39 replies to this topic

[sixela]

Calibration just confirmed the graphs I posted earlier.

One thing noted with the 2.5nm Antlia: the effective refraction index for the passband shift indeed does not seem overly high (the astropolis.pl estimate of 1.81 seems to match fairly well).

Even though the bandpass curve for 0° angle is well nigh identical to that Chroma published for their 3nm, there is more shift with respect to angle of incidence.

Coupled with 0.25nm less slack that means the Antlia will perform less well under f/4 than a Chroma probably does (measurements in other threads confirm this; people also have used Chromas to f/3 and Gavster reports less bandshift at 1x with his Chrome to the field edge than I see with the Antlia, pointing to the same direction).

Edited by sixela, 05 February 2024 - 09:54 AM.

[sixela]

For those who are not averse to Google translated text from Dutch:

https://www.astrofor...oscoop.1478821/

[sixela]

There is now a discussion of the Chroma too in there. TL;DR: you get what you pay for. Whether you need it depends on how lucky you are with an Antlia 2.5 and how fast your scope is (and of course, if you have the money to burn on the Chroma).

Edited by sixela, 09 February 2024 - 05:55 PM.

[Jethro7]

There is now a discussion of the Chroma too in there. TL;DR: you get what you pay for. Whether you need it depends on how lucky you are with an Antlia 2.5 and how fast your scope is (and of course, if you have the money to burn on the Chroma).

Hello sixela,

All said and done, my Antila 2.5nm Ha did not come with any test Data but by simple observation, everything appears to work quite well. I have used this filter on scopes from F/5 to F/8 and the Antila 2.5nm Ha filter works pretty darn well and is a nice upgrade from my 6nm Ha filter. 

 

HAPPY SKIES AND KEEP LOOKING UP Jethro

[sixela]

Yes -- as I said, with the variation I saw in samples of that filter, they all work perfectly (and with an even larger contrast than a Chroma!) on f/4.5 scopes and above; it's only on faster scopes that you need a bit more luck, and it's from f/4 that the lower effective refractive index (and larger blueshift that yields) leads to transmission losses on one of the better ones that you don't seem to have on a Chroma.

[sixela]

Here's the Chroma and a final validation of the spectrometre calibration.

 

Light green: graph for the H-alpha filter class on the Chroma web site

Dark green: my measurement of my filter. The resolution of my spectrograph isn't quite as good, so I expect the FWHM to be somewhat wider (here 0.2nm) and the transmission peak to be somewhat lower (but really that's buried in the noise)

Red: H-alpha emission

Claret: solar spectrum taken with the same calibration, with H-alpha absorption line clearly visible.

 

In light grey the measured bandpass for a 7.66° angle of incidence, which is the half cone angle for the extreme edge of an f/3.72 scope (i.e. transmission over any part of the surface is better than this). Estimated transmission of H-alpha over the aperture is close to 96%.

 

The cherrypicked Antlia 2.5nm has a similar bandpass setting, but it's even more narrow and the refractive index of the filter layers is lower, with more bandpass sihft, so its transmission over my f/3.72's aperture is estimated at 80%. Which gives approximately the same contrast with respect to the background as the Chroma, given the passband is also 20% less wide, but at a slightly lower surface brightness for both background and object.

Edited by sixela, 10 February 2024 - 03:45 PM.

[JohnB1989]

Has anyone made curves for SPCC yet for the 2.5nm filters?

[sixela]

Not me.

[Thierry Legault]

Just received the replacement Optolong L-Ultimate filter, measured in Halpha at 89% (much better than the other one at 68% !)

Edited by Thierry Legault, 15 February 2024 - 03:38 PM.

[sixela]

There is an enormous variation in passband setting. Here are two measured recently:

https://www.astrofor...-46-png.1504557

The green one works OK on my scopes (but not on slower scopes than f/5) but it isn’t mine. The orange I sent back, waiting for a third…

Edited by sixela, 15 February 2024 - 04:52 PM.

[Thierry Legault]

There is an enormous variation in passband setting. Here are two measured recently:

https://www.astrofor...-46-png.1504557

The green one works OK on my scopes (but not on slower scopes than f/5) but it isn’t mine. The orange I sent back, waiting for a third…

no access without account

[sixela]

OK, now with three filters:



The orange is the one that would beat your 89%, alas it's also the one I can't use on anything except my Lunt because it's too blueshifted ;-).

I kept the blue one, which gives me 84% -- beggars can't really be choosers with these.

One wonders why a company that has this much variation in bandpass setting manufactures "3nm" filters (4nm actually), at least if they also claim the filters work to f/4 (although one of them actually works optimally for f/4.5 and faster!!).

Edited by sixela, 16 February 2024 - 07:16 PM.

[ytserrof]

For all your scopes that filter is a no-brainer.

 

 

I am quite certain that I don't get the whole picture, but would assume that this statement is only true when he uses his scope at f5, right? I mean afocally he can go way below than f5. Do I overlook something here?

Edited by ytserrof, 22 February 2024 - 02:02 AM.

[sixela]

Yes. In an afocal setup you use the filter in front of the eyepiece, on the light bundle from the telescope, before the reduction by the combination of the eyepiece and NVD objective. What matters is the f/ratio of the telescope.

Incidentally, a Paracorr is more or less telecentric —it does not make the central ray of a bundle more diverging— but also raises the f/ratio behind the Paracorr by 1.15x, so for non-preshifted filters it can be beneficial to put the filter between the Paracorr and eyepiece rather than in front of the Paracorr.

The effective f/ratio at the photocathode only matters for filters between the photocathode and the NVD objective (and you definitely do need wider passband and well preshifted filters there).

Edited by sixela, 22 February 2024 - 03:19 AM.

[sixela]

Some more data:

After having to get three Optolong Ultimate filters to get a good one for my f/3.72, I now have an Altair Astro. I tested both filters myself now, and here are the results:

The Altair Astro is slightly less preshifted, but the effective refractive index is a lot higher (2 vs. 1.75) so it's better for really fast scopes.

Graphs with measurements for 0° angle of incidence and 7.66° angle of incidence (not coincidentally the maximum AoI on my scope).

The Optolong fares quite well on my scope (with 83% transmission of H-alpha, 87% transmission of OIII) but this one still beats it (92% transmission in H-alpha and 88% in OIII).

More importantly, it came with a filter test report that matches our own measurements completely (while when buying the Optolong you need your own spectrometer and you need to play Chinese roulette).

Edited by sixela, 25 May 2024 - 01:32 PM.