Upgrade to a Thermal Scope or Wait for Better Resolution

[RichardAllen]

For 4 years I have frequently been using a Pulsar Helion XQ38F monocular as a thermal spotter. My problem even after years of experience is that target identification is too uncertain to shoot with an equivalent thermal scope. (I already have a IR NV, but want to go full thermal).

My main assumption is that the quality of the thermal picture for identification purposes mostly depends on the resolution of the sensor (microbolometer), though the digital post-processing will help. The XQ38F sensor I have been using is 384 x 288.

Taking the Thermion XP50 as a candidate scope, its sensor resolution is 640 x 480. So the XP38 sensor is about 1.7 times resolution in each dimension than the XQ38F. I believe 1024 x 768 sensors are available, but not yet in a rifle scope.

So does that 1.7 times 'better' resolution give me thermal target identification certainty, or do I have to wait until better sensors become available for the next generation ? For example, the Fieldsports video () at 3:40, has a rabbit shot at 40 yards. For my permission, that possibly rabbit-shaped blur, even if you had seen it move, is nowhere near certainly a rabbit. At 100 yards, I expect it would not certainly be anything other than a blob. I am not criticising the Fieldsports shooter as I was not there and don't know his ground, but that video is not convincing enough for me to drop well over £4k.

However, thermal scopes seem to sell well. Am I missing something ?

 

[Freeforester]

Your assumption re the core spec is not the whole picture, of you'll pardon the pun!

The quality and fast-ness of the lens are both factors, and it is well accepted that some Scopes using identical cores to some spotters nevertheless give a superior picture, owing to the design parameters being somewhat different between the two. I'm certainly no expert in this particular field, but there are a number of factors that determine the view your eye eventually sees of the quarry. Basically you'd be best off to try before you buy. To assume an XP cored scope is great but dear, against an XQ cored scope of otherwise identical spec which is toward half the cost is half as good, is not a correct assumption.

 

[mealiejimmy]

There's a very simple way to calculate the resolution of a thermal scope
Simply divide the pixel size in microns by the focal length of the objective lens in mm
The answer to that calculation is an angle in mrad - the smaller the number, the higher the resolution
The mrad number is also known as the instantaneous field of view (IFOV) and represents what a single pixels "sees"
So, your Pulsar XQ38 has an IFOV of 17/38 = 0.447mrad
Putting that number into context means that at 100m. one pixel on your scope can "see" a square with sides 44.7mm
So for an XP50 the IFOV is 17/50 = 0.34mrad, or a square with sides 34mm at 100m
That allows the XP50 to detect targets about 25% smaller than the XQ38
The number of pixels in the sensor has no effect on IFOV, but it does affect field of view and therefore magnification.
For a given size of objective lens, a physically smaller sensor will give more magnification and a narrower field of view
You can have a smaller sensor either by having fewer pixels (384x288 v 640x480) or by having smaller pixels (12 micron v 17 micron)
I've been shooting foxes with a thermal scope for over 4 years now and because of previous experience with thermal spotter, I knew that I would need the biggest lens that I could afford and that led me to a scope with a 75mm lens and a 17 micron 640x480 sensor.
That scope has an IFOV of 17/75 = 0.226 mrad or a 22.6mm square at 100m or more than 50% better resolution than an XQ38
That thermal scope is quite big and heavy and sits on top of a rifle with a 22 inch heavy barrel - so it's not a walking about gun!
A lighter solution is the new Pulsar FXG50 with a 50mm lens and 12 micron (640x480) sensor giving an IFOV of 12/50 = 0.24 mrad, or a 24mm square at 100m
12 micron sensors is where we will be for a few years to come, meaning that the only way to improve resolution will be what it has always been - use bigger lenses
Higher resolution sensors such as 1024x768 won't provide more target detail, but they will give the same level of detail as sensors with the same pixel size and lens size but over a wider field of view and therefore lower magnification.


Cheers

Bruce

 

[Mr. Gain]

Thermal gear will keep getting better and good gear will keep getting less expensive as the performance bar is raised... and all the while the sands of time keep flowing... which means there's no time like the present. And despite the evolutionary process I've just mentioned, I still believe you have to go top-end on a thermal scope for CF use.
Even then, you have to accept that they are nothing like as precise as a good day scope, which means unambitious ranges and boiler-room shots are the order of the day. A look at the part of the manual that tells you what each numerical point on the adjustment scale represents at 100m will confirm Bruce's formula.
At the risk of stating the obvious, the key issue is being able to know with a high degree of certainty where the shot is going to go.
A bit of time spent watching any animal, backed up by some general knowledge of behaviours and habitats, will solve the identification issue. You don't need top-end performance for that.
But if between them the sensor, software and screen are stacking up tolerances to the point where you can't be sure your bullet is going to strike the target within 10cm -or whatever- of where the crosshairs seem to be when you pull the trigger, then you're probably best not pulling it at all.

 

[fallow me]

I would take Mealiejimmy advise, completely baffled me, I'm sticking with my quantum xq38 till he says change it. lol

 

[compo]

wow

 

[George1982]

Very interesting indeed

 

[finnbear270]

Feel like I just had a visit to night school class, ........... but at least this time I understood the lesson .

 

[Boosh]

Also Richard it may be worth considering the base mag settings of the scopes.
Your Pulsar Helion XQ will have a base setting of 3.1x whilst a lot of XP's are only just over 2.0x base setting, so if you start zooming you are reducing the pixel detail.
The XQ riflescopes start out with a 3.5x base mag, so although the XP'S image may be better ln base mag, the XQ'S image may not even need zooming in for the majority of shots.
Having the extra field of view though on base mag is always a better thing when you get behind a rifle scope on a shifty target like a fox.

 

[norma 308]

Love the thermal spotter but see no need at all to go thermal on the scope side after several years of using a thermal unit to spot and using nv to sensible ranges for foxs I’d rather save my ££ but somewhere down the line I may change my mind just not yet

 

[slider]

no idea about the particular kit but like all technology there is always better technology just around the corner.and The latest and newest always has a premium price.

If what is on the market will do your job well then go for it.

 

[243varmint]

There's a very simple way to calculate the resolution of a thermal scope
Simply divide the pixel size in microns by the focal length of the objective lens in mm
The answer to that calculation is an angle in mrad - the smaller the number, the higher the resolution
The mrad number is also known as the instantaneous field of view (IFOV) and represents what a single pixels "sees"
So, your Pulsar XQ38 has an IFOV of 17/38 = 0.447mrad
Putting that number into context means that at 100m. one pixel on your scope can "see" a square with sides 44.7mm
So for an XP50 the IFOV is 17/50 = 0.34mrad, or a square with sides 34mm at 100m
That allows the XP50 to detect targets about 25% smaller than the XQ38
The number of pixels in the sensor has no effect on IFOV, but it does affect field of view and therefore magnification.
For a given size of objective lens, a physically smaller sensor will give more magnification and a narrower field of view
You can have a smaller sensor either by having fewer pixels (384x288 v 640x480) or by having smaller pixels (12 micron v 17 micron)
I've been shooting foxes with a thermal scope for over 4 years now and because of previous experience with thermal spotter, I knew that I would need the biggest lens that I could afford and that led me to a scope with a 75mm lens and a 17 micron 640x480 sensor.
That scope has an IFOV of 17/75 = 0.226 mrad or a 22.6mm square at 100m or more than 50% better resolution than an XQ38
That thermal scope is quite big and heavy and sits on top of a rifle with a 22 inch heavy barrel - so it's not a walking about gun!
A lighter solution is the new Pulsar FXG50 with a 50mm lens and 12 micron (640x480) sensor giving an IFOV of 12/50 = 0.24 mrad, or a 24mm square at 100m
12 micron sensors is where we will be for a few years to come, meaning that the only way to improve resolution will be what it has always been - use bigger lenses
Higher resolution sensors such as 1024x768 won't provide more target detail, but they will give the same level of detail as sensors with the same pixel size and lens size but over a wider field of view and therefore lower magnification.


Cheers

Bruce

I think that's the answer everyone has been waiting for. A simple explanation of the difference between sensors and lenses.

 

[mealiejimmy]

Also Richard it may be worth considering the base mag settings of the scopes.
Your Pulsar Helion XQ will have a base setting of 3.1x whilst a lot of XP's are only just over 2.0x base setting, so if you start zooming you are reducing the pixel detail.
The XQ riflescopes start out with a 3.5x base mag, so although the XP'S image may be better ln base mag, the XQ'S image may not even need zooming in for the majority of shots.
Having the extra field of view though on base mag is always a better thing when you get behind a rifle scope on a shifty target like a fox.

Further to my earlier post about resolution, it might be worth discussing magnification in thermal (and NV) spotters and scopes
The four numbers (in mm) needed to calculate magnification in a thermal scope or spotter are:
A - The focal length of the objective lens assembly -this is the number that is part of most thermal model types e.g the 50 in XP50 or XQ50
B - The diagonal size of the sensor- the size of a pixel multiplied by the number of pixels in each direction will give the length and width of the sensor and a little bit of pythagoras will give the diagonal size
C - the diagonal size of the near eye display
D - the focal length of the ocular lens assembly
The base magnification is given by (A/B)*(C/D)
From that simple equation it can be seen that the magnification can be increased by:
Increasing the focal length of the lens
Reducing the size of the sensor
Increasing the size of the display
Reducing the focal length of the ocular lens
So, an XQ50 has a higher magnification than an XP50, because it has a physically smaller sensor (17 micron 384x288 pixels v 17 micron 640x480 pixels)
An XM50 has a higher magnification than an XQ50 because it has a physically smaller sensor, both in the size of the pixels (12 micron v 17 micron) and the number of pixels (320x240 v 384x288)
However, using different sensors and lenses is not the only way to change the magnification
The Thermion XQ38 thermal scope has a base magnification of x2.5, while the Axion XQ38 has a base magnification of x3.5
Both devices use the same lens, sensor and display, the difference comes from how they are used.
In use, the Axion XQ38 spotter is held close to the eye, while the Thermion requires eye relief to prevent the user from getting "scoped"
That means the Thermion needs a longer focal length ocular lens and that leads to lower base magnification.

Field of view is also an important parameter in thermal and NV scopes, with field of view getting smaller as magnification increases and vice versa
Field of view only depends on the focal length of the objective lens and the diagonal size of the sensor - the display and the ocular lens have no effect.
Sticking with same notation as in the magnification formula, the field of view (in radians) is given by 2*arctan(B/2A)
Similar to the example in my earlier post, a field of view of 0.1 radian would equate to 10 metres at 100 metres or approximately 30 feet at 100 yards.

Digital zoom works by using a smaller number of pixels to create the image seen on the display. This has the effect of making the sensor smaller and thus increasing magnification.
However, digital zoom does not improve resolution, in the way that optical zoom improves resolution because, in a thermal (or NV) scope, resolution is fixed by the size of the pixels and the focal length of the objective lens.
When digital zoom is used, each sensor pixel is displayed on a larger number of display pixels, which does make the target appear larger, but less sharp.

You may have noticed that, while optical zoom is common in glass scopes, there are no NV or thermal rifle scopes with optical zoom, and that the only type of zoom available is digital zoom
The reason for this is cost - making a zoom lens which will maintain it's optical axis (and therefore the scopes zero) is extremely expensive. There are plenty of zoom lenses available for cameras, but they don't have to maintain their exact optical axis throughout their zoom range.
People who fitted zoom lenses to their Yukon Photons to increase its magnification soon found that they could zero the scope at a given magnification, but changing the magnification shifted the zero.
Cost is the reason why we don't have NV scopes with optical zoom, and that cost issue is magnified greatly when it comes to thermal scopes.
The germanium objective lens assembly is the single most expensive item in a thermal scope, so making a zoom lens system with multiple, perfectly made and aligned germanium lenses would put such thermal spotters and scopes out of reach of all except the military and millionaires.

Cheers

Bruce

 

[Boosh]

Now I want to read another section on germanium lenses Bruce.
Thanks for the info in those two posts, cannot say I took it it all in, but I will read it again and again until no more will fit in.

 

[25 Sharps]

I don’t think I’ll ever go to thermal scope, have used a spotter for years but I don’t think I’ll ever switch. The difference in what you can see sight picture wise is huge, you will see a whole animal the thermal only to switch to NV and see it is completely obscured by undergrowth.

Only takes one small twig to deflect a bullet for a miss or worse a badly shot animal

 

[Lateral]

At what point do you decide the current offerings, will do what you want, so you can get out, use, and enjoy them ?

There will never be a time, where something newer, better isn't in the pipeline !

 

[RichardAllen]

Thanks guys, that is really really helpful, I am a retired engineer (albeit not in optics), so Bruces analyses make real sense.

The one point I would take issue with is: "A bit of time spent watching any animal, backed up by some general knowledge of behaviours and habitats, will solve the identification issue. You don't need top-end performance for that." My buddy and I have been thermal spotting for 4 years and ID is still an issue over 100 yards unless you can wait for ages for definitive movement. Our best one was a certain sighting of a badger at approx 120 yards moving gently along. When we moved up to about 80 yards, it stuck it's head up - we had been spotting the arse end of a fallow buck grazing in a dip ! Still not a fox though.

 

[The Singing Stalker]

I am of the opinion that this year will see a marked improvement in kit coming out. The Krypton is the first unit that has been released that has made me think I would move to thermal. But unfortunately as I can’t go shooting I may as well not buy anything until this corona cack has been sorted.

 

[Freeforester]

If Lynred have already produced a 640/480 12micron sensor, it's surely just a matter of time before a European version of the Thermion XG or similar comes along, its not as though we don't have some Chinese makers already circumventing ITAR by producing (and selling to whoever) their own similarly specced units?

 

[BLUEROLL]

Thanks guys, that is really really helpful, I am a retired engineer (albeit not in optics), so Bruces analyses make real sense.

The one point I would take issue with is: "A bit of time spent watching any animal, backed up by some general knowledge of behaviours and habitats, will solve the identification issue. You don't need top-end performance for that." My buddy and I have been thermal spotting for 4 years and ID is still an issue over 100 yards unless you can wait for ages for definitive movement. Our best one was a certain sighting of a badger at approx 120 yards moving gently along. When we moved up to about 80 yards, it stuck it's head up - we had been spotting the arse end of a fallow buck grazing in a dip ! Still not a fox though.

And thats why I keep a Pard 008 LRF/Dark Engine combo in my my pocket for positive ID of suspect Thermal targets when using the Thermion.