In this rather longish post, I am going to answer the elusive question of Meade's back focus and how flat a field does the scope produce - read on if interested!
I recently bought a used Meade LX850 10in F8 ACF telescope with the view to use it for remote DSO imaging. Being excited, I started a post on this forum to announce to the world that I was finally moving away from refractors and also to ask a couple of questions. Additionally, I was wanting to put Moonlite's NC on the back of the scope (I already own the NC, just required a Meade ACF 10in scope's flange from Ron at ML).
I was met with a couple of questions about the scope's specifications (primarily back focus and field curvature). I have to be honest, when I purchased the scope, I had no clue what I was getting in to, so those questions came as a jolt and I started questioning my decision to purchase the Meade scope for remote imaging. The two biggest questions were - what is the published back focus for the scope (and by extension, if I could use a decent focuser and camera system with this telescope that would be operated remotely) and whether the scope produces a flat field for larger sensors (again, I had no clue that these scopes were not corrected for field curvature). It really started to look like a bad decision, to buy the scope.
To answer the above questions, I searched the web. I first started looking for images on Astrobin. That resulted in bringing up images by 2 people. One of them has exceptional images and even the second person's images are decent (at least the underlying data seems to be really good). They both seemed to be using a 4/3 sensor and achieving a fairly flat field. I don't know any other specifics - like if they are using a reducer or a flattener in their imaging train. Of the two people, one is using the STT8300 with FW and AO. That system by itself uses up 103mm of back focus, but I don't know what focuser that person is using - so there is no way for me to know what the actual back focus is. Regardless, I would still wanted to know if Meade had published the scope's back focus anywhere - but there is nothing on the web or on Meade's website.
Next, I called up a couple of my imaging friends - primarily Chris F. Both live in the Austin area. Chris actually uses a Meade F10 scope for imaging (he recently made a post about resurrecting his Meade F10 for DSO imaging on this forum. While Chris did not have an exact answer for the back focus (he said his camera hangs about a foot from the flange of the scope) - he did mention that when he used a full frame camera (DSLR) for testing, he could see that most of the field was pretty flat (maybe just a bit of curvature at the extreme edges of the field, but nothing that couldn't be cropped out). His current set up includes a 4/3 sensor (ZWO1600) and that produces a flat field. That seemed to be consistent with the images that I was seeing on the web. He also mentioned that he uses two reducer/flatteners - AP27TVPH and Optec Lepus. Given that his scope hangs at about a foot from the flange, that changes the focal ratio at which he images - makes it F12 (native). With the two reducers that he uses, he can bring that down to around F8 or F5, depending on how he uses it. However, again, while encouraging, this did not give me Meade's take on the back focus for the scope itself. This being critical because I wasn't sure if I could use the NC and achieve automation. One thing that was becoming quite apparent was that a 4/3 chip would be the most workable chip with this scope without having to worry about field curvature.
Now, I have heard about the non-existent Meade's customer service. So, I grudgingly put a call in to Meade, not sure what the expect, but hoping to find out the back focus for this particular scope (Meade's website lists the back focus as "TBD"). I got through to customer service, however, the person on the other end did not know the back focus - but he promised to call me back within 30 minutes after consulting their engineering department. A couple hours pass by and I am thinking, the customer support guy had punted and I would not be getting a call back, but after about 3 hours. I had a VM - the back focus is 4.56in from the back flange. They were even courteous to send me a diagram (attached)! So much for the "non-existent" customer service. I am not sure why that is not published on their website, but I am a happy camper as of now.
Coming back to my decision though, getting to know that the back focus is just 4.56in was a big bummer - the NC is 4.5in by itself. No way I can attach a camera to the back of this scope and hope to achieve anything. So, I was back to square one - the decision to get the scope was looking bad.
That is when I decided to call Tolga. I had heard that Tolga had recently set up a system using the 14in version of this scope and NC - I wondered how he'd done that. Tolga mentioned he was not aware of the 4.56in back focus requirement, however, it was a fact that he had set up the system. But given that Meade had claimed the back focus to be 4.56in, he too confirmed that the NC would not be a viable option. In desperation, I asked him, what could be done to automate focusing - and viola - Tolga mentioned that the existing focuser on the Meade LX850 system is rather robust and that I can use a solution provided by Optec - DirectSync ACF to automate focusing. I have to admit, I had no clue such a thing existed. There just isn't material on the web regarding these scopes. So, this came as a pleasant surprise.
So, I am hoping that the Optec solution (thanks Tolga) along with me settling on using a 4/3 sensor (most likely using the STT8300) will help me use this scope for remote DSO imaging. I am looking forward to receiving the scope and testing it before commissioning it at the remote observatory. Hopefully, I will start using it later this summer at which point, I hope to provide images once I have it fully commissioned. Keeping my fingers crossed! More to follow ......