So here it came – new kid on the block. 130mm triplet went to new happy owner, and was exchanged with this 10″ barrel. There were several reasons for this, but mainly I wanted bigger aperture that will still fit my astroshed and my EQ6 mount and allow me to grasp more light for photometry. In the same time I wanted still to have some pretty imaging possibility and also visual ability. All these criteria narrowed choice to a few instruments, and eventually I have chosen LX200 Meade ACF 10″ f/10 optical tube. I also chose Astro Physics CCDT 0.67x reducer and ACUN 3.3″thread Crayford focuser as accessories. There is also f/8 version with integrated Crayford focuser, but it is significantly heavier and more expensive.
ACF 10″ tube is pretty large and heavy (12.5 kg OTA with Losmandy dovetail). Main mirrors in ACF telescopes are little bit oversized – that is one reason for bigger weight. One inch larger Celestron SCT have similar weight. Why ACF? ACF stands for Advanced Coma Free. In this construction secondary mirror shape is different (hyperbolical), so all coma was eliminated from the image. However field curvature remains. Tube looks very much like classic Cat – there is nothing special in the front of telescope – corrector plate, secondary mirror cell with collimation screws, and that’s it. Also in the back there is pretty much nothing impressive. Rear cell thread is 3.3 inch with adapter to classic small SCT thread. Plus one handle and two knobs – for focusing and mirror lock. Tube was assembled in Mexico. Inner tube is well blackened and has nice small baffles inside. Clear aperture is 2 inches, so for example 2 inches telecompressor can be inserted into it. Secondary mirror cell obstruction is 87mm by diameter.
Next step was installing tube in the shed. EQ6 mount was already equipped with WO Losmandy saddle, I would not even try to put such telescope on Vixen dovetail. Two standard counterweights were not enough to balance telescope with camera and dewcap. I did not wanted to put more weights there, I just extended counterweight rod and that’s how I gained these few missing centimetres to move. Telescope on the EQ6 mount looks and behaves pretty good. It is stiff and stable enough and also motors have no problem with moving such weight. The 10″ ACF tube is more less the same weight as my 130mm triplet before, but due to larger diameter the balance is moved away from the mount head. But on the other hand the tube is shorter and momentum in declination axis is lower. No problem popped up while installing tube in the shed. OTA with installed dewcap takes some significant amount of space, but there is still much room behind for comfortable position when observing visually or operating with camera.
One week after OTA installation I had a chance for first light (so far only visual). I installed finderscope, 2″ diagonal and put 34mm eyepiece to it. I aligned mount with three stars: Rigel (pretty split to two components, very nice view), Castor (also split with ease) and Caph. I needed to adjust collimation a little with 1/2 turn of one collimation screw, then it was all perfect. Sky was not quite clear this night, I estimated NELM to 5 mag. Then I spent about two hours on observations: magnificent M42 with large Trapezium, M78 first time visible, few planetary nebulae and small open clusters in Gemini and most prominent galaxies in Andromeda and Triangulum. I am pretty happy with the image quality I get from this tube. Image is sharp and stars are pinpoint almost to the edge of field of view (I used ES 68* eyepieces). Largest eyepiece I have is 34mm and that gives 3.3mm exit pupil. For the suburban sky I had at my backyard I think it is the largest useful exit pupil. The sky background is already pretty bright at this value. But for darker sky some longer eyepieces would be recommended, however it comes at a cost of field of view. 10″ f/10 SCT has pretty long focal length, and the clear aperture is limited to 2 inches. So there is no way to achieve large real field of view without vignetting. You can gain some percent using reducer inserted into the telescope back, but not so much.
One thing that I did not like is unfortunately mirror shift during refocusing. It is barely visible when temperature is high, but at lower temperatures the grease thickens and mirror shift becomes obvious. So for some serious work you need to consider additional focuser. This is what I am going to purchase, because there is no way of comfortable imaging without good focuser.
Meade ACF f/10 10″ is definitely not a tube for wide fields observing and imaging. This telescope (as much as all SCT type telescopes) are pretty convenient for small and medium sized targets and of course for planetary observations and imaging. They are also relatively lightweight and compact for given aperture. So it will actually work well under my mediocre suburban sky, where there is no way to catch large and faint fuzzies anyway.