APO/ED refractors are great, however they do need a flattener to give you those pinpoint stars, or, even better, a reducer/flattener to also give you faster exposures and wider field at the same time. Some refractors will have a matched reducers (especially the “slow” ones), but, for those that do not the expensive TeleVue TRF-2008 is often recommended. I thought I’d test how it actually performs using the two ED doublets I have right now, the 600mm f/7.5 Evostar 80ED and the 500mm f/6.25 Equinox 80 ED, and pit it against an inexpensive Altair Lightwave 0.8x that had gotten me curious for a while now (it looks a bit like the Orion 0.8x – no idea how similar they are).
|Reducer Model||Reduction: Spec / Actual||Weight||Compatibility||Price|
|TeleVue TRF-2008||0.8x / 0.785x||347g||400-600 focal length||$305 / £306|
|Altair Lightwave 0.8x||0.8x / 0.801x||170g||f/6+||£75|
One more difference is that the TRF-2008 barrel has a groove between two full width segments. As I found out evaluating the images I took for this test, this is probably a good design for avoiding tilt, compared to the Altair’s slanting top-part of the barrel.
As for their compatibility, the TeleVue mentions 480-700mm on the box (and most recent boxes) and 400-600mm on the website, while the Altair asks for f/6 or slower. These are quite “wide” claims IMHO, when I know refractors of different focal lengths require different correction, so we’ll put them to the test with my 2 refractors which both fit the claimed requirements.
With the f/6.25 SkyWatcher Equinox 80 ED
The Equinox, as a relatively fast refractor at 500mm focal length has a lot of field curvature and a matched flattener is available for it. However, if you want an even bigger field and brighter image, you’ll have to find a reducer that matches it yourself. First of all, let’s see what we are working with, using a Canon DSLR. 100% crops at the edges of a Canon APS-C sensor and a 4/3″ sensor (simulated from the same image) are provided, as well as the full image at 50% resize:
I have to admit that I tried to make a session where all images were centered on Ruchbah (δ Cas), which is a decent starfield, but I did not notice that the adapter I used added some tilt, so I had to swap it with an image from a previous session with no tilt (around Alnitak). Hence less stars (and the different colors, since it was a modded 600D). In any case you can see the curved field that we will try to correct. If you want to compare the field of view, this is the starfield used in this session, with the tilt worsening the left part of the image:
- TeleVue TRF-2008
Here is the Cas region with the TeleVue 0.8x:
As you can see, there is a serious improvement, although the edges of a large sensor do show some comet-like stars. Remember, that there is an effective reduction of about 0.785x (i.e. 393mm focal length, calculated from the image), so the stars that were at the edges of the APS-C sensor without the reducer, are now well inside the edges of even the 4/3″ sensor, so they are mostly corrected. In effect, after stacking and processing and perhaps cropping out a little of the extreme edges, the results are very pleasing:
- Altair Lightwave 0.8x
The Atair does not give you little “comets”, however the stars are more bloated than the TeleVue. In addition, there is some tilt. I did not notice it when shooting, so perhaps it might be alleviated using some tricks, e.g. to try and fasten the tube without pushing it fully in so that the focuser screw is not at the slanted part. But the point is that if you have a common focuser like the Equinox’s, with a simple screw+brass ring, you can get tilt with this reducer if you don’t pay attention. Still, even at the “better” left side, the reducer seems to overcorrect. It is certainly an improvement over the original (again, remember the sensor-edge stars without a reducer are now much closer to center, so they are relatively corrected), but you’d probably want to avoid using the edges of the APS-C. A nice thing is that the reduction here was 0.801x, which is pretty much right on spec (more than the TeleVue). In general the reducer might be performing exactly on spec, just not be that great of a match for the Equinox.
- Flat frame comparison
Both reducers give us quite decent flats – they deviate from the no-reducer flat only at the extreme edges, and, actually the Altair is doing a bit better than the TeleVue.
- Other Reducers/Correctors
Just for fun I gave a try to my SW 0.9x reducer for newtonians and the Celestron 0.63x reducer for SCTs. Neither could reach focus. I also gave a try to a “trick” I read now and then, which involves focusing with a star between the center and the edge of your field of view. That’s also a no-go, as the Equinox is a fast doublet, which means it shows some CA and this “trick” introduced blue halos at the center of the field.
As for a simple corrector/flattener, you can have a look at how the TSFlat2 performs with the Equinox from my older post.
With the f/7.5 SkyWatcher Evostar 80ED
The 600mm fl Evostar does have a 0.85x reducer available, so not many people would normally try to match it with the TRF-2008. However, the Altair costs less than half the SkyWatcher reducer, plus it is a 0.8x, so it would make an interesting candidate. First, let’s see what we are dealing with, curvature-wise, with a shot without any reducer/corrector. The Evostar, unlike the Equinox, allows you to focus a bit off-center without messing up the stars, so in this shot I focused at about 1/3rd of the way to the corner of the sensor which improved corner stars very lightly (other imagers would “push” it even more, but I wanted perfect central stars):
Since this is an f/7.5, the field curvature is less (larger curve radius + deeper plane of focus), however even on a 4/3″ we get comet like stars at the edges (despite focusing a bit off-center).
- TeleVue TRF-2008
Even though the 600mm Evostar 80ED is within the specified limits for the TRF-2008, they are not a good match at all. It is a bit strange, since the TeleVue is made for the TV 85, which is an 85mm f/7 doublet, which would seem closer to the Evostar than the Equinox. Let’s hope the Altair does better.
- Altair Lightwave 0.8x
Unlike the TeleVue, the Altair seems like a great match for the 80ED. The stars are not perfect to the edges of the APS-C sensor, but they are not too bad, and I’d say they are pretty OK by the time we reach the edges of a 4/3″ sensor. We got no tilt this time either, as the 2-screw focuser seems to work better with the type of barrel the reducer uses. This is great performance/cost for 80ED users, as it is less than half the price of the dedicated SkyWatcher 0.85x reducer. We can actually have a look at how the matched reducer performs in comparison:
- SkyWatcher 0.85x reducer for Evostar 80ED
This is an M31 frame with the Evostar 80ED + the matched 0.85x reducer, provided by my friend Pantelis Samiotis. The field is a bit less heavy on stars, but you can still see that, as expected, the matched reducer is even better than the Altair. It is still not perfect, especially towards the upper left where some tilt shows up (Pantelis tells me it seems to be the grip of the Evostar-reducer threads which are not tight enough), but I expect the difference would not be enough to justify the price for quite a few people, with the Evostar 80ED being a budget APO. If you want to add a 2″ filter to the SkyWatcher reducer, the cost rises even more as you’ll need a special adapter.
- Flat frame comparison
As with the Equinox, both the TeleVue and the Altair very little additional vignetting on the APS-C sensor, with the Altair still having a small edge over the TeleVue.
- Other Reducers/Correctors
As with the Equinox, I could not focus with the SkyWatcher 0.9x reducer for Newtonians. However, I did manage to focus with the Celestron 0.63x reducer for SCTs, when I brought the reducer closer to the sensor, at a distance of less than 90mm (instead of the normal 105-110), where it performed as a 0.66x reducer. The result was not great, I’ll only include the 4/3″ sensor edges, but you can open the full APS-C frame:
It does take the scope to a very fast f/4.95, but the stars are not good for reasonably sized sensors.
As for a simple corrector/flattener, you can have a look at how the TSFlat2 performs with the Evostar from my older post.
The TeleVue TRF-2008 has a reputation of working with small APOs that don’t have matched reducers. Its performance with the Equinox seems to validate that point. It is built well, as everything “TeleVue” that I’ve owned, and it is also very expensive. I would only add a caveat: the “400-600mm” (or “480-700mm” if you prefer according to the box) focal length specification doesn’t tell the whole story, as it was pretty badly matched with the 600mm Evostar. Perhaps it doesn’t do well with scopes slower than f/7, I don’t know, but better give it a try first before splurging for it.
The Altair Lightwave 0.8x was a pleasant surprise. It is a “lightweight” next to the TeleVue, yet it hits its “0.8x” spec better, adds no vignetting (marginally better than the TV even!) and when matched well, it performs great. I know several people who could not afford something more expensive than the modestly priced Evostar 80ED and are without a reducer/corrector, since the SkyWatcher one is very expensive, at about half the price of the 80ED OTA (£169 vs £350 on FLO currently)! Well, the Altair is a great solution, it flattens the field reasonably well even on an APS-C sensor, takes filters, gives you a wider/faster scope than the SkyWatcher 0.85x, all for a low price. It is a bit disappointing that it didn’t do better with my Equinox (with which I actually intended to use it), both in respect to flattening and the barrel that can cause tilt, but, as we saw with the TeleVue, reducers are just not fit-all solutions.