nm to do a similar side by side.

If anyone is interested - the original subs are here : 

https://www.dropbox.com/s/i8wpwkp90y7hjmy/3nm%20Ha%20%20frame3%202015-11-08%20234132%20Horse%20Head_600sec_-15.1C%201x1.fit?dl=0

https://www.dropbox.com/s/uym3nz7vv1vh0ur/Ha%20%20frame1%202015-11-08%20235959%20Horse%20Head_600sec_NA%201x1.fit?dl=0

Steve

[ChrisLX200]

"CCD Inspector shows the FWHM of the Chroma image to be 3.02" v 2.79" for the Baader (? - I'm trying to think that one out) and the backgrounds to be 516 v 666."

I've seen this mentioned before and I believe the answer lies in the fact that sampling is quite different between the two images - they are not the same stars but merely a random selection of what CCD Inspector can see in the images. Only if you use a sample of stars which appear on _both_ images will you get a valid comparison. The average FWHM for the 7nM image will be including lots of small stars which are simply missing from the 3nM image.

ChrisH

[Steve 1962]

Thanks Chris - that makes sense.

[riklaunim]

It would also be interesting to compare those filters on a nebula with significant [N II] emission, like M27 - chow much less signal 3nm will catch?

[PhotoGav]

At first glance that looks impressive. Does more than three times the cost give more than three times better an image? Well, maybe yes! A very interesting look at the 'unknown' Chroma filter, thank you for posting. It would definitely be interesting to add the Astrodon 3nm to the comparison.

What I would also like to see is how the OIII & SII 3nm filters cope with varying degrees of moonlight pollution... Given that the only slightly improved skies we seem to have are when Mr Blobby is out to play.......

[Steve 1962]

Here's 11 x 10 min 3nm Ha subs 2x drizzle integrated with no Flats, Darks or Bias frames  - it's just been stretched,  cropped, some TGVDenoise and a touch of wavelets.

[ollypenrice]

Very good post and images. Thanks.

One thing we discovered last week was the moon-proofness difference between 3 nan and 7 nan. On parallel scopes on the same gibbous moon night the 3 gave data that worked well. The 7 went straight in the dustbin as being entirely useless. That was on the Cave nebula.

Olly

[opticalpath]

One thing that I found amazing is that the focus is more than 200 steps of focuser travel (nearly 5% of the total travel) difference. 

I notice that you use an OAG.  The focus difference (caused by the different thickness of the filters) can be a nuisance if you are mixing different brands in the filter wheel. Whilst it's no big deal to have to refocus the imaging camera when you change filters, it's the fact that the OAG guiding camera also goes out of focus that can be the problem; it's generally not so easy to change the focus of the OAG camera back and forth as required!  You can still guide on a slightly out-of-focus guide star, but you may have to find a half-way position for the OAG focus so that the guider is about equally out of focus for the different filters. 

I had this problem when mixing an Astrodon NB filter with Astronomik LRGBs.  The Astrodons are 3mm thick (same as Chroma) and the Astronomiks are only 1mm.  For me, the focus difference was so great that I could not find a compromise focal position for the OAG that would provide a sharp enough star to guide on with both filter types.  In the end, it was such a pain that I had to get matching-thickness Astrodon LRGBs.  Your Baader filters are 2mm thick, I think, so hopefully the focus difference is small enough that a compromise position for the guide camera focus will be OK.  

Adrian

[PhotoGav]

I notice that you use an OAG.  The focus difference (caused by the different thickness of the filters) can be a nuisance if you are mixing different brands in the filter wheel. Whilst it's no big deal to have to refocus the imaging camera when you change filters, it's the fact that the OAG guiding camera also goes out of focus that can be the problem; it's generally not so easy to change the focus of the OAG camera back and forth as required!  You can still guide on a slightly out-of-focus guide star, but you may have to find a half-way position for the OAG focus so that the guider is about equally out of focus for the different filters. 

I had this problem when mixing an Astrodon NB filter with Astronomik LRGBs.  The Astrodons are 3mm thick (same as Chroma) and the Astronomiks are only 1mm.  For me, the focus difference was so great that I could not find a compromise focal position for the OAG that would provide a sharp enough star to guide on with both filter types.  In the end, it was such a pain that I had to get matching-thickness Astrodon LRGBs.  Your Baader filters are 2mm thick, I think, so hopefully the focus difference is small enough that a compromise position for the guide camera focus will be OK.  

Adrian

Interesting issue and one that is avoided with an integrated oag where the guide star is picked off pre filters, so no problems arise!

Well, no filter thickness problems anyway...........

[Horwig]

Interesting issue and one that is avoided with an integrated oag where the guide star is picked off pre filters, so no problems arise!

Well, no filter thickness problems anyway...........

Ummm, not too sure about that, with the OAG after the filters, both cameras are adjusted to be parfocal, and then moved in tandem to achieve focus, but with the OAG ahead of the filters, don't differing filter thicknesses essentially change the relative camera distances?

Huw

[Steve 1962]

Thanks for the comments everyone.

  I notice that you use an OAG.  The focus difference (caused by the different thickness of the filters) can be a nuisance if you are mixing different brands in the filter wheel.

I was worried about the OAG being out of focus - because, as Adrian has said, because the OAG pick-off is in front of the filters in my set up, the focus off set between the two filters causes the guide camera to go out of focus.

However - PHD2 coped beautifully with the out of focus stars - and there was absolutely no difference in guiding performance (although the 10 Micron hardly needs guiding - it's just something I like to do).

Another BIG however is that I'm pleased that the OAG is in front of the filters - these things are opaque! I wonder what it'd do to my ability to find a guide star if the filters were in front of the OAG?.

 moon-proofness difference between 3 nan and 7 nan. On parallel scopes on the same gibbous moon night the 3 gave data that worked well. 

That's great news Olly - " moon-proofness" is one the main reasons I invested in these filters - so I'm relieved!

Cheers

Steve

[opticalpath]

Ummm, not too sure about that, with the OAG after the filters, both cameras are adjusted to be parfocal, and then moved in tandem to achieve focus, but with the OAG ahead of the filters, don't differing filter thicknesses essentially change the relative camera distances?

Huw

Exactly!  I'm using a QSI wsg camera where the integrated OAG pick-off comes ahead of the filters and, as you said, in that case changing the imaging camera focus position puts the OAG camera out of focus - precisely the problem I experienced.  Hope it's not such an issue for the OP.

Adrian

[PhotoGav]

Ummm, not too sure about that, with the OAG after the filters, both cameras are adjusted to be parfocal, and then moved in tandem to achieve focus, but with the OAG ahead of the filters, don't differing filter thicknesses essentially change the relative camera distances?

Huw

I stand corrected! Though thankfully as Steve says, PHD can cope with the slight focus alteration of the guide star.

[Fukinagashi]

Anyone tried to compare Astrodon 3nm versus Chroma 3nm?