20 replies to this topic

[Mauro Da Lio]

Last year, I rented an NVD (an OVNI-M) with a Harder Digital tube and tested it in several different conditions, especially under a dark sky (https://www.cloudyni...bserving-needs/). The unit provided excellent performance for H-alpha targets, but I was unhappy with low-contrast, very faint objects (e.g., galaxies, many PNs, reflection nebulae, etc.). On many occasions, these types of objects, which I could "detect" with glass in dark sites, were not visible at all in the NVD.

 

Hence, I considered Photonis tubes, which, in theory, should have shifted their sensitivity towards the blue. 

 

One issue I found was excessive sensitivity in the 800+ nm wavelengths, which are strongly polluted by natural airglow (https://www.cloudyni...under-dark-sky/). Another issue was that the environment is much brighter in IR than it is in the visual band. With a truss tube Dobsonian, preventing stray (IR) light is difficult, and this can cause a loss of contrast. 

 

On the other hand, many interesting objects are blue-rich (e.g., spiral arms). Hence, a sensitivity shifted towards the blue (even at the expense of the peak performance in H-alpha) looked interesting to me.

 

I could not find a NV with a Photonis tube to rent, so I took the risk and bought a PVS-14 unit with a Photonis Echo tube (https://www.cloudyni...onis-echo-tube/). Recently, the shop that rented me the OVNI-M last year gave me the unit again to make a side-by-side comparison.

 

The following figure shows the OVNI-M on the left and the PVS-14 on the right.

 

The former mounts a Harder Digital Tube with the following specs:

Luminous gain at 10^-6 fcd: 59800
EBI: 0.6 
Resolution: 70 lp/mm
SNR: 31.2 
FOM 2184

 

The latter mounts a Photonis Echo tube with the following specs:

Luminance gain 10540 (33112 in Gen3 units).
Resolution 66 lp/mm
EBI 0.07 (0.7 in Gen3 units)
SNR 29.69

FOM 1960

 

The price paid for the PVS-14 with that tube was 4179 Euro. I could have bought the unit on the left from the shop as an used item for approximately the same price (it is still listed in the used items inventory).

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Edited by Mauro Da Lio, 01 September 2024 - 12:23 PM.

[Mauro Da Lio]

My expectation was to find a significant difference in H-alpha (i.e., Hard Digital significantly outperforming the Photonis) and, on the opposite side, the Photonis significantly outperforming the Harder Digital in low surface brightness broadband bluish objects (in the tests I did with the new device, I maintained this opinion, but I was aware that only side-by-side comparison could have provided a m ore precise evaluation).

 

Spoiler. However, when I got the two un ties side-by-side under the same sky at the same time, I found really minor differences that I am now going to show.

 

... continue...

Edited by Mauro Da Lio, 01 September 2024 - 12:26 PM.

[Mauro Da Lio]

Before I explain my findings, I'd like to comment on certain aspects of the construction of the devices that affected how I conducted the tests.

 

The PVS-14 is made of sealed metal, while the OVNI appears to be made of plastic and is not sealed. Sealing is crucial for military use (mine can be submerged), but not necessary for astronomy.

 

I was informed that I could remove the objective of the PVS-14 for prime focus, but I am hesitant to do so because I'm concerned about dirt getting in. In fact, there is some dirt visible in the other unit (on the phosphor screen). It can be cleaned, but I did not touch it.

 

For prime focus, as shown in the next picture, the two devices require a similar back focus if used with a 1.25 nose. In that case, the PVS-14 must clear the battery lid (bottom), and the OVNI must clear the on/off switch.

 

However, I have discovered that the OVNI can be used with a 2-inch nose, allowing me to leverage the cut in the 2 inches focuser (for the 2 to 1.25 adapter) and fit the OVNI more closely (second and third photo).

 

I can only achieve focus in this scenario. To achieve focus with the 1.25 nose, I would need to shorten the truss poles, which has been another factor holding me back from using it in prime.

So, until now I have used the PVS-14 in afocal mode alongside with various eyepieces, such as the Baader Zoom 8-24, which enables quick adjustments to magnification.

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Edited by Mauro Da Lio, 01 September 2024 - 02:43 PM.

[Mauro Da Lio]

I conducted the tests in focal mode as per the explanations above.

 

To differentiate between the two units, I held the devices in front of the eyepiece with my hands touching the eyepiece cup to quickly swap them and better perceive the minor differences.

While both units could have benefited from being set to "prime," the time required to switch them would have hindered the detection of subtle differences, even if I had the capacity to use the PVS-14 in prime mode.

 

One thing to note is that the PVS-14 has an objective that filters blue light (as is common in many PVS-14 implementations). Therefore, in afocal mode, I may be filtering part of the presumed better response of the Photonis tube at blue. The objective of the OVNI does not appear to filter blue to the same extent (photo).

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Edited by Mauro Da Lio, 01 September 2024 - 02:53 PM.

[Mauro Da Lio]

Now the comparisons....

 

The main difference between the two tube specs is the luminance gain: 60k for the Harder and 33k for the Photonis.

 

The following picture confirms this, as both devices are facing the same white, dimly lit wall.

 

The Photonis (on the right) appears lightly yellowish, while the Harder appears whitish. The eyepiece lens of the Photonis seems to be larger.

 

I roughly measured the apparent surface brightness with an SQM and obtained around 12.9 mpsas for the Photonis and around 12.0 for the Harder, making the former about 0.9 magnitudes brighter, approximately 2.3 times brighter (even though it should be 1.8 times brighter based on the luminance gains).

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Edited by Mauro Da Lio, 01 September 2024 - 03:03 PM.

[Mauro Da Lio]

However, the increased surface brightness did not result in a proportional (0.9 magnitudes) improvement in star visibility.

 

In reality, the brighter unit projected brighter stars against a brighter sky background with the same contrast. I could see the same stars in both units. The difference was that the stars appeared against a darker sky with the Photonis.

 

The OVNI includes an external gain control, which is activated with a push button. Every time the button is pressed, the gain is reduced, seemingly by a factor of two. This control modifies the internal brightness control loop.

The PVS-14 is equipped with a tube featuring automatic gain control (only internal brightness adjustment is available).

 

By pressing the gain control button one time, the Harder sky was still a bit brighter. Pressing it two times made it dimmer than the Photonis. 

 

The level of perceived noise when the Harder is at full gain is higher than the noise of the Photonis at its full gain. The Photonis can be used at full gain on several occasions, while the Harder needs to be used one step dimmer under similar conditions.

There have been instances where I needed to further reduce the brightness of the images. This was achieved by pressing the gain button twice on the Harder and using a 50% ND filter on the eyecup of the Photonis.

Edited by Mauro Da Lio, 01 September 2024 - 03:21 PM.

[Mauro Da Lio]

Filters.

 

I used the following filters.

 

Baader UHC-S. It. has two large passbands (50 nm wide) centered on H-alpha and OIII/H-beta. It works well as a dual band filter under dark sky, with plenty of visible stars and a realistic contrast.

 

SVBONY UV/IR cut. It passes a band from 400 to 700 nm. The purpose is to cut the IR airglow in dark sky as explained in the threads linked in the first post. It also works against stray IR light in light polluted sites.

 

SVBONY IR pass. It passes everything longer than 685 nm (apparently including the wavelengths of the strongest airglow).

 

Astronomik ProPlanet 642. It passes a band between 642 nm and 800 nm, i.e., including H-alpha and excluding the strongest test airglow regions over 800 nm.

 

Explore Scientific 12 nm H-alpha. For dark sky I prefer a 12 nm wide band, which shows more stars.

[Joko]

In addition to your comparisons, it might be worth mentioning :

 - this OVNI-M you rented was built in early 2020 and is one of the 1st made by our company. It belongs to an Italian astro shop that was one of our dealer and who rent it to astronomers.

 - its has a lower FOM and other specs that we no longer manufacture since 2021 (current OVNI-M entry level has a minimum FOM 2200).

 - it also has different wavelenght sensitivities, with less sensitivity in the blue spectrum since 2021 and no sensitivity to ultraviolet. (current OVNI-M has an extended range from 390nm to 1000nm. They also have good sensitivity to Ultraviolet which older tubes like the one you tried don't have).

 - it doesn't have an OVNI Night Vision "Astronomy Grade" tube that we manufacture since 2021. (our current tubes are built for astronomy, which was not the case with the OVNI-M you rented).

- for the best views we provide a range of accessories and filters but you didn't use any of them.

- OVNI-M is better suited for astrophotography. Here are some pics : www.ovni-nightvision.com/en/content/52-astrophotography-gallery

For visual observation we recommend the OVNI-B (binoviewer).

 

In conclusion, this OVNI-M was good but is very far from representing the actual performance/specs nor visual observations that our NV eyepieces currently provide.

Edited by Joko, 03 September 2024 - 04:52 AM.

[Mauro Da Lio]

@joko, send me a better unit. I will be more than happy to make the comparison with mine (and the old one).

 

Observing sessions.

 

So far I had only two observing sessions (none under a really dark sky).

 

1) Saturday, August 24 at home. SQM 18.55. Moon was rising. Atmospheric Optical Depth (AOD) 0.2 which translates to zenith extinction equa to 0.31 magnitudes (not very good).

 

2) Thursday, August 20 at (44.815491, 12.399387). SQM 20.9, AOD 0.25.

 

I wanted to visit a 21.5 sky last Saturday, but it was cloudy.

 

Telescope: 10 inches Sumerian.

Edited by Mauro Da Lio, 02 September 2024 - 04:40 PM.

[Mauro Da Lio]

Performance on H-alpha.

 

Target M27: After comparing both sessions, I found that the nebula appeared slightly better in the Harder tube. Quantitatively, the difference is smaller than the gap between 10 and 12-inch telescopes, and is similar to day-to-day visibility variations.

 

The Dumbbell nebula has a bright star on one side (bottom right here https://en.wikipedia...Dumbbell_Nebula).  In session 1,vthe star was visible in both tubes. In session 2, the six stars on the opposite side were equally visible in both tubes. Additionally, the nebula has a brighter knot near the first star, which was slightly more distinct in the Harder tube.

 

It's worth mentioning that the central star was consistently visible in the 24-inch Dobsonian.

 

Filter Comparison: When using the Proplanet filter, the Harder tube performed slightly better than the Photonis. However, with the UHC-S filter, both tubes performed equally well under a dark sky).

 

O3 Filter: I also tested the O3 filter at home, and it gave a slight advantage to the Photonis, even with the blue-filtering objective.

 

With the H-alpha filter the Harder tube was slightly better.

 

 

Performance on galaxies.

 

Targets: NGC 147, 185, and 248. These are galaxies with decreasing surface brightness. Galaxy 248 was visible in both tubes, showing the core and the halo. Galaxy 185 appears as a faint fuzzy object in both tubes. Galaxy 147 becomes invisible against the sky background.

 

The main issue with NV (night vision) devices is that while they increase surface brightness, they also introduce some noise (the sky background looks clearer with glass), which in turn reduces contrast. Consequently, very faint objects may not be visible with NV devices, even though they might be visible with glass and a larger aperture.

 

As a side observation, the Harder tube works better with the Proplanet than with the IR-cut. Conversely, the Photonis works better with the IR-cut.

 

The Photonis IR-cut combination works equally well as the Hard Proplanet combination.

 

Stray light prevention.

 

Another important consideration is preventing stray light. In the infrared (IR) spectrum, the environment is much brighter than in the visible band, even in dark sites. Therefore, it is crucial to thoroughly inspect truss tube dobsonians for stray light in the IR. Some shrouds and baffles may reflect IR light, so it's important to use a Night Vision Device (NVD) to confirm the absence of stray light in the tube.

Edited by Mauro Da Lio, 02 September 2024 - 05:15 PM.

[Mauro Da Lio]

I forgot the night time temperature: 23 °© on session 1 and 27 °C!! on sessione 2.

 

Globular clusters.

 

Target: M15. Session 1. Zoom Baader 8-24. At the focal length of 8 mm (156x, 1.6 mm exit pupil), without night vision, the brightest stars are barely discernible. With night vision, many stars are visible on both devices. The limiting magnitude is the same. However, the Photonis has a slight edge because the glow of the unresolved stars is more realistic. Globular clusters, in regular eyepieces with sufficient aperture, show many stars floating over the glow of the faintest unresolved stars. This glow was less clear with the Harder tube (I already noticed this effect last year). It was more realistic and visible in the Photonis.

[WheezyGod]

Thanks for sharing. I didn’t read through everything you mentioned since it’s a lot but think I got the big picture.

If I understand what you’ve explained correctly, the performance difference between two similarly spec’d tubes that have a different type of tube is very minor. Conversely, this would mean the differences in specs between two different tubes is more important. Is that right?

[sixela]

They're similarly spec'ed except for gain (mainly on the red end, I don't think the difference is that pronounced on the blue end). The OVNI-M has more gain, or at least it should have if it were new (and this one isn't, although it's hard to know how it compares to a new one).

 

Mauro, if you're ever in Belgium or I'm ever close to you (where in Italy would I need to be? Respond in a PM) feel free to borrow my OVNI-M. We can even compare to yet other devices too, but I won't let you borrow these (I either observe with them or they aren't mine).

Edited by sixela, 05 September 2024 - 03:52 AM.

[Mauro Da Lio]

The two devices have two main differences: gain and spectral sensitivity.

 

The Harder tube has about twice the gain (59800 vs 33112). I would say that post 5 confirms  this ( https://www.cloudyni.../#entry13660262 ) : the output brightness is 0.9 magnitudes greater.

 

However, part of the extra gain also brings higher perceived noise. To achieve similar noise levels, one needs to step down one level. Therefore, for similar perceived noise, the brightness levels are closer.

 

At full gain and no filter, the sky appears unnaturally bright in the Harder. One step down, it looks darker, similar to the Photonis. Aesthetically, I would compare the Harder with reduced gain to the Photonis at full gain.

 

I often use the Photons with a 50% ND filter on the eyecup and the Harder between 1 and 2 steps less. In my opinion, the contrast is slightly better for the Photonis, which balances the somewhat dimmer image. It's worth noting that the limiting magnitude is the same.

 

Regarding spectral sensitivity, the Harder tube performs better on H-alpha, but only slightly (see the description of M27). Conversely, I believe the Harder tube works better with the Astronomik Proplanet filter because the filter cuts off above 800 nm, where a lot of airglow is present. The Photonis tube performs better with the IR-cut 685 because it is probably less sensitive in the far IR, and cutting at 685 nm eliminates the artificial light pollution tail.

Edited by Mauro Da Lio, 05 September 2024 - 03:27 AM.

[sixela]

It should be noted that Photonis at one point sold a limited series with gains very much in line with those of Gen 3 devices.

 

Alas, gavster (who got one) reported that ,despite similar SNR numbers as fpr the 'regular' Photonis, the subjective noise also made the highest gain settings less usable, just as on most Gen 3s.

[WheezyGod]

The need to reduce the brightness because of the added noise makes sense. It just reaffirms the importance of SNR. I think I had read some reports that SNR is measured at a tube’s full gain, but if that were true then the supergain tubes would be superior.

Most of us tend to observe at least slightly below max gain if not a a step or two below. I wonder what SNR value would be needed for most of us to want to observe at max gain for a device with 65k gain which is typical for Gen3 devices over the last few years.
Let’s say a SNR of 45 would have most of us prefer the view at max gain. For a supergain tube that has a gain value of 110k, a tube would need a SNR of above 75 to hold that max gain preference. If there’s any possibility of this being real after 15-20yrs I would imagine there will be some other better technology that comes along.

[Mauro Da Lio]

Yesterday, I could go to one dark site and complete the comparison (under a dark sky).

 

The conditions were: SQM 21.4, excellent transparency (AOD<0.05), Temperature 4°C, elevation 1800 meters, telescope: Sumerian 10" dobson—afocal use.

 

The conclusions are the same as the previous posts (under light-polluted or semi-dark conditions). In particular, the Harder tube has a very tiny advantage in H-alpha. The Photonis has a slightly tiny advantage on the sky darkness on galaxies. In afocal mode, the stars in the Photonis are pinpoints to the point that I do not feel the need for prime focus (thus, I can use the Baader 8-24 zoom to get the optimal magnification). At the entire gain, the Harder tube is brighter but noisier. One gain step down (*) is still a tiny, brighter, and noisier. Two steps down, it is darker and shows less. The best compromise is, often, one step down. The best view with the Photons is either full gain or using a 50% neutral density filter (at the eyepiece).

(*) The OVNI gain control is obtained via a push button that (presumably) halves the gain at each pressure.

 

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I considered these targets:
- NGC 6760, a globular that's visible but unresolved in glass. Both devices showed the same stars (more point-like in the PVS-14 - afocal).
- NGC 6749, a tough globular that was invisible in the glass. Both devices showed a similar glow.
- Sh 2-71, allegedly a planetary nebula (https://www.imagingd...com/sh2-71.html). Using an H-alpha and an Astronomik Proplanet filter, the Harder tube returned a slightly brighter image (-1 step), but the details visible were the same. I could see an elongated elliptical-rectangular shape with an elongated hole inside (matching the picture).
- NGC 7479, a barred spiral galaxy with hard-to-see arms. The bar was visible on both devices, with a star on the bar and another on the side. An elliptical glow where the arms should be was slightly better visible in the Photonis.
- NGC 7814, an edge-on galaxy with a tiny dust lane. The lane was not visible in both devices. The borders of the galaxy were elusive.
- ARP 78 (NGC 772) a galaxy showing distorted arms. Both devices showed a glow, with a clear nucleus and fading into the sky (no arms). 

Galaxies are the most challenging things for NV. The IR-cut filter did not improve the views. The sky was slightly darker in the Photonis, but only by a tiny amount.

Edited by Mauro Da Lio, 29 September 2024 - 09:10 AM.

[John Vogt]

How superior are the supergain tubes over the standard L3 Harris WP Gen 3 tubes?

 

The pictures Mike Lockwood recently posted comparing supergain vs standard

show a dramatic improvement over the standard version?

 

Would this improvement appear the same visually?

[Mauro Da Lio]

I compared the two tubes that are available in the EU, with similar cost and specs. I do not know how others could compare.

[Joko]

I compared the two tubes that are available in the EU, with similar cost and specs. I do not know how others could compare.

The OVNI-M FOM 2100 you've tested is no longer available (discontinued since 2021)... and have very different specs from what we offer now.

 

As we already told you many times, the last time was here #8 :

www.cloudynights.com/topic/935333-comparison-photonis-vs-harder-digital/#entry13661665

Edited by Joko, 30 September 2024 - 01:35 PM.

[Mauro Da Lio]

As said, I compared two devices I could buy for a similar price, at around 4k. One was brand new, while the other was a demo unit used in outreach events. It was never sold and sat in a drawer until I rented it. Both devices have a similar figure of merit (FOM) of about 2000-2100.

 

It's worth noting that there may be tubes with better performance, but they typically come with a higher price tag. This applies to brands like Photonis and Harder Digital. For instance, my Photonis tube is an "Echo" tube, a third-tier choice in the 4G line.

 

Your comment would be valid if I had compared a higher-spec, more expensive 4G tube with a cheaper one. The comparison is fair, given that the devices have the same price and specs. It wouldn't be fair to compare a 4k device with a 7k one (and I wouldn't buy a 7k Harder and a 7k Photonis tube just for the sake of comparison).

 

In addition, although this is not crucial, the Alpha-mod PVS-14 is constructed with a sturdy metal case and gaskets that protect the optics and tube from dirt and water. The optics are good and provide pinpoint stars when used in an afocal mode, which allows me to use a Zoom eyepiece and easily find the optimal magnification.

Edited by Mauro Da Lio, 30 September 2024 - 04:25 PM.