Enhanced Guiding Performance for cheap! Use a #25 red planetary filter!

I've had quite poor seeing for the past 2 weeks and have been using an AM5N mount. I usually use 2s exposures but when the seeing is poor, even with multi-star guiding the guide star moves too much frame to frame and the guiding performance suffers.

I was looking at ways to fix this and learned that longer wavelengths are less susceptible to seeing and I did not have an IR pass filter but I did have a #25 red filter that is meant for planetary viewing. Looking at the spectrum for Agena it seems like a less aggressive IR pass that lets in some red light and gave it a try!

My guide performance immediately doubled from 1.2" to 0.6" under the same poor seeing conditions. With my pixel scale using a Rokinon 135 and a 533MC pro, this makes no difference to the final image but it was very cool to see such an improvement for free! It is also a cheaper alternative to a traditional IR-pass filter, lets in more light, and I bet many people have these lying around never being used.

That's an interesting result, free money so it's worth a try!

Hm. I'm curious as to whether your guiding performance truly improved. Because you can't compare the guiding numbers from two different setups.

Imagine if I smeared petroleum jelly onto the front glass of my guide scope and got better guiding numbers as a result. Is it because my guiding really improved? Or is it because I'm now measuring the guiding with a different -- perhaps less accurate -- tool?

To correctly measure whether this is giving you better guiding numbers, you need to use a truly independent measurement. For example, you could issue guiding corrections via your guide camera but measure guiding performance via your main (unchanged) camera.

Noah Tingey:
To correctly measure whether this is giving you better guiding numbers, you need to use a truly independent measurement. For example, you could issue guiding corrections via your guide camera but measure guiding performance via your main (unchanged) camera.

True; but the theory here makes sense, doesn't it: the IR signal is less impacted by atmospheric turbulence; hence, if you limit your guiding to that signal (or you make it the prominent component you use), your guiding wouldn't chase (or it would chase "less") the atmospheric turbulence (for which it would be too slow to correct anyway), but just the actual target.

Noah Tingey:
To correctly measure whether this is giving you better guiding numbers, you need to use a truly independent measurement. For example, you could issue guiding corrections via your guide camera but measure guiding performance via your main (unchanged) camera.

True; but the theory here makes sense, doesn't it: the IR signal is less impacted by atmospheric turbulence; hence, if you limit your guiding to that signal (or you make it the prominent component you use), your guiding wouldn't chase (or it would chase "less") the atmospheric turbulence (for which it would be too slow to correct anyway), but just the actual target.

The theory makes sense, but I'm unsure whether cutting out a significant portion of the visible spectrum (and therefore having lower SNR in your guide exposures) is outweighed by this benefit.

Idk! I'd be curious to see some direct testing on it. Would make for a good writeup.

Noah Tingey:
Noah Tingey:
To correctly measure whether this is giving you better guiding numbers, you need to use a truly independent measurement. For example, you could issue guiding corrections via your guide camera but measure guiding performance via your main (unchanged) camera.

True; but the theory here makes sense, doesn't it: the IR signal is less impacted by atmospheric turbulence; hence, if you limit your guiding to that signal (or you make it the prominent component you use), your guiding wouldn't chase (or it would chase "less") the atmospheric turbulence (for which it would be too slow to correct anyway), but just the actual target.

The theory makes sense, but I'm unsure whether cutting out a significant portion of the visible spectrum (and therefore having lower SNR in your guide exposures) is outweighed by this benefit.

Idk! I'd be curious to see some direct testing on it. Would make for a good writeup.

I don’t have enough focal length to verify with my main camera but the guide stars Im using are still nearly saturated so I don’t think the snr on the stars Im using is making that much of a difference. Would definitely be interested to see the results if someone tries it though!

The only thing I know for sure is that when watching the guide star it moves far less frame to frame than before.

Yes, that's right! It is well known that the Kolmogorov turbulence model is wavelength dependent and measurements show that the seeing is indeed better at a longer wavelengths. I talked about this effect in my "The Quest for Aperture" talk on the Astro-Imaging Channel. In fact, when I perform FWHM analysis on the data from my 24" scope, the red channels (R, Ha, and SII) always show smaller values. This is something that helps guiding accuracy a bit with an ONAG as well since guiding in done in the NIR. The reduced tracking errors in the NIR tends to damp out "over-corrections", which helps to produce slightly better results even with the blue filter inserted in the imaging path.

John

Interesting. I wonder if there is a way to place a NIR or Red small filter in front of a guide camera that is inserted in an OAG.

Astro with RoRo made a video about exactly this subject:

https://www.youtube.com/watch?v=9uNsccMuwvk

Ashraf AbuSara:
Interesting. I wonder if there is a way to place a NIR or Red small filter in front of a guide camera that is inserted in an OAG.

If you are using a color camera for guiding you would just need to convince PhD to only work with the red channel.
Other than that, I think you can get a cheap IR pass plastic filter for next to nothing off the internet to stick on front of the sensor.

Ashraf AbuSara:
Interesting. I wonder if there is a way to place a NIR or Red small filter in front of a guide camera that is inserted in an OAG.

If you are using a ZWO mini camera or similar in the OAG you could put a 1.25" filter screwed in to your guide cam.

My guide performance immediately doubled from 1.2" to 0.6" under the same poor seeing conditions.

Did you also see an improvement in your subs? For me, though at a much longer FL, the 4 to 1 rule seems pretty accurate i.e. if seeing is 4”, then anything below 1” total RMS is just bragging rights….

Cheers,
Scott

I'm not sure about that Scott. If seeing is on average 4" and guiding is 1" it can still have a visible effect. Lots of factors here but unless the error is perfectly distributed it will impact the roundness of the stars.

D. Jung:
Ashraf AbuSara:
Interesting. I wonder if there is a way to place a NIR or Red small filter in front of a guide camera that is inserted in an OAG.

If you are using a color camera for guiding you would just need to convince PhD to only work with the red channel.
Other than that, I think you can get a cheap IR pass plastic filter for next to nothing off the internet to stick on front of the sensor.

*How do you "convince" PHD2 to use only the red channel if using a color guiding camera?...which I do ;)

Ashraf AbuSara:
Interesting. I wonder if there is a way to place a NIR or Red small filter in front of a guide camera that is inserted in an OAG.

I did try some time ago adding a IR pass filter in front of the 174mm OAG guiding camera. It didn't go well, not enough signal was coming through - even at max gain.
Not sure if it was because the OAG itself already makes things dimmer for the guiding camera, or because the camera's IR sensitivity is too low, or because the camera's stock glass sensor cover is cutting IR, or because the IR pass filter was too aggressive - or a combination of all/some of the above. But I wasn't willing to waste time troubleshooting it.
I am thinking of giving it (or the #25 filter as mentioned by the OP) a second try when using the guiding scope - in "hyperstar configuration".

Tony Gondola:
I'm not sure about that Scott. If seeing is on average 4" and guiding is 1" it can still have a visible effect. Lots of factors here but unless the error is perfectly distributed it will impact the roundness of the stars.

Can poor seeing alone cause RA rms and DEC rms to vary from each other? Generally when seeing is bad, my stars are still round, just fat, but you're right, I'm assuming RA rms and DEC rms remain relatively equal, or at least no worse.

Cheers,
Scott

D. Jung:
Ashraf AbuSara:
Interesting. I wonder if there is a way to place a NIR or Red small filter in front of a guide camera that is inserted in an OAG.

If you are using a color camera for guiding you would just need to convince PhD to only work with the red channel.
Other than that, I think you can get a cheap IR pass plastic filter for next to nothing off the internet to stick on front of the sensor.

*How do you "convince" PHD2 to use only the red channel if using a color guiding camera?...which I do ;)

Why would you need to convince it of using the Red channel, if the only photons making it to your sensor are red or NIR only because of the filter? Shouldn't really matter if it is color or monochrome. The only issue is that your color sensor would be significantly less sensitive than a monochrome camera because of the Bayer matrix.

Paul Picazo:
Ashraf AbuSara:
Interesting. I wonder if there is a way to place a NIR or Red small filter in front of a guide camera that is inserted in an OAG.

If you are using a ZWO mini camera or similar in the OAG you could put a 1.25" filter screwed in to your guide cam.

That's a great idea. I have a IR pass but not a red filter.

Might give them both a try.

I could see this being far more helpful for harmonic drive mounts that use shorter exposures.

Ashraf AbuSara:
D. Jung:
Ashraf AbuSara:
Interesting. I wonder if there is a way to place a NIR or Red small filter in front of a guide camera that is inserted in an OAG.

If you are using a color camera for guiding you would just need to convince PhD to only work with the red channel.
Other than that, I think you can get a cheap IR pass plastic filter for next to nothing off the internet to stick on front of the sensor.

*How do you "convince" PHD2 to use only the red channel if using a color guiding camera?...which I do ;)

Why would you need to convince it of using the Red channel, if the only photons making it to your sensor are red or NIR only because of the filter? Shouldn't really matter if it is color or monochrome. The only issue is that your color sensor would be significantly less sensitive than a monochrome camera because of the Bayer matrix.

*The point being that the red channel is already filtered, so you wouldn't need to add an extra filter.

D. Jung:
Ashraf AbuSara:
D. Jung:
Ashraf AbuSara:
Interesting. I wonder if there is a way to place a NIR or Red small filter in front of a guide camera that is inserted in an OAG.

If you are using a color camera for guiding you would just need to convince PhD to only work with the red channel.
Other than that, I think you can get a cheap IR pass plastic filter for next to nothing off the internet to stick on front of the sensor.

*How do you "convince" PHD2 to use only the red channel if using a color guiding camera?...which I do ;)

Why would you need to convince it of using the Red channel, if the only photons making it to your sensor are red or NIR only because of the filter? Shouldn't really matter if it is color or monochrome. The only issue is that your color sensor would be significantly less sensitive than a monochrome camera because of the Bayer matrix.

*The point being that the red channel is already filtered, so you wouldn't need to add an extra filter.

Ah I see. Misunderstood that part.

So to test this I need a #25 red filter 1.25" and screw it into front of my 120mm mini?

TiffsAndAstro:
So to test this I need a #25 red filter 1.25" and screw it into front of my 120mm mini?

Thats exactly what I did!

Naveen Ravindar:
TiffsAndAstro:
So to test this I need a #25 red filter 1.25" and screw it into front of my 120mm mini?

Thats exactly what I did!

little harm me trying thanks

Ashraf AbuSara:
Interesting. I wonder if there is a way to place a NIR or Red small filter in front of a guide camera that is inserted in an OAG.

I did try some time ago adding a IR pass filter in front of the 174mm OAG guiding camera. It didn't go well, not enough signal was coming through - even at max gain.
Not sure if it was because the OAG itself already makes things dimmer for the guiding camera, or because the camera's IR sensitivity is too low, or because the camera's stock glass sensor cover is cutting IR, or because the IR pass filter was too aggressive - or a combination of all/some of the above. But I wasn't willing to waste time troubleshooting it.
I am thinking of giving it (or the #25 filter as mentioned by the OP) a second try when using the guiding scope - in "hyperstar configuration".

I thought to go that route as well. The 174 isn't sensitive enough at that wavelength.

Darryl Ackerman:
Ashraf AbuSara:
Interesting. I wonder if there is a way to place a NIR or Red small filter in front of a guide camera that is inserted in an OAG.

I did try some time ago adding a IR pass filter in front of the 174mm OAG guiding camera. It didn't go well, not enough signal was coming through - even at max gain.
Not sure if it was because the OAG itself already makes things dimmer for the guiding camera, or because the camera's IR sensitivity is too low, or because the camera's stock glass sensor cover is cutting IR, or because the IR pass filter was too aggressive - or a combination of all/some of the above. But I wasn't willing to waste time troubleshooting it.
I am thinking of giving it (or the #25 filter as mentioned by the OP) a second try when using the guiding scope - in "hyperstar configuration".

I thought to go that route as well. The 174 isn't sensitive enough at that wavelength.

On the other hand, this evening I am testing the 220mm with a guidescope (running with Hyperstar these nights), and the combo does work quite well with the IR pass filter. I did have to increase the gain (from my usual 200 to 320 - keeping exposures at 1.5s), but I am impressed about how well the filter is shielding the camera from the pretty bad atmospheric turbulence we have tonight in the Boston area. Guiding consistently well below .50, and you can see the graph is much "smoother" than equivalent filter-less nights I have gone through in the past.

Related discussions