Review of the saxon FCD100 127mm triplet APO

Bill provides a review of the saxon FCD100 127mm triplet APO refractor. 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. In this review, I’m referring to the big one here.

This is a variant of the scope sold as the Explore Scientific 127ED, however there are a few differences. The focuser on the saxon is the standard barrel-type. There’s a hexagonal focuser on the Explore Scientific variant. This focuser supports more weight.

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, making the scope excellent for larger deep sky objects.

Market placement

The saxon FCD100 127 is a triplet apochromatic refractor.

Doublets and triplets

Simply put, “triplet” means is that there are three separate lenses in a group at the front of the scope. It’s distinct from scopes with two lenses in the front group, which are called “doublets”. Three lenses give the telescope designers better ability to tune the refraction of the group. This means that red, green and blue light all focus at the same point. Triplets, having this single focus, area called “apochromatic” (APO). Doublets are called “achromatic”, which generally means that only the red and blue light focus at the same point.

This is confusing, but doublets have a blue coma, or fringe around them. Of course, doublets are much less expensive refractors. Triplets are highly technical and expensive, but it’s important to get rid of this “chromatic aberration”.

Other saxon FCD100 scopes

The 127 is the largest of the saxon FCD100 range, which comes in a very cute 80mm as well as a 102mm version. I think the 80mm would be a fantastic grab-and-go triplet APO. It’d be just the right size for an HEQ5 mount with a small autoguider. That’d be a great and highly portable rig. We’ve got one but didn’t talk about it in this review.

Other triplets

There are lots of other triplet APO refractors around, and all are very pricey. However, the saxon FCD100 127 is significantly less expensive than other triplet APOs.

As a comparison with another triplet APO, 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. It appears to me that the company’s general approach is to support aspirational and learning members of the astronomy community. Of course, I haven’t actually spoken with saxon about this.

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 or eventually upgrade weak parts. They’re all managable. Eventually, they might upgrade 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; 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 the telescope delivers in both quantity and quality. Dim objects show up in short exposures, colours are precise, and stars are sharp and free of chromatic aberration.

Oh, 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 Hoya FCD100 ED (extra low dispersion) lenses. 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) bend diffently in glass. 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. This is especially the case at the edges of the field of view.

The triplet design uses three different shaped lenses in the front of the scope. This eliminates 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. Lower dispersion further decreases chromatic aberration. I don’t know if there’s a measurable difference, but I do know that they’re both considered to be very good.

NGC 4755 (Jewel Box) test image

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. (Prime focus is when there’s nothing between the sensor and the telescope’s objective). They’re also heavily cropped, so a pixel on the image is close to a pixel on your computer’s display. This will also depend on your browser and some other things, of course.

First, this is the unprocessed image.

Jewel Box out of the camera

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. I also moved the median towards the left, which brightened the whole image.

Jewel Box with some processing

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 are colourful and the colour response is very good. There’s not a trace of chromatic aberration, with the stars not showing that tell-tale blue halo. It’s a top effort.

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. This was a non-linear stretch, so the background remained dark. Still, there’s no sign of chromatic aberration or blue fringing. However, there may be a bit of an orange fringe towards the bottom of the brightest stars. I think this may be atmospheric dispersion or it might indicate some flop in the focuser.

Jewel Box with heavy processing

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 huge amount of light.

NGC 3372 (Eta Carina) test image

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. It had also just popped out from behind 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. It’s just a star field, which is what you would expect for a 10 second exposure.  Carina Nebula out of the camera

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. The yellow star on the left contrasts 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.

Carina Nebula - single image with processing

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

I decided to see what else I could get. I took 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 very closely, I think that I have over-sharpened in Astro Pixel Processor.

Carina Nebula 170x10s stack processed in APP

Overall, both test images show that the scope would produce some first class images. You need to do two things: take it to a dark sky site and get more experience.

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. Even without photographic equipment, it’s pushing the photographic limits of either a saxon HEQ5 pro or Celestron Advanced VX mount.

My NEQ6 had no problem though.

The air spaced triplet cell is a huge piece of glass. This means that the balance of the tube is significantly to the front, and this does cause problems. I balanced the scope on my mount by pushing 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 fully. So now I couldn’t reach the lens cover to take it off.

I juggled about with the positioning of the tube rings for a while. I did find a position where I could retract the hood and balance the scope. If I were to be using this scope on a regular basis, I think I would add a diver’s weight. On the rear of the tube, this would provide better balance.

Dovetail

Continuing on the theme of balance, the scope is supplied with a 200mm Vixen sized dovetail. This is used to clamp the scope 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 have the focuser set in with a 2″ diagonal and a short eyepiece. Then you’ll want to put in spacers and a heavy DSLR. Your imaging equipment might include an intervalometer or a refrigerated CMOS with a filter wheel and an off axis guider with a second camera.

This means that the balance for the whole scope in these two scenarios is going to be very significant. Simply sliding the dovetail up and down the mount’s saddle is not going to be enough. The dovetail needs to be longer.

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.

350mm Losmandy and 200mm Vixen dovetails

Focuser

This is probably the telescope’s weak point. A number of people have discussed the focuser that comes with the saxon variant of this scope. Two aspects of the focuser I had for review did bug me. Other reviewers have discussed this as well.

Focuser 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. I don’t know what the cause is, but similar focusers imported into Australia have had the same problem. I’m hoping it’s a bad batch of focusers and that the manufacturer will solve the problem soon.

Focuser 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.

I focused on infinity with my little 20mm 1.25″ test eyepiece and the diagonal. To do this, I needed one of the two 37.5mm spacers supplied with the scope. I found focus at about 26mm.

Focus at infinity with eyepiece

It’s best to have as little between the sensor and the objective as possible for prime focus photography, so ditch the right-angle. To focus on infinity with my DSLR I needed both spacers. Even then I had to rack the focuser all the way out to 44mm. This is probably the cause of the tiny bit of flop that was evident in the test photos.

Focus at infinity with DSLR

Having a spacer on the draw tube provides the ability for the user to add equipment to the train. If you add a filter wheel and off-axis guider, you simply remove a spacer. 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 as part of a future upgrade. The replacment would be smoother and have a longer travel. But seriously, the current focuser is not a deal breaker for someone who wants glass of this quality at this price.

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

One person I spoke with complained that there was a crunch just where the image was focused. In this case, I’d advise adding a small spacer to the train. The spacer will move the focus point. This workaround will fix the problem for just a few dollars.

Diagonal

We received a couple of diagonals with our FCD100 samples (the 80 and the 127). During the setup, I wedged one of the diagonals in the draw tube (it’s a very snug fit). As I was “easing” it out, the eyepiece holder came away. It turns out that this when they made the part, they didn’t attach it to the casting with a thread. Instead, they held it in place with five grub screws.

It was an easy fix and the diagonal is back in perfect working order. However, it’s curious that the manufacturer didn’t attach the circular parts to the casting with 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.

Further reading

Starizona give a good explanation of apochromatic and achromatic refractors, and ask “why are refractors so expensive?” (Starizona are the makers of Hyperstar)

Bill is Optics Central’s expert on astrophotography, telescopes and bird watching. You’ll find him in the Mitcham store on Mondays, Fridays and Saturdays. Come in for advice on how to get the best out of your current telescope, what your next telescope should be, or how to take photos of the sky. He can even help you to see some rare birds.

Share Button

Leave a Reply

Your email address will not be published. Required fields are marked *