To be honest, I never thought I'd have the privilege of mucking about with a 5" triplet refractor. I love this job!

Introduction

The saxon (note the lower case) FCD100 series comes in three sizes, an 80mm, a 100 and the monster 127. It's this last one (www.saxon.com.au/telescopes/ed-refractor-telescopes/saxon-deluxe-127mm-apochromatic-air-spaced-ed-triplet-refractor-telescope.html) I'm talking about here.

This is a variant of the scope sold as the Explore Scientific 127ED (https://explorescientificusa.com/products/127mm-fcd100-apochromatic-refractor), however there are a few differences. The focuser on the saxon is the standard barrel-type, where the ES model has a hexagonal focuser, which may be rated to a higher weight limit.

First, the basics. This is a five-inch air-spaced triplet apochromatic telescope with Hoya FCD100 ED glass. That sentence alone should have you either bewildered or drooling. Suffice to say, it's a large, high end refractor.

Its focal length is a long (for a refractor) 952mm, giving it a focal ratio of f/7.5. This gives the scope a good level of flexibility - longish focal length for magnification, and low focal ratio for dim objects.

Extending this, you can get a 0.7 reducer flattener for the scope as well, and this reduces the focal length to f/5.25, potentially making the scope excellent for larger deep sky objects.

Market placement

The saxon FCD100 127 is a triplet apochromatic refractor.

Simply put, "triplet" means is that there are three separate lenses in the cell up the front. This gives the telescope designers the ability to tune the refraction of the group so that red, blue and green light all focus at the same point. Having three lenses gives the telescope its "apochromatic" ability, rather than "achromatic", which means that only the red and blue light focus at the same point.

Without this "apochromatic" (APO) ability, blue light focuses closer to the objective lens and red light focuses further from it. This causes bright stars to have a blue coma, or fringe around them, when the whole image is in focus. Getting rid of this "chromatic aberration" is highly technical and expensive.

The 127 is the largest of the saxon FCD100 range, which comes in a very cute 80mm as well as a 102mm version. I haven't played with the 102, but I think the 80mm would be a fantastic grab-and-go triplet APO, the right size for an HEQ5 mount with a small autoguider. That'd be a great and highly portable rig.

There are lots of other triplet APO refractors around, and all are very pricey.

As a comparison, the Sky-Watcher Esprit 120ED has a smaller aperture and sells for about $1000 more than the saxon. However, the Esprit comes with a field flattener that is an extra on the saxon for around $500.

The verdict

This scope is an entry by saxon into the high-end refractor market. At the same time, and this will become obvious shortly, it's also an example of "the cheapest house on the best street" strategy. I haven't spoken with saxon about this, but it appears to me personally that the company's general approach is to support the lower, but aspirational and learning members of the astronomy community.

This product is about the cheapest way of getting you a gigantic triplet APO. There are drawbacks, which I'll get to, but users will either manage these (and they're all very manageable) or eventually upgrade parts or the whole scope to a Takahashi, Astro-Physics or other super high-end brand. Thinking about this, it's my belief that this strategy is good for the whole market, and good for astronomers in general. A discouraged beginner is bad all around.

Overall, the telescope is very impressive. It delivers in both quantity (with a huge 127mm aperture) and quality, with the optics being superb. There are a number of issues, however, none of which, for me, at least, would be a deal breaker. The scope is tricky to handle and convert between visual and photographic configurations; you cannot easily use a DSLR with the flattener; and a couple of cost-saving efforts are noticeable.

Glass

Oh my, this is the part where this telescope sings.

I took some test photos from my front yard in Melbourne. Because I haven't had much experience with this particular scope, the images would improve with repetition. However, they show that the telescope sucks in a large amount of light and puts it accurately and crisply on your sensor.

Overall about the glass

If you get this telescope you're going to get some seriously high quality glass. And there's acres of it. The scope comes with a cell made up of three five-inch lenses, one of which is a Hoya FCD100 ED (extra low dispersion) lens. Both the triplet design and the low dispersion glass are intended to reduce chromatic aberration.

A single lens made of glass (or pretty much anything) can't focus white light to a true point. This is because the different wavelengths (grouped as red, green and blue) don't refract (bend) to the same amounts in the one lens. The red light focuses further away than the green light, and the blue light focuses closer than the green. This means that there's going to be colour fringes around bright and contrasty stars, especially at the edges of the field of view.

The triplet design (using three different shaped lenses in the front of the scope) is intended to eliminate chromatic aberration by making the different colours focus at the same point.

Hoya claim that their FCD100 glass has a lower dispersion than Ohara's FPL53, and this lower dispersion further decreases chromatic aberration. I don't know if that's measurably the case, but I do know that they're both considered to be very good.

NGC 4755 (Jewel Box )

I set up the scope to look at a star cluster (in this case NGC 4755, the Jewel Box). Here are a few 4 second test shots at ISO 800 taken with my Pentax K3-II at prime focus (that is, with nothing between the sensor and the telescope's objective). They're also cropped to 1200x628 pixels, meaning a pixel on the image is close to a pixel on your computer's display - depending on your browser and some other things.

First, this is the unprocessed image.

Next, I pushed the exposure a bit using the levels tool in Photoshop. I pushed the left slider a single stop to fractionally darken the background and moved the median towards the left, brightening the whole image.

The stars are brighter and more numerous, and there's more noise in the background, as you'd expect. Because I'd deliberately underexposed to preserve colour, the stars aren't blown out and the colour response is very good. There's not a trace of chromatic aberration, but I suspect it may either be atmospheric in origin (I'm living with considerable light pollution) or other dispersion.

In the third photo I've pushed the processing to the point where the stars were starting to blow out. Instead of using Photoshop's levels tool, I used the curves tool to give a non-linear stretch, so the background remained dark. Still, there's no sign of chromatic aberration or blue fringing, although there may be a fringe towards the bottom of the brightest stars, which may indicate some flop in the focuser.

Overall I'm very impressed with this test. Remember it's a 4 second exposure onto a DSLR. The saxon performs well in both quality and quantity. Clearly a 127mm aperture delivers a hell of a lot of light.

NGC 3372 (Eta Carina )

As an encore, I decided to see what it was like on a nebula. The Carina Nebula was the obvious test subject, not being far from the Jewel Box, and also not being my neighbour's roof.

Again, this was a single exposure (in this case 10 seconds) at ISO 800 with my Pentax K3-II DSLR. Out of the camera there 's no nebulosity evident, just a star field, which is what you would expect for a 10 second exposure.

With some very basic processing, the nebula just pops out at you. However, light pollution has affected the image fairly badly. You can see it in the dark areas, which aren't really dark. Of course, this is not the telescope's fault. The colours that the optics delivers are great, with the yellow star on the left contrasting the nebula's pink hydrogen and blue oxygen clouds. The Keyhole Nebula (the bubble in the middle left) is nicely defined and there are bok globules above the Keyhole.

To say I was impressed by a single 10-second exposure like this from my own front yard would be an understatement.

To see what else I could get, I set up a sequence of 170 similar 10 second images, and processed them using Astro Pixel Processor. The result isn't hugely different, although I was able to eliminate some of the light pollution and most of the noise. Again, it isn't really the telescope doing the heavy lifting here, but the post processing. If you look closely, I think that I have over-sharpened in Astro Pixel Processor.

Overall, the test shows that taking the scope to a dark sky site and having some experience with it would produce some first class images.

Ergonomics

This is a big heavy scope, there's no other way to describe it. Seriously, it's a monster. The tube alone is nearly 8kg, so even without guide scope, diagonal or DSLR it's pushing the photographic limits of either an HEQ5 pro or Advanced VX mount.

The air spaced triplet cell is a huge piece of glass, and it means that the balance of the tube is significantly to the front. This does cause problems. When I tried to balance the scope on my mount, I found that I had to set the tube rings a long way forward. This only became a problem when I tried to take the lens cover off. With the rings in that position, the retractable hood couldn't retract, meaning I couldn't reach the lens cover to take it off.

It took quite a bit of juggling about with the positioning of the tube rings before I could retract the hood and balance the scope. If I were to be using this scope on a regular basis, I would consider adding a diver's weight to the rear of the tube to rebalance it.

Dovetail

Continuing on the theme of balance, the scope is supplied with a 200mm Vixen sized dovetail to clamp on to mounts. However, this telescope is meant for both visual and photographic use.

As such, it's going to have quite a lot of equipment swapping going on - one minute you'll be racked in with a 2" diagonal and a short eyepiece, and the next you'll be racked way out with spacers, and on the end of the train you'll have a heavy DSLR with an intervalometer or maybe a refrigerated CMOS with a big filter wheel and an OAG with a second camera.

The difference in balance for the whole scope for these two scenarios is going to be significant, and I can easily see a situation where simply sliding the dovetail up and down the mount's saddle is not going to be enough.

I happen to have a 350mm Losmandy-sized dovetail that would be perfect for the job. Having this rather than the 200mm Vixen-sized dovetail would significantly improve the ergonomics of the whole scope.

Focuser

This is probably the telescope's weak point. There are a number of reports around about the focuser that is supplied with the saxon. There are two problems that I've noticed with the test scope, and which have been discussed with other focusers.

Smoothness

First, the movement of the focuser isn't smooth. When you run the coarse wheel in or out, you can feel a bit of grinding. I want to stress that this does not affect the operation of the focuser, and I don't know what the cause is, but other focusers have also had the same problem, both in Australia and the United States. I'm hoping that it's a bad batch of focusers and that the problem will be solved in the near future.

Travel

Second, travel - there's not enough. The focuser only racks out 48mm. That's possibly what you'd expect from a Newtonian, but this is a refractor. My own scope is a 107 triplet, and it racks out to 110mm.

To focus on infinity with my little 20mm 1.25" test eyepiece and the diagonal, I needed one of the two 37.5mm spacers supplied with the scope. The focuser was racked out to about 26mm.

It's best to have as little between the sensor and the objective as possible for prime focus photography. So to focus on infinity with my DSLR I needed both spacers and even then the focuser was racked all the way out to 44mm. This is probably the cause of the tiny bit of flop that was evident in the test photos.

Having a spacer on the draw tube provides the ability for the user to add a filter wheel and off-axis guider without having to have a long draw in the focus. However, I would have expected the focuser in a telescope of this level to have a longer travel.

Focuser overall

The focuser works adequately as it is. Someone who bought this telescope would possibly consider upgrading the focuser to one with longer travel and a better action as part of a future upgrade. However, it's not a deal breaker.

The smoothness isn't evident when you're using the fine adjustment, just the coarse. Perhaps it would be a useful focuser for autofocus, where the coarse wheel is turned by a stepper motor that has masses of torque.

As a workaround, if it turns out that there's a crunchy bit right where you're focusing and it's really bothering you, I'd advise adding a small spacer to the train so that the focus happens at a different point. This would fix the problem at a very small cost.

Flattener

The saxon FCD100 range is essentially a clone of the Explore Scientific telescope. There are a few differences, such as the focuser, but one thing that is the same is the flattener. Saxon and ES don't really have a flattener available for the 80mm or the 102mm versions, but there is one for the 127mm.For this test, I wasn't able to have the flattener - they're not all that common.

The flattener for the 127 probably does a good job, but it's not easy to get it attached to give you good results. Out of the box, the camera side ends in a male M54, which is an unusual thread for a flattener. The flattener comes with an M42 adapter, but this adds 3mm to the imaging train. This is a problem.

If you have a DSLR and a standard M42 t-ring, this arrangement provides a 55mm gap between the flange of the t-ring and the camera's sensor. This is the standard design.

The problem is that the flattener provided has a 55mm back focus - from the end of the flattener, not including the 42mm adapter. This means - as far as I can tell - it is impossible to attach a DSLR to the flattener to get the correct back-focus. I have seen test images taken using this equipment anf they are horrible.

This does not apply if you are using a CMOS or CCD camera, of course. You simply use spacers to suit. Similarly, if you have a mirrorless camera you can get a short t-ring, commonly known as a t-minus, and add spacers as required.

I would consider that having a 52mm back focus (from the M42 adapter) is a serious design flaw.

Diagonal

We received a couple of diagonals with our FCD100 samples (the 80 and the 127). During the setup, one of the diagonals got a bit crooked in the draw tube (it's a very snug fit) and as I was "easing" it out, the eyepiece holder came away. It turns out that this part is not attached to the triangular casting with a thread, but it's slotted in and held in place by no fewer than five grub screws.

It was an easy fix and the diagonal is back in perfect working order. However, it's curious that the male and female 2" circular parts are not attached to the casting by a thread. After all, that would seem to be the obvious way of attaching them.

Either the diagonal we got was from a bad batch, or it is part of a cost-saving effort in manufacturing. The astrophotographer in me doesn't care about diagonals anyway, I just throw them away. However, a visual astronomer might consider replacing this with a better 2" diagonal.

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The Scruffy Astronomer

Review of the saxon FCD100 127mm triplet APO