The Explore Scientific 0.7x Focal Reducer Corrector, aka Field Flattener - .7x Focal Reducer or FFFR, has no provision for mounting either 1.25" or 2" filters. Is this an issue for anyone else? How does ES suppose that customers attach filters when using their ES Field Flattener-Focal Reducer with a stock ES ED127APO, or other ES telescopes ?
Background and DIY Solution
I recently purchased an Explore Scientific ED127 APO refractor with stock 2" R&P focuser that I use for visual, EAA and to experiment with AP. I wanted to bring the ED127 down to F/5.25 for camera use, so I bought a used, complete and current model Explore Scientific 0.7x Focal Reducer Corrector. To bring my ZWO ASI224MC into focus I need two extension tubes on the focuser. The FFFR threads directly onto these extension tubes with an included adapter, and the camera attaches directly to T-threads at the other end. The two extension tubes were included with the ED127. Everything else is included with the FFFR to easily and securely attach a camera and use it with the stock 2" focuser and extension tubes. This set-up is 100% Explore Scientific except for the camera and is a very sturdy, rigid system. The FFFR kit also includes a 3" adapter for use with ES 152mm refractors.
However, there's one major obstacle. There is no way to attach a 1.25" or 2" filter. There are no internal threads included inside the focuser, extension tubes or the FFFR. Since the 1.25" ZWO IR-cut filter for the 224MC normally threads into the 1.25" T-threaded nose piece, it cannot be used with the FFFR in the above configuration. Also, I saw no way to place my 2" L-Pro filter in the light path.
I contacted ES Tech support to inquire about a solution to my filter mounting dilemma. According to the technician, they do not have an adapter or method of attaching filters. They offered to machine a flange with internal threads for a 2" filter, but this would cost me approximately $200. (ES ticket Reference: 556501-T8015). I declined and thanked them for their assistance.
Here are two suggested low-cost solutions for using 1.25" and 2" filters with the stock ED127 APO refractor, 2" stock R&P focuser, and ES FFFR.
To attach a 1.25" filter to a stock Explore Scientific FFFR set-up, ScopeStuff has an adapter that works. This 1.25 Inch Filter Adapter for T-Thread Capture, #TFA1, places the filter neatly below the T-threads at the camera. It doesn't add any distance between the sensor and FFFR optics. It fits perfectly and there's adequate clearance so the filter will not hit the FFFR lens. There is no interference with the light path.
Attaching a 2" filter is slightly more challenging. I used a ScopeStuff 2" M48 Filter Ring with Retainer, #TIFR, to place threads inside a spare extension tube. The inside diameter of the ES extension tube is just a hair over 2" (2.01" or 51.10mm). The OD of the filter ring is a hair under 2" (1.996" as per ScopeStuff). This M48 filter ring fits perfectly inside the extension tube. After removing some of the blackening inside the extension tube I carefully epoxied it inside the tube. A 2" filter sits nicely inside the tubes when threaded together. This cuts down the light path from 2" to 1.7", which is the actual diameter of the filter glass. I haven't tested this yet in use, so I'm not sure if this will cause vignetting. The 224MC has a small 1/3" sensor, so probably not. With a large sensor it might cause vignetting.
The total cost of these two solutions is about $25 for the parts from ScopeStuff. If you have a spare or damaged 2" filter, the ring might fit inside the extension tube to provide the threads for mounting a filter.
I can't imagine why the Explore Scientific design engineers did not include any 1.25 or 2" filter threads or some type of filter adapter for their $299 Focal Reducer Corrector. If this was a mix and matched agglomeration of various parts from several vendors, I would understand the absence of filter threads. But this set-up is 100% pure, stock Explore Scientific and intended for use during AP. I wasn't about to pay them $200 to custom fabricate an adapter to correct a missing feature in their product.
Edited by N2KEN, 08 February 2017 - 08:54 PM.