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How Good Are ED-APO Telescopes for Imaging?
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How Good Are ED-APO Telescopes for Imaging?
This article comes about because I purchased a Lunt 152mm ED-APO a few years ago. While it is excellent for visual observing - read the Cloudy Nights review by Bill Paolini - many have said that it would not be satisfactory for imaging. The reason for this is that these kinds of telescopes use 2 rather than 3 lenses, though made of the newer exotic glasses. It is possible with this design to correct three wavelengths (See Telescopes, Eyepieces, Astrographs by Smith, Ceragioli, and Berry, ch. 6), making them apochromatic, though this is difficult to realize at shorter focal ratios. The color “errors” will fall in the deep blue and far red wavelengths, where the human eye is much less sensitive. The following image from Wikipedia shows the extreme range of the eye’s color sensitivity:
Figure 1. Sensitivity of the Human Eye
So since I own the Lunt, I decided that there is no point in talking about the problem when you can actually investigate in order to find out how serious it is! I decided to compare three telescopes to determine how they image two stars: Vega (type A0) and Albireo (types K3 and B9). Albireo turns out to be the difficult test because of the color contrast of its two components. The fainter component, Albireo B, is coincidentally a B-type star, which accounts for its readily observed blue color. The brighter star, Albireo A, is a K type star, with a yellowish-orange color typical of such cooler stars. Vega, as a type A star, is not as hot and not as blue as Albireo B. This turned out to be apparent in course of the tests that I did.
I did imaging with the Lunt 152mm, an Explore Scientific 102mm FCD APO (their flagship scope in the 102 aperture range), and a Celestron 8” (200mm) Edge HD SCT. The result that I expected was that the SCT would be best, followed by the APO, and then the Lunt, with respect to image quality. The actual results were a little different. We’ll start with the Albireo images here because they tell much of the story.
I took photos using a Mallincam DS16c Skyraider, at exposure times set to ensure equal light collection by each scope. This camera has fairly small pixels (3.8 microns), with the result that the images are very slightly oversampled. The camera was fitted with a Baader UV/IR/L cut filter for all images (essential for imaging with any refractor, APO or not, because no objective lenses are corrected for UV and IR light). Of greatest interest are the photos based on a 10 second exposure with the Lunt, which corresponds to 22.2 seconds with the APO, and 6.9 seconds with the SCT. To see the difference, I enlarged the images so that the image scale is the same on all. I used a 250% enlargement of the Lunt image, and corresponding image sizes on the others. The images are shown below, starting with the SCT. Together with each image is a line profile drawn though the axis of the A-B pair. (Maxim DL has this great feature). No one would ever make such an enlargement, which clearly show pixilation, and normal size images will be shown below. But the enlargement allows us to really see the differences among these scopes with respect to images. The image and profile for the SCT are given in Figures 2 and 3; for the Lunt 152mm, in Figures 4 and 5; and for the ES 102mm, in Figures 6 and 7.
Figure 2. Celestron 8” Edge HD Image of Albireo
Figure 3. Line Profile for Celestron 8” Edge HD Image
Figure 4. Lunt 152mm Image of Albireo
Figure 5. Line Profile for Lunt 152mm Image
Figure 6. Explore Scientific FSD-100 102mm Image of Albireo
Figure 7. Line Profile for ES 102mm Image
Some interesting things emerge from these images. First, the SCT has the best color correction, as expected. The stars have fairly uniform color rings that are not wide. The line profile shows extremely good color correction for both stars, very symmetric, with a nice, wide separation between them. The Lunt image is good but not quite as good. First notice the wide blue ring around very blue Albireo B. This is due to the fact that blue is not brought to the same focus as most other colors. When blue is focused, other stars become defocused—I did this test. That is the main color penalty. There is also a very slight blue patch on the bottom of Albireo A, noticeable because of the predominantly gold color. The line profile shows this as well: there is an extra wide blue outline on the left star, and the slight blue deviation on the falling side of the right star profile. Visually the separation of the stars is not quite as great on the Lunt image, though still very good. This shows on the line profile as well. Visually, none of this can be seen—the image is very sharp and the colors well defined. Partly the difference in the separation is due to diffraction, since we are comparing telescopes of different aperture. However, this is likely only a small part of the difference, because the resolution of even the smallest of the scopes, the ES102 mm, is close to 1”, and the separation of the two stars is 35.3”.
The real surprise was the APO image. Albireo B illustrates the problem with the color correction—it is reddish at the top, blue at the bottom. The color error distorts the shape of the star somewhat, as can be seen. The star to the left of Albireo A indicates that there was a small tracking error, about 1 pixel, but that would not be enough to account for the shape of Albireo B, or the color glitch. Curiously, a shorter exposure, slightly out-of-focus image showed a rounder Albireo B, though still with the red at the top. The problems, whatever they are, also affect the separation, which shows visually and in the line profile. The line profile shows a problem bringing red and green to the same focus as blue. The stars appear much fatter as well, especially as compared to the separation. At first I thought that this might be a tracking error, or a focus error, but it shows up in shorter exposures as well. I did the images of Albireo with this scope three times, focusing carefully; and I checked the collimation. I send the image to Explore Scientific, but never heard from them, so I assume that this is within specifications. Visually, however, the images in the eyepiece are sharp and clear, no suggestion of any color problem. In fact, the refractor images appear better and sharper than those of the SCT.
Turning now to lower magnification images, we consider first a full-frame image from the SCT, which is approximately a reduction by a factor of 4 from 100% mapping. This is more like what an image printed or displayed would look like. Refer to Figures 8, 9, and 10. Figure 8 shows the SCT image, full frame, though considerably reduced for reproduction here. An image from the Lunt, cropped to show about the same area, is given in Figure 9. This image, naturally, is reduced less. Finally, consider the corresponding image from the 102mm APO, shown in Figure 10. What these three images show is that at “normal” display magnifications, there is very little difference between the scopes. If you look closely, Albireo B looks slightly bluer in the Lunt image, due to the focus issue discussed above. But it is barely noticeable. The APO image does show the two stars as larger, though the effect is not great.
When looking at the images of Albireo, I discovered another double, much smaller, about 25 arc minutes to the NW of Albireo itself, and at the upper left corner of the image. This turned out to be SAO87325, curiously another blue-gold double, though with the colors reversed. It has a separation of 5.4” and magnitude 7.6. A blowup of the Lunt image of this little double is shown in Figure 11, showing the two stars clearly resolved, demonstrating the flat field of this telescope for imaging with modest size sensors (the Mallincam has a 22.2mm diagonal sensor, giving a 23 x 31 arc minute field). The pair is not resolved in the ES 102mm image (see Figure 12). Unfortunately this star is out of the field of the SCT images.
Figure 8. Full Frame Image from 8” SCT
Figure 9. Lunt 152mm Image Showing Same Field as SCT Image
Figure 10. ES 102mm Image Showing Same Field as SCT Image
Figure 11. Blowup of SAO87325 from Lunt 152mm Image.
Figure 22. Blowup of SAO87325 from ES 102mm Image.
The Vega images show much less variation, and in particular, the Vega images taken with the ES102 do not seem to have the problems seen in the images of Albireo B, presumably due to the fact that Vega is not as blue as Albireo B, and so the blue color convergence problem is not as severe. Figure 13 shows the Vega image from the Celestron 8”, and Figure 14 a line profile through it. Both the image and the line profile show good color control. The line profile is very symmetric. The image shows the ray pattern typical of SCT images. Figure 15 shows the Lunt 152mm image of Vega, where the blue edge is less pronounced than in the Albireo B image, as would be expected with a less-blue star. The line profile in Figure 16 shows good color control, with slightly more asymmetry than the SCT image but not much. Figure 17 shows the ES 102mm Vega image. The line profile is given in Figure 18. It shows good symmetry, slightly better than the Lunt 152mm. I do not know why this image looks so much better than that of Albireo B; the fact that Vega is less blue than Alibireo B explains part of the difference, but would not seem to explain all of it. In any case, the image of Vega is quite good.
Deep Sky Images with the Lunt 152mm
Just as an example of some longer deep-sky imaging, Figure 17 shows M13 taken with the Lunt 152mm and a QHY8L camera, 3 30 second images stacked. The red lines are due to a readout problem in the camera. Figure 18 shows the Crescent Nebula with the same equipment, however, it utilized a focal reducer, giving a wider field of view but some vignetting, which is not completely corrected in this image.
What are the conclusions from this exercise? (1) The Lunt 152mm and presumably other ED-APO designs can be used for astrophotography, provided that significant enlargements are not envisioned. Since almost no one does this, and no one likes to see pixels in photographs, it is not a serious problem. For Electronically Assisted Astronomy (EAA, aka “video astronomy”), it would be fine. However, if absolutely the best color fidelity is required, a reflector or top quality APO may be preferable. I did not have at my disposal a TEC or TAK or AP APO, so I cannot speak for their image quality, though I am sure that it is extremely good. Whether the full APO scopes are worth the 3x price is something that each purchaser will have to determine. (2) Not all APOs are created equal. The performance of my ES 102mm may not be typical of other scopes made by ES, or even of other copies of the FCD-100. Still it is surprising that this level of performance passed their quality control. (3) If you’re not sure about the quality of your imaging system with respect to color, try shooting Albireo and inspecting the image in Maxim DL, using its fine toolset.
Figure 13. Celestron 8” Image of Vega
Figure 14. Line Profile for Celestron 8” Image of Vega
Figure 15. Lunt 152mm Image of Vega
Figure 16. Line Profile for Lunt 152mm Image of Vega
Figure 17. ES 102mm Image of Vega
Figure 18. Line Profile for ES 102mm Image of Vega
Figure 19. M13 Imaged by Lunt 152mm
Figure 20. Crescent Nebula, 780 seconds, with Lunt 152mm and QHY8L camera
- Ken Sturrock and the Elf like this