What's the best camera for my H-Alpha Telescope? I try to provide some guidance here.
Let's look at some familiar H-alpha telescopes and available 2018 astro-cameras.
With the ZWO ASI178MM and my 350mm focal length Lunt LS50, the diameter of the sun with some space around it is about 1660 pixels... 1660 x 2.4µm = 4.0mm. e.g. Solar diameter at 350mm = 4.0mm. Using this basic information, one can make a table of focal length versus solar diameter on the sensor.
Focal length (mm) Solar Diameter (mm) on Sensor
50 0.57
75 0.86
100 1.14
150 1.17
200 2.29
250 2.86
300 3.43
350 4.0
400 4.57
450 5.14
500 5.71
600 6.86
700 8.0
800 9.14
900 10.29
1000 11.43
1100 12.57
1200 13.71
1500 17.14
One can use this table to determine if a full solar disc will fit on any given sensor.
One can take this a step further to determine the suitability of a camera to work with an etalon on a given H-Alpha telescope.
IMAGE RESOLUTION
Image resolution depends upon the diameter of the Airy disc.
The Diameter of the Airy disc = 2.44 x wavelength x f#, where f# is called the f-number.
f# = focal length of the telescope / aperture of the telescope
The smaller the Airy disc, the smaller the details that can be resolved by the telescope,
Resolution is inversely proportional to the size of the Airy disc; high resolution requires a small Airy disc.
A small Airy disc means a small f#. A small f# means a large aperture and/or short focal length.
To resolve (reasonably image) the Airy disc, how many pixels are needed across the diameter of the Airy disc?
The answer is that at least 4.5 pixels are needed across the diameter of the Airy disc.
Diameter of the Airy disc = 2.44 x wavelength x f#
For teal-green light of wavelength 0.527 microns, we calculate...
Diameter of the Airy disc = 2.44 x 0.527 microns x f# = 1.2859 microns x f#
Placing 4.5 pixels across the diameter of the Airy disc corresponds to a pixel size of
(1.2859 microns x f#) / 4.5 or f# / 3.5.
For good resolution in white light, the Maximum Pixel Size = f# / 3.5
However, here one is concerned with Hydrogen Alpha imaging.
For Ha wavelength of 6562.8 Angstroms (0.65628 microns), we calculate...
Diameter of the Airy disc = 2.44 x wavelengh x f#
Diameter of the Airy disc = 2.44 x 0.65628um x f# = 1.601 x f#
Placing 4.5 pixels across the diameter of the Airy disc corresponds to a pixel size of
(1.601 microns x f#) / 4.5 or f# / 2.8.
For good resolution in H-Alpha light, the Maximum Pixel Size = f# / 2.8.
Some H-Alpha examples...
NOTE: Similar calculations could be performed for white light telescopes - use Maximum Pixel Size = f# / 3.5.
Given the available (2018) H-Alpha telescopes, what cameras are best suited to capture a full solar disc?
1. What's the best camera for a Coronado PST?
PST: 400mm focal length, 40mm objective lens, f/10.
Maximum Pixel Size = f# / 2.8 = 10 / 2.8 = 3.57µm.
ZWO ASI174 (1936x1216 pixels @ 5.86µm)
No, the large 5.86µm pixels are too big (>3.57µm required for good resolution).
ZWO ASI178 (3096 x 2080 pixels @ 2.4µm)
The small 2.4µm pixels would be OK (less than 3.57µm) but over-samples the image.
According to the table, at 400mm focal length, the solar disc would be 4.57mm in diameter. This requires 4.57mm / 2.4µm = 1904 pixels.
Do we have that? Yes, the ZWO ASI178 has 2080 pixels on the vertical.
ZWO ASI290 (1936x1096 @ 2.9µm)
The 2.9 µm would be OK (less than 3.57µm) but over-samples the image.
At 400mm focal length, the solar disc would be 4.57mm in diameter. This requires 4.57mm / 2.9µm = 1576 pixels.
Do we have that? No, the ZWO ASI290 has only 1096 pixels on the vertical.
ZWO ASI120 (1280 x 960 @ 3.5µm)
The 3.5µm pixels are an excellent match to the ideal 3.57µm pixel size.
At 400mm focal length, the solar disc would be 4.57mm in diameter. This requires 4.57mm / 3.5µm = 1306 pixels.
Do we have that? No, the ZWO ASI120 has only 960 pixels on the vertical.
ZWO ASI1600 Mono (4656 x 3520 @ 3.8µm)
The 3.8µm pixels are larger than the ideal 3.57µm pixel size.
At 400mm focal length, the solar disc would be 4.57mm in diameter. This requires 4.57mm / 3.8µm = 1203 pixels.
Do we have that? Yes, the ZWO ASI1600 has 3520 pixels on the vertical.
ZWO ASI183 (5496 x 3672 pixels @ 2.4 µm)
The 2.4 µm would be OK (less than 3.57µm) but over-samples the image.
At 400mm focal length, the solar disc would be 4.57mm in diameter. This requires 4.57mm / 2.4µm = 1904 pixels.
Do we have that? Yes, the ASI183 has 3672 pixels on the vertical.
Celestron NexImage Burst Monochrome (1280 x 960 @ 3.75µm)
The 3.75µm pixels are larger than the ideal 3.57µm but might be OK.
At 400mm focal length, the solar disc would be 4.57mm in diameter. This requires 4.57mm / 3.75µm = 1219 pixels.
Do we have that? No, the camera has only 960 pixels on the vertical.
Celestron Skyris 236M (1920 x 1200 @ 2.8µm)
The 2.8 µm would be OK (less than 3.57µm) but over-samples the image.
At 400mm focal length, the solar disc would be 4.57mm in diameter. This requires 4.57mm / 2.8µm = 1632 pixels.
Do we have that? No, the Skyris 236M has only 1200 pixels on the vertical.
Mallincam SSIc (1270 x 1030 @ 3.63µm)
The 3.63µm pixels are just over the size limit of 3.57µm but would likely be OK.
At 400mm focal length, the solar disc would be 4.57mm in diameter. This requires 4.57mm / 3.63µm = 1259 pixels.
Do we have that? No, the Mallincam SSIc has only 1030 pixels on the vertical.
For the Coronado PST to image a full solar disc, suitable cameras appear to be the low cost ZWO ASI178 or higher cost ZWO ASI1600 and the ZWO ASI183.
2. What's the best camera for a Lunt LS50?
Lunt LS50: 350mm focal length, 50mm objective lens, f/7.
Maximum Pixel Size = f# / 2.8 = 7 / 2.8 = 2.5µm.
ZWO ASI174 (1936x1216 pixels @ 5.86µm)
No, the large 5.86µm pixels are too big (>2.5µm) required for good resolution.
ZWO ASI178 (3096 x 2080 pixels @ 2.4µm)
The 2.4µm pixels would be excellent, a good match to the ideal 2.5µm.
At 350mm focal length, the solar disc would be 4.0mm in diameter. This requires 4.0mm / 2.4µm = 1667 pixels.
Do we have that? Yes, the ASI178 has 2080 pixels on the vertical.
ZWO ASI290 (1936x1096 @ 2.9µm)
The 2.9 µm pixels would be oversize (>2.5µm required for good resolution).
At 350mm focal length, the solar disc would be 4.0mm in diameter. This requires 4.0mm / 2.9µm = 1380 pixels.
Do we have that? No, the ASI178 has only 1096 pixels on the vertical.
ZWO ASI120 (1280 x 960 @ 3.5µm)
The 3.5 µm pixels would be too large (>2.5µm required for good resolution).
ZWO ASI1600 Mono (4656 x 3520 @ 3.8µm)
The 3.8µm pixels are too large (>2.5µm required for good resolution).
ZWO ASI183 (5496 x 3672 pixels @ 2.4 µm)
The 2.4µm pixels would be excellent, a good match to the ideal 2.5µm.
At 350mm focal length, the solar disc would be 4.0mm in diameter. This requires 4.0mm / 2.4µm = 1667 pixels.
Do we have that? Yes, the ASI183 has 3672 pixels on the vertical.
Celestron NexImage Burst Monochrome (1280 x 960 @ 3.75µm)
The 3.75µm pixels are far larger than the ideal 2.5µm.
Celestron Skyris 236M? (1920 x 1200 @ 2.8µm)
The 2.8 µm pixels would be oversize (>2.5µm required for good resolution).
At 350mm focal length, the solar disc would be 4.0mm in diameter. This requires 4.0mm / 2.8µm = 1429 pixels.
Do we have that? No, the Skyris 236M has only 1200 pixels on the vertical.
Mallincam SSIc? (1270 x 1030 @ 3.63µm)
The 3.63µm pixels are far larger than the size limit of 2.5µm,
For the Lunt LS50 to image a full solar disc, suitable cameras appears to be the low cost ZWO ASI178 or the higher cost ZWO ASI183.
3. What's the best camera for a Lunt LS60?
Lunt LS60: 500mm focal length, 60mm objective lens, f/8.3.
Maximum Pixel Size = f# / 2.8 = 8.3 / 2.8 = 3.0µm.
ZWO ASI174 (1936x1216 pixels @ 5.86µm)
No, the large 5.86µm pixels are too big (>3.0µm required for good resolution).
ZWO ASI178 (3096 x 2080 pixels @ 2.4µm)
The 2.4µm pixels would be over sampling.
At 500mm focal length, the solar disc would be 5.71mm in diameter. This requires 5.71mm / 2.4µm = 2327 pixels.
Do we have that? No, the ASI178 has only 2080 pixels on the vertical.
ZWO ASI290 (1936x1096 @ 2.9µm)
The 2.9 µm pixels would be an excellent match to the required 3.0µm.
At 500mm focal length, the solar disc would be 5.71mm in diameter. This requires 5.71mm / 2.9µm = 1969 pixels.
Do we have that? No, the ASI290 has only 1096 pixels on the vertical.
ZWO ASI120 (1280 x 960 @ 3.5µm)
The 3.5 µm pixels are oversize (>3.0µm required for good resolution).
At 500mm focal length, the solar disc would be 5.71mm in diameter. This requires 5.71mm / 3.5µm = 1631 pixels.
Do we have that? No, the ASI120 has only 960 pixels on the vertical.
ZWO ASI1600 Mono (4656 x 3520 @ 3.8µm)
The 3.8µm pixels are too large (>3.0µm required for good resolution).
ZWO ASI183 (5496 x 3672 pixels @ 2.4 µm)
The 2.4µm pixels would be over sampling (ideal is 3.0µm).
At 500mm focal length, the solar disc would be 5.71mm in diameter. This requires 5.71mm / 2.4µm = 2327 pixels.
Do we have that? Yes, the ASI183 has 3672 pixels on the vertical.
Celestron NexImage Burst Monochrome (1280 x 960 @ 3.75µm)
The 3.75µm pixels are larger than the ideal 3.0µm.
Celestron Skyris 236M (1920 x 1200 @ 2.8µm)
The 2.8 µm pixels would be close to the ideal 3.0µm required for good resolution.
At 500mm focal length, the solar disc would be 5.71mm in diameter. This requires 5.71m / 2.8µm = 2039 pixels.
Do we have that? No, the Skyris 236M has only 1200 pixels on the vertical.
Mallincam SSIc (1270 x 1030 @ 3.63µm)
The 3.63µm pixels are larger than the size limit of 3.0µm,
For the Lunt LS60 to image a full solar disc, the only suitable camera appears to be the ZWO ASI183.
4. What's the best camera for a Coronado 60?
Coronado 60: 400mm focal length, 60mm objective lens, f/6.7.
Maximum Pixel Size = f# / 2.8 = 6.7 / 2.8 = 2.4µm.
ZWO ASI174 (1936x1216 pixels @ 5.86µm)
No, the large 5.86µm pixels are too big (>2.4µm required for good resolution).
ZWO ASI178 (3096 x 2080 pixels @ 2.4µm)
The 2.4µm pixels would an excellent match to the 2.4µm required for good resolution.
At 400mm focal length, the solar disc would be 4.57mm in diameter. This requires 4.57mm / 2.4µm = 1904 pixels.
Do we have that? Yes, the ZWO ASI178 has 2080 pixels on the vertical.
ZWO ASI290 (1936x1096 @ 2.9µm)
The 2.9 µm pixels are larger than the maximum size of 2.4µm.
At 400mm focal length, the solar disc would be 4.57mm in diameter. This requires 4.57mm / 2.9µm = 1576 pixels.
Do we have that? No, the ASI178 has only 1096 pixels on the vertical.
ZWO ASI120 (1280 x 960 @ 3.5µm)
The 3.5 µm pixels are too large (>2.4µm required for good resolution).
ZWO ASI1600 Mono (4656 x 3520 @ 3.8µm)
The 3.8µm pixels are too large (>3.0µm required for good resolution).
ZWO ASI183 (5496 x 3672 pixels @ 2.4 µm)
The 2.4µm pixels would be an excellent match to the ideal 2.4µm.
At 500mm focal length, the solar disc would be 5.71mm in diameter. This requires 5.71mm / 2.4µm = 2327 pixels.
Do we have that? Yes, the ASI183 has 3672 pixels on the vertical.
Celestron NexImage Burst Monochrome (1280 x 960 @ 3.75µm)
The 3.75µm pixels are larger than the ideal 2.4µm.
Celestron Skyris 236M (1920 x 1200 @ 2.8µm)
The 2.8 µm pixels are larger than the maximum size of 2.4µm.
At 400mm focal length, the solar disc would be 4.57mm in diameter. This requires 4.57m / 2.8µm = 1632 pixels.
Do we have that? No, the Skyris 236M has only 1200 pixels on the vertical.
Mallincam SSIc? (1270 x 1030 @ 3.63µm)
The 3.63µm pixels are larger than the size limit of 2.4µm.
For the Coronado 60 to image a full solar disc, suitable cameras appear to be the ZWO ASI178 or the ZWO ASI183.
5. What's the best camera for a Quark?
A Quark operates at at about f30.
Maximum Pixel Size = f# / 2.8 = 30 / 2.8 = 10.7µm
If one does not want to image a full solar disk, any of the above cameras will work but the ASI187 with 5.86µm pixels would be the closest match. However, many of the cameras can be binned 2x2, 3x3 or 4x4 to achieve a larger pixel size and may be a better match.
ASI174 (1936x1216 pixels of 5.86µm)
- binned 2x2 becomes 968x606 pixels of 11.7µm.
ASI178 (3096 x 2080 pixels of 2.4µm):
- binned 2x2 becomes 1548 x 1040 pixels of 4.8µm, or
- binned 3x3 becomes 1032 x 693 pixels of 7.2µm, or
- binned 4x4 becomes 774 x 520 pixels of 9.6µm
ASI290 (1936x1096 @ 2.9µm)
- binned 2x2 becomes 968 x 548 pixels of 5.8µm, or
- binned 3x3 becomes 645 x 365 pixels of 8.7µm, or
- binned 4x4 becomes 484 x 274 pixels of 11.6µm
ZWO ASI120 (1280 x 960 @ 3.5µm)
- binned 2x2 becomes 640 x 480 pixels of 7.0µm, or
- binned 3x3 becomes 427 x 320 pixels of 10.5µm
ZWO ASI1600 Mono (4656 x 3520 @ 3.8µm)
- binned 2x2 becomes 2328 x 1760 pixels of 7.6µm, or
- binned 3x3 becomes 1552 x 1173 pixels of 11.4µm
ZWO ASI183 (5496 x 3672 pixels @ 2.4 µm)
- binned 2x2 becomes 2748 x 1836 pixels of 4.8µm, or
- binned 3x3 becomes 1832 x 1224 pixels of 7.2µm, or
- binned 4x4 becomes 1374 x 918 pixels of 9.6µm
Celestron NexImage Burst Monochrome (1280 x 960 @ 3.75µm)
- binned 2x2 becomes 640 x 480 pixels of 7.5µm, or
- binned 3x3 becomes 427 x 320 pixels of 11.35µm
Celestron Skyris 236M (1920 x 1200 @ 2.8µm)
- binned 2x2 becomes 960 x 600 pixels of 5.6µm, or
- binned 3x3 becomes 640 x 400 pixels of 8.4µm, or
- binned 4x4 becomes 480 x 300 pixels of 11.2µm
Mallincam SSIc (1270 x 1030 @ 3.63µm)
- binned 2x2 becomes 635 x 515 pixels of 7.26µm, or
- binned 3x3 becomes 423 x 343 pixels of 10.89µm
ASI183 (5496 x 3672 pixels @ 2.4µm)
- binned 2x2 becomes 2748 x 1836 pixels of 4.8µm, or
- binned 3x3 becomes 1832 x 1224 pixels of 7.2µm, or
- binned 4x4 becomes 1374 x 918 pixels of 9.6µm
Suppose one wants to image the full solar disc with a Quark; is that possible? Yes, it is.
The original Quark has an internal 4.2X Barlow. The Quark operates best at about f30.
Without additional optical devices, Maximum Pixel Size = f# / 2.8 = 30 / 2.8 = 10.7µm
Suppose one couples the Quark to a 100mm focal length photography lens with the iris set to f8.
From the table, at 100mm the solar disk image is 1.14mm in diameter. Coming out of the Quark, the system is now a 4.2 x 100mm = 420mm H-Alpha telescope of f# = 4.2 x 8 = f33.6. The diameter of the solar disc on the sensor is 4.2 x 1.14mm = 4.8mm. Many of the above cameras, with binning, may be suitable.
It is also possible to place a focal reducer after the Quark. For example, one could use a 200 mm focal length photography lens with the iris set to f8. Coming out of the Quark, the system is a 840mm H-Alpha telescope of aperture f33.6. The diameter of the in focus solar disc is 4.2 x 2.29 mm = 9.6mm. Place a 0.5X focal reducer between the Quark and the camera and the system becomes a 420mm H-Alpha telescope of f# = f16.8. The diameter of the solar disc on the sensor is 0.5 x 9.6mm = 4.8mm. Here the Maximum Pixel Size = f# / 2.8 = 16.8 / 2.8 = 6.0µm. Many of the above cameras may be suitable, with or without binning.
I hope this article may assist you in choosing the best camera for your H-Alpha Telescope.
V/R
Rick