The Meade LX850 ACF f/8 are a class of telescopes that have not been widely popular among astrophotographers, but they deserve attention. While these optical tubes were released a few years ago, information is very scarce and there has not been an in-depth review of how they actually perform. As a result, the user base has remained small.
Description of the tube
The LX850 ACF f/8 is a modified Schmidt Cassegrain telescope that features an aspherical secondary mirror which delivers Advanced Coma Free (ACF) performance. Standard Schmidt-Cassegrains have a heavily curved focal plane as well as coma, while the Meade ACF design offers a well corrected field for APS-C format and smaller. This is a crucial point for astrophotography. Additionally, the focal ratio of f/8 makes this tube feasible for astrophotography as the exposure time is almost halved compared to the classic f/10 Schmidt Cassegrain configuration.
The secondary mirror is large, and the obstruction is higher than the classic Schmidt-Cassegrain, makes this tube less ideal for high-resolution planetary photography but still usable due to its large diameter.
The tube itself is rock solid, and the dovetail bar connects to the tube using heavy duty curved supports that redistribute the overall weight and reduce flexure. In contrast, most Schmidt-Cassegrain telescopes have a much narrower interface between the bar and tube.
The focuser includes an 8:1 reduction gear for micrometric focusing, however there isn’t an adapter to connect the focusing knob to a focusing motor, and therefore it is necessary to install a complete robotic unit in order to achieve the maximum focus sharpness. Precise focus can be reached by hand but nowadays modern autofocusing solutions have become a must-have feature for serious astrophotography.
The weight of the tube on its own is 18.6 kg. Once my Eagle, camera, focuser and filter wheel were added, I probably surpassed the 21kg mark, however that weight is still manageable.
Accessories and mount used
I had the chance to try the 12” OTA which has a focal length of 2438mm, making it ideal for small objects but also for close-up pictures of popular larger objects. Such a long focal length demands a capable mount. For this test, I used the 10Micron GM2000. Its precision achieved by absolute encoders allows you to avoid guiding while maintaining sharp and round stars during long exposures.
For a camera, I used the QHYCCD 268C. Its 3.76µm pixels give a pixel scale of only 0.33”/pixel. While this scale is too small for any real-world seeing conditions, it puts the telescope under scrutiny, revealing any defects and aberrations.
During my test, I used the Prima Luce Lab Esatto 3.5” which has a small back focus footprint. The telescope is also compatible with the Esatto 3”, however that has a much larger optical thickness. Unfortunately, Meade did not disclose the minimum and maximum back focus requirements, although it is sufficient for most accessories. With a focal ratio of f/8, the back focus is smaller when compared to the back focus of the classic Schmidt Cassegrain telescopes but it is still manageable.
To be able to comfortably reach focus, firstly I placed the Esatto 3.5” at half of its travel and then I shifted the mirror using the focusing knob until I reached approximate focus. In this way the Esatto has sufficient travel leftover to achieve fine focus.
Living in Sydney under a relatively polluted sky, I used the Antlia 5nm Golden Dual Band filter that has a band pass of only 5nm for both the OIII and Ha.
Using a telescope with such a long focal length blew my mind! Initially I pointed at the Eagle Nebulae; while with my Newtonian (640mm focal length) I could barely see the shape of the Eagle, with this setup the Eagle was large and full of details.
Then it was the Helix nebula’s turn, which filled the entire field of the camera’s APS-C sensor. As you can see in the image below, the details are very sharp.
The Helix Nebula (NGC 7293) captured using the Meade 12" ACF f/8, Antlia ALP-T Golden Filter, QHYCCD 268C, 54x300"
Compared to a Ritchey Chretien, the tube is enclosed by the correcting plate, so it is easier to keep the mirrors clean. Also, a closed tube is less affected by air currents and turbulence which can negatively affect the picture. The Meades do not show the typical diffraction spikes from the spiders of the secondary mirror as you would find on a Newtonian, which is advantageous as it makes the images appear sharper and the post processing easier.
In one of the nights of testing, the humidity was so high that I decided to use a dew heater around the correcting plate at maximum power. This was a mistake! The heat generated was sufficient to cause the stars to appear elongated due to the air currents! Once I turned off the dew heater and let the tube cool down, the stars became round.
While slewing the telescope to different areas of the sky, the image remained almost in perfect focus. This means that the mirror flop is minor. Also, during my 300s subs, stars remained perfectly round. If you will not use a 10Micron for tracking, you will need to guide, preferably with an off-axis guider: long focal length telescopes, especially if they employ mirrors, cannot be guided with a regular guide scope as that will not take into account differential flexure.
The field is well corrected for APS-C sensors, but collimation has to be spot on (as with every Schmidt-Cassegrain). Initially, I collimated the telescope using the OCAL, followed by fine-tuning using an out of focus star. In this way, it is possible to check if the secondary mirror shadow is centred, hence the tube is collimated. Lastly, I took a 30 second exposure to check the quality of the stars at the borders of the field. One of the corners of the frame showed some elongated stars, so I fine-tuned the collimation with the corresponding secondary mirror screw. The adjustment needed is only about a 1/8th turn of the screw. To determine the screw to adjust, it is needed to look at how the camera is oriented, identify the corner of the field that shows elongated stars, and identify the secondary mirror collimation screw that it is on that same corner.
This is going to be a keeper, and once you will go for long focal length telescopes, you will rarely go back. The natural evolution of astrophotography goes toward longer focal lengths because we all want to capture galaxies and planetary nebulae. There are very few objects that are so wide to require a lower magnification, but with longer focal length is possible to capture detailed views of these wide field objects (and you can eventually construct highly detailed mosaics).
Using a long focal length telescope is exciting. As with any other long focal length telescope, you need to take into consideration several factors that can affect your imaging:
- Excessively hot dew heaters that can disturb the image, causing the stars to appear elongated
- An off-axis guider must be utilized
- You can forget taking any decent pictures when there is wind
- If the seeing is very poor, then images will not be as sharp as other nights
But as far as long focal length telescopes go, the Meade LX850 ACF f/8 is an excellent choice: solid, sharp, closed tube, no diffraction spikes and much cheaper than any f/8 astrograph with a similar focal length.