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.
That makes sense, look at the QE on the 220 vs the 174   |
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Darryl Ackerman:
That makes sense, look at the QE on the 220 vs the 174 Yep; too bad the 220mm's sensor is way too small for a C8 oag...
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Interesting! Well, this method would certainly produce better red alignment. The question is whether other colors would also improve…or be worse because they aren’t contributing to guidance feedback.
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Dave Rust:
Interesting! Well, this method would certainly produce better red alignment. The question is whether other colors would also improve…or be worse because they aren’t contributing to guidance feedback. The color is irrelevant here; once you have a fix on the star, you have your guide - no matter which color your main camera is shooting.
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Darryl Ackerman:
That makes sense, look at the QE on the 220 vs the 174
Yep; too bad the 220mm's sensor is way too small for a C8 oag...
Try the 585
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Mark McComiskey:
Darryl Ackerman:
That makes sense, look at the QE on the 220 vs the 174
Yep; too bad the 220mm's sensor is way too small for a C8 oag...
Try the 585 That's just color, isn't it? I'd rather stay monochrome for guiding, like the 678. That said, the form factor of the mini cameras is so convenient... The perfect solution would be a 678mm mini... In the meanwhile - once I switch back to "full" focal length configuration on the c8 - I'll see if the #25 filter on the 174mm is workable and helps.
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I just checked and I have an Orion, made in Japan, #23A. The transmission looks similar to the #25 so I may get the 220 and give that a try.
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Mark McComiskey:
Darryl Ackerman:
That makes sense, look at the QE on the 220 vs the 174
Yep; too bad the 220mm's sensor is way too small for a C8 oag...
Try the 585
That's just color, isn't it? I'd rather stay monochrome for guiding, like the 678. That said, the form factor of the mini cameras is so convenient... The perfect solution would be a 678mm mini... In the meanwhile - once I switch back to "full" focal length configuration on the c8 - I'll see if the #25 filter on the 174mm is workable and helps.
The 678 is available in a monochrome mini format from QHY, isn’t it? It would be a great option, but the chip is smaller than the 174 and the 585, so if at a long focal length, that is a negative. Is using a color camera problematic?
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Mark McComiskey:
Mark McComiskey:
Darryl Ackerman:
That makes sense, look at the QE on the 220 vs the 174
Yep; too bad the 220mm's sensor is way too small for a C8 oag...
Try the 585
That's just color, isn't it? I'd rather stay monochrome for guiding, like the 678. That said, the form factor of the mini cameras is so convenient... The perfect solution would be a 678mm mini... In the meanwhile - once I switch back to "full" focal length configuration on the c8 - I'll see if the #25 filter on the 174mm is workable and helps.
The 678 is available in a monochrome mini format from QHY, isn’t it? It would be a great option, but the chip is smaller than the 174 and the 585, so if at a long focal length, that is a negative. Is using a color camera problematic? You are right, the 678 is indeed smaller (7.7 x 4.3) than the 174 (11.34 x 7.13) and the 585 (11.2 x 6.3); probably still big enough though - and it's a very sensitive camera, which I like for the oag. I run AsiAir, so I'm stuck to the ZWO world - until I have a very good reason to move away from it. Nothing wrong with the color camera - just more of an irrational position by me when it comes to guiding cameras...
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Mark McComiskey:
Mark McComiskey:
Darryl Ackerman:
That makes sense, look at the QE on the 220 vs the 174
Yep; too bad the 220mm's sensor is way too small for a C8 oag...
Try the 585
That's just color, isn't it? I'd rather stay monochrome for guiding, like the 678. That said, the form factor of the mini cameras is so convenient... The perfect solution would be a 678mm mini... In the meanwhile - once I switch back to "full" focal length configuration on the c8 - I'll see if the #25 filter on the 174mm is workable and helps.
The 678 is available in a monochrome mini format from QHY, isn’t it? It would be a great option, but the chip is smaller than the 174 and the 585, so if at a long focal length, that is a negative. Is using a color camera problematic?
You are right, the 678 is indeed smaller (7.7 x 4.3) than the 174 (11.34 x 7.13) and the 585 (11.2 x 6.3); probably still big enough though - and it's a very sensitive camera, which I like for the oag.
I run AsiAir, so I'm stuck to the ZWO world - until I have a very good reason to move away from it.
Nothing wrong with the color camera - just more of an irrational position by me when it comes to guiding cameras...
I suppose one issue with the color camera is that only 1/4 of your pixels would be able to get signal using a Near IR filter….
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If one wants to stick with the ZWO ASI174mm mini as guide cam (I have two of them, one on a C9.25 OAG and a C14 OAG) and the guide stars are too dim using a red or NIR filter, an orange or even yellow filter can be used instead. They pass a whole lot more light and still block the blue end of the spectrum where most of the seeing-induced turbulence occurs.
Arnie
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I see concerns about quantum efficiency. Cameras with older sensors e.g. ZWO ASI1600MM have the same quantum efficiency at 850nm* as ZWO ASI120MM Mini – around 30%. However, the stars picked by the guiding software are bright objects and as such have high signal-to-noise ratio. This makes me wonder whether the lower quantum efficiency, even with the short guiding exposures, will matter enough to be a major problem. Edit: I guess the main problem would be atmosphere and humidity as water vapor absorbs infrared light. An IR pass filter at 685nm would be less susceptible. I'll give it a try. * 850nm being popular in IR pass filters |
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You will absolutely get better guiding results using a red or IR pass filter in bad seeing, as red/infrared wavelengths are less affected by seeing and other atmospheric disturbances…
This is not anecdotal, this is physics… if you get a sensor particularly sensitive in the IR end of the spectrum like a imx585, you could be guiding on stars with a 650-700nm cut off, and your guide stars will stay quite still even under very average seeing.
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Alex Nicholas:
You will absolutely get better guiding results using a red or IR pass filter in bad seeing, as red/infrared wavelengths are less affected by seeing and other atmospheric disturbances...
This is not anecdotal, this is physics... if you get a sensor particularly sensitive in the IR end of the spectrum like a imx585, you could be guiding on stars with a 650-700nm cut off, and your guide stars will stay quite still even under very average seeing. +1
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Now that I am fully awake, the 585 from QHY has the mini form factor and seems to have 50% QE at 850nm in all three channels. The chip is the same size as the 174MM, which is as big as my OAG can illuminate. So seems like an ideal candidate for a NIR pass filter application set at 800 or so as a path to enhanced guiding? I believe the ONAG, from Innovations Foresight, used to great effect by John Hayes, operates above 750nm. I am liking this as an upgrade for my system, as the ONAG itself is not an option - it can’t handle my imaging chip size.
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Yup, the 585 is a great and versatile sensor, cooled and uncooled.
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Which IR pass filter would be optimal for my ASI220 with the ZWO OAG on a SkyWatcher 200 PDS?? Would something like this one https://www.astronomik.com/en/infrarot-passfilter-infrared-pass-filters/proplanet-642-bp-ir-passfilter.html be a good choice?
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Christian Bennich:
Which IR pass filter would be optimal for my ASI220 with the ZWO OAG on a SkyWatcher 200 PDS??
Would something like this one https://www.astronomik.com/en/infrarot-passfilter-infrared-pass-filters/proplanet-642-bp-ir-passfilter.html be a good choice? I'm using the 742 version of the same with my ASI220, it has been working quite well during the last 3-4 sessions. https://www.highpointscientific.com/astronomik-proplanet-742-ir-pass-filter-1-25-p742-1 |
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Naveen Ravindar:
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.
 . I just tried this technique last night and it worked extremely well. My guiding average for the night was 0.14" RMS and at a few points it dropped to below 0.10" RMS. I have never seen numbers anything like that, to the point that I almost don't believe it. Normally, with just a Lumicon deep sky filter on a great night, I would average around 0.45" so if true, this is an amazing improvement. Now, this is just one run and it might have been exceptional seeing so I'll have to see if it holds up. From what I see so far, for the $22.00 (ZWO 25 red filter) spent, it's a pretty good upgrade. For those who are interested by guiding setup consists of an Orion ST-80 guide scope with ZWO585mc guide camera. The mount is a factory stock EQ6-R pro. Polar alignment as measured by PHD2 is 30".
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I’m having some trouble finding a red ZWO filter with that transmission curve on their website. Where does one get such a thing?
Also, may I ask how long your guide
exposures are? Mine are 5” and I’m wondering if a longer exposure averages out the seeing anyway, reducing the benefit of this approach. Obviously still potentially beneficial, though.
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Actually, it's a GSO #25A red filter. I don't have a curve for this exact filter but the band pass of these filters is a photographic standard so the curve below should be close. My exposure time for PHD2 is 2 sec.  |
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Mark McComiskey:
I’m having some trouble finding a red ZWO filter with that transmission curve on their website. Where does one get such a thing?
Also, may I ask how long your guide
exposures are? Mine are 5” and I’m wondering if a longer exposure averages out the seeing anyway, reducing the benefit of this approach. Obviously still potentially beneficial, though. 5s exposures are usually long enough to average out seeing and even on large telescopes 0.5m and larger, 5s is the seeing averaged rule of thumb where exposures 4-5s or longer are expected to be near identical. One thing the filter may help with is managing chromatic dispersion allowing you to reach a sharper focus by letting in less wavelengths. If chromatic dispersion is not a concern (oags or well corrected refractors) then I would expect any filter with 5s exposures to show minimal if any improvement.
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My ST-80 of course is just an achromatic doublet so this would be true, the stars in that scope are certainly tighter. I still don't think that fully accounts for last night's results.
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