54 replies to this topic

[Marcsabb]

Looking for a portable refractor that I could carry also on airplane, last September I stumbled upon this: http://www.omegon.eu...bon-ota/p,21266

 

The specs are impressive and matched almost all I was looking for. With a carbon fiber body, despite the 10cm/3.9" aperture, it weights only 3.5 Kg. With accessories and inside its aluminium case (which doesn't exceed IATA cabin luggage restrictions) it tips the scale at 7Kg. It has a generous 100mm (3.9") aperture (most of the other scopes I considered were 80mm) and, being F/6, it's quite fast which makes this scope useful also as a specialized high power telephoto lens. Last but not least, while it's only a doublet, it uses a combination of FPL-53 and SNBM-51 glass; at least on paper this sounds pretty good especially for a refractor with a fast focal ratio.

 

As I couldn't find a review online, I did some research on my own: Omegon it's a fabless manufacturer that put its brand on OEM scopes also sold under other names: for example, they have in catalogue a 127/952 refractor that looks similar to the one sold by Explore Scientific and a 5" Mak F/15 similar to the one sold by Bresser and now also by Meade. I could trace the origin of this 100mm F/6 refractor back to Long Perng, a well known Taiwanese manufacturer: on their site they list an OTA which is identical in all the characteristics except that it is said to employ FPL-51 glass in the low dispersion element of the lens pair. A few more days spent browsing the web and I came up with an old review of a Teleskop-Service refractor which also looked very similar to this Omegon. Even if it had a metal tube, the rest of the specs were identical to the Omegon refractor and it also used FPL-53 glass. While it's no longer listed on TS website, I could retrieve the original webpage via the Internet Archive: named TSAPO1006 I've no doubt it's the same scope manufactured by Long Perng and sold by TS, except now it uses a carbon fiber tube. 

 

Satisfied by my investigation, I placed an order for the scope and waited.

 

The scope was delivered to me recently and put under test as soon as possible. The exterior finish and quality of workmanship is very good. The carbon tube is covered with a hard, glossy enamel coating that had no scratches or imperfections. The mounting rings are solid pieces of machined aluminium; the large screws that clamps the rings are also machined. A small but nice detail is the lens cap made of a thick aluminium plate and which is very sturdy. The rear portion of the tube, including the massive focuser, are all made of metal and finished with a black glossy paint.

 

The Crayford styled focuser  is almost oversize for such a small tube; it has a 1:11 reduction ratio and it's very smooth in it's operation. It has a locking knob that allows to change the stiffness of the focusing mechanism or to lock the movement in place: a must when using heavy cameras. The complete focuser assembly is also freely rotatable around, which is a nice feature to have with equatorial mounts. On the eyepiece side, there is a M72 thread making the scope compatible with a lot of accessories made for Takahashi refractors (including the excellent Baader Clicklock 2" adapter). The scope also comes with M72 to 2" and 1.25" visual back; these use all-around brass compression rings to lock the barrel into place: a nice touch for those, like me, who are constantly worried to scratch the barrels of their accessories. In the end, after a detailed visual inspection, I was positively impressed by this scope. It was time to check if it delivered also quality views.

 

I put it on the mount before sunset so I was able to use it visually; the sky was still vividly bright so I targeted some distant tree branches that dotted the horizon. I tried with different magnifications between 15x to about 100x and the scope always delivered crisp views; I checked the trees and other silhouettes around the edges to see if I could pick up some chromatic aberration but there was none I could see. There is no doubt that this scope can be made into a good telephoto lens; Omegon also sells a telecompressor that works also as a field flattener and that is recommended if one wants to take pictures with this scope using large sensors.

 

The sun went down; in these months Venus is quite high and bright in the sky after the sunset and it's also the best target to check for chromatic aberrations. For this test I used the new TS UWAN, 7 elements and 82° eyepieces recently introduced with focal lengths of 7mm and 4mm (which would yield respectively 85x and 150x magnification). At 85x Venus is already visible as a bright, half moon shaped object; the borders are generally very sharp but there is a slight violet haze visible on the top half of the disk. At 150x, chromatic aberration becomes very intrusive with a distracting, purple-violet fringe present on about half of the visible borders of Venus. I must confess that I was really depressed after seeing this! I know: it's a doublet and, on top of it, it's also quite a fast refractor, but I was expecting a bit more from a lens system realized with what is supposedly the best non fluorite glass available! The first (irrational) reaction was to verify if the scope cell was somehow misaligned. I did a Ronchi test, then the "torch and cardboard test" but everything seems to be in order. To be at peace with my mind I would  need to check if the ED glass is indeed FPL-53 and not the cheaper FPL-51, but I don't think there is an easy and accessible way to determine this. So, I must take the manufacture word on this or have the scope checked by some longtime refractor user to hear from somebody I trust, if my expectations were a bit too high. 

 

After Venus, I pointed the scope towards Mars and this appeared like a nice, bright orange small disk. At least on this planet I couldn't see aberrations at the borders but I suspect these days it's too small and dim to be useful for a comparison. In ten days the moon will be again high and bright enough to be another good benchmark of what this scope can and cannot do. At least it didn't disappoint when it comes to DSO photography. I still haven't got the time to use it for a long time but I took a couple of shots of Orion from my Balcony with my DSLR. I also tested this scope with a CCDT67 telecompressor. While this focal reducer is designed to work with scopes having a ratio of F/8 or higher, many individuals used this on faster refractors - with generally good results. I attached the telecompressor to a 2" to T2 nosepisece with a 5mm spacer in-between, so that the optical path would be exactly 85mm for a 0.67x ratio. This way I have a 400mm F/4 scope. Field curvature is not taken care of as this is not a combined reducer/flattener; also the compressor reduces the image circle to 29mm which is still enough for an APS-C sized sensor but I loose the ability to take full frame shots. The compromises however are well worth, as I do sky imaging from an alt-az mount, so long exposures are out of question.

 

Overall, I've mixed feelings about this purchase. Built quality is surprisingly good. For DSO and terrestrial photography it looks promising. As a ultra-portable moon and planetary scope it's still unproven. At 1300€ new this isn't cheap and, for a few hundreds € more, one can buy a 127/952 triplet (from ES or somebody else) or a tried and true SW 120 doublet, which is universally kept in high esteem. These are not 'portable' the way I needed, though. On the other side, there are some refractors of comparable size and weight from premium brands, but the price gap widens considerably.

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Edited by Marcsabb, 29 December 2016 - 09:51 AM.

[Edwin]

Nice and honest review ! Seems like and interesting telescope, especcially for grab-and-go and travel purposes. It is a nice combo with that Ioptron-mount. I agree with you that it is rather expensive, for EUR 600 extra you could have bought a TS 130 FPL53 triplet, although that doesn't fit your travel requirements.

 

Venus is a very demanding target when speaking about CA. Being such a fast doublet, you can expect a little CA on Venus, even if it is using FPL53 glass. My F5.6 FPL 53 doublet also shows some at higher magnifications. To be completely colourfree on Venus, you need a FPL53 triplet or fluorite doublet and a good design and execution of that design. So, don't worry and enjoy!

Edited by Edwin, 29 December 2016 - 06:02 AM.

[Marcsabb]

This is a quick and dirty stacking of 15x15" pictures taken with a Sony A7 mark 2 @ ISO1600, CCDT67 telecompressor and a Baader UHC-S filter to block some of the light pollution. For this kind of work, the scope certainly delivers within the constraints of taking short poses in a light polluted environment and with an alt az mount. I still haven't got the chance to try it with my ASI astro camera although, since that one has only a tiny 1/3" sensor, the scope would be under-utilized with such a small chip! 

 

EDIT: I've replaced the picture with a wider view. I've also taken 10 minutes to add a bit of post processing to make it more pleasant.

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Edited by Marcsabb, 29 December 2016 - 09:33 AM.

[orlyandico]

Point it at a star field with many bright stars (the Pleiades are good) and take a photo with a DSLR.  If there are blue halos around the brightest stars with 30s - 1 minute exposures, it's FPL-51.

 

An FPL-51 doublet does not show much CA, but a long-ish DSLR exposure will make it pop out.

[daveCollins]

I agree that this is an honest review which is appreciated. First light reviews almost universally will say, "Perfect star test, no color, great contrast, pinpoint stars". This is done so much now that reading first light reports probably leads to false impressions of scopes' actual abilities. This is similar to the general exaggerations in everyday speech which make many words meaningless. In our case, I think the terms "pinpoint stars, great contrast, and no color" are generally meaningless. Rigel renders as a pinpoint star in your scope ... really? It’s too bad, but that is the way it is.

 

Your report is an exception and I appreciate the honesty. Thanks.

Edited by daveCollins, 29 December 2016 - 08:28 AM.

[Marcsabb]

Thanks for the compliments. Well, I tried to be as objective as possible within my limited knowledge of the subject. In the case of this scope, either my expectations were a bit too high or I'm a too harsh judge. If I had much more money to spend, I would have bought a Borg of similar specs without second thoughts. I also gave up looking at the second hand market because refractors usually maintain the price more than other types of scopes and especially quality portable OTAs are much sought after.

 

Today is cloudly but I'll try to listen to orlyandico suggestion. Do I have to pick a star withing a certain magnitude range (i.e. not too bright, not too dim) and what about the settings for the camera? If I use my astrocamera  which has smaller pixels (3.75u vs 5.97u of the Alpha 7) will the aberration be more evident?

[Erik Bakker]

Marcsabb,

 

Thanks for posting your first impressions. Your scope looks nice in it's carbon fibre dress. 

 

Sorry to hear your disappointment about the performance of your new scope. A new scope should bring joy!

 

For better quality views, don't go up in size, but in quality. There are good reasons why experienced and sane amateur astronomers buy similar size scopes for double the price and more. 

[Derek Wong]

The sun went down; in these months Venus is quite high and bright in the sky after the sunset and it's also the best target to check for chromatic aberrations. For this test I used the new TS UWAN, 7 elements and 82° eyepieces recently introduced with focal lengths of 7mm and 4mm (which would yield respectively 85x and 150x magnification). At 85x Venus is already visible as a bright, half moon shaped object; the borders are generally very sharp but there is a slight violet haze visible on the top half of the disk. At 150x, chromatic aberration becomes very intrusive with a distracting, purple-violet fringe present on about half of the visible borders of Venus. I must confess that I was really depressed after seeing this! I know: it's a doublet and, on top of it, it's also quite a fast refractor, but I was expecting a bit more from a lens system realized with what is supposedly the best non fluorite glass available! The first (irrational) reaction was to verify if the scope cell was somehow misaligned. I did a Ronchi test, then the "torch and cardboard test" but everything seems to be in order. To be at peace with my mind I would  need to check if the ED glass is indeed FPL-53 and not the cheaper FPL-51, but I don't think there is an easy and accessible way to determine this. So, I must take the manufacture word on this or have the scope checked by some longtime refractor user to hear from somebody I trust, if my expectations were a bit too high. 

 

...

 

Overall, I've mixed feelings about this purchase. Built quality is surprisingly good. For DSO and terrestrial photography it looks promising. As a ultra-portable moon and planetary scope it's still unproven. At 1300€ new this isn't cheap and, for a few hundreds € more, one can buy a 127/952 triplet (from ES or somebody else) or a tried and true SW 120 doublet, which is universally kept in high esteem. These are not 'portable' the way I needed, though. On the other side, there are some refractors of comparable size and weight from premium brands, but the price gap widens considerably.

 

I think that there may be nothing wrong with your scope and that you are being too harsh without further testing..  Even when Venus is relatively high, there is a combination of atmospheric dispersion, eyepiece lateral color (sometimes) and chromatic aberrations that can make the effect you saw, color on one side of the planet and not the other.  The smaller aperture scopes will not show red on the opposite side from atmospheric dispersion as well as the larger scopes.

 

It is difficult to judge the scope without looking at it.  However, any f/6 doublet that is not some super exotic unobtainable design is going to have some color error on Venus, regardless of whether the abnormal dispersion element is FPL-51, FPL-53, or fluorite.  You can see some minor color error on such exotic scopes as the Televue 140 and the larger Takahashi fluorite doublets; I even see a bit on the FC-100N f/10.  Those scopes are a lot more money than yours and most are slower.  You mentioned a Borg; those will show some false color.  You can make reflectors show purple on Venus at some altitudes and with some eyepieces.

 

I would look at Sirius and Rigel in focus, and at Jupiter.  If you can, get together with other owners and compare scopes.

 

Derek

[Erik Bakker]

I agree with Derek.

[orlyandico]

I only really tried this with the Pleiades. So magnitude 1-2 should be OK.

 

Around 30 second exposures should be plenty.

 

If your astrocamera is monochrome, you won't see the CA.  But if it's an OSC you most definitely would.

 

The thing is many of the FPL-51 doublets are optimized for visual so you can't see the blue CA.  But it's there, just too faint. A sufficiently long camera exposure will show it.

 

That said even an FPL53 doublet at f/6 would have some CA. I have no experience with FPL53 doublets that short. I can only say that my old Orion 100ED f/9 FPL53 doublet was pretty much color-free even with a camera. This is also true of the FPL53 and OK4 f/6 triplets that I've owned.

 

That said even an FPL51 triplet can be color-free...

 

Today is cloudly but I'll try to listen to orlyandico suggestion. Do I have to pick a star withing a certain magnitude range (i.e. not too bright, not too dim) and what about the settings for the camera? If I use my astrocamera  which has smaller pixels (3.75u vs 5.97u of the Alpha 7) will the aberration be more evident?

[Daniel Mounsey]

Derek is right. It's premature at this point.

[beanerds]

Nice looking scope .

 

I understand where you are coming from with these Long Perng scopes , brilliant build quality but the optics are good but not great .

 

I know this because a few years ago I got the 90mm 500mm fl version of this scope and as it was advertised as an APO , I was shocked to find my ED80 doublet using FPL53 I had at the time was head and shoulders better on CA control than the 90mm LP , a few of us grabbed these down here when on sale  by a local supplier and all came to the conclusion that these were FPL51 at best with sub standard mating element  , good scopes but sadly not an APO .

 

Shame really as if LP had invested a little more in the optics  ( true FPL53 and better mating element ) they would have had a real winner as the figure , poliish , cell and coatings are top notch  with that superb build quality they could have been a real winner .

 

Brian.

Edited by beanerds, 29 December 2016 - 11:03 PM.

[orlyandico]

AFAIK LP doesn't make the optics, they are purely on the mechanicals side.. also AFAIK William Optics actually rebadges LP mechanicals, but the optics come from elsewhere.

[Marcsabb]

Thanks for all the feedback. This evening, if the skies are clear I will try with Rigel. Unfortunately, the Pleiades are a bit too high to be seen from my balcony without being blocked. I'll give a shot at them the first time I'll be able to move the setup to my country home. First I'll try visually with my eyepieces and I'll throw in also a couple of short focal length Orthos I've borrowed from a friend, to exclude it's a problem with my eyepieces. These are of a new design and while some people who have tested them say they're good, even better than the older UWANs, I don't think they have been tested yet with such a fast telescope.

 

I have a color astrocamera so I will play around with gain and exposure till I can get the best result and I'll post a picture here Monday or Tuesday.

 

The manufacturer clearly states it's FPL-53 glass (plus S-NBM51, another 'premium' glass type) and the same design, with same optics, was also sold by TS a few years ago. I understand not all glass blanks are equal and the lower quality ones usually end up in cheap products but I was under the impression that blank glass quality didn't impact specifically the chromatic aberration performance of the lens.

 

I've attached the spec sheet of the TSO I found on the Internet Archive, if anybody is curious. My Omegon is identical except for the material of the tube.

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Edited by Marcsabb, 30 December 2016 - 01:56 AM.

[Derek Wong]

FYI I tried a 180mm f/6 Mak-Newt which has almost no color (a super low amount from the corrector but effectively a reflector).  There was obvious atmospheric dispersion with purple on the straight upper edge of the half Venus and orange on the lower convex edge.  The amount of purple was somewhat less than when I viewed with the Televue 140 yesterday, and that lends credence to the hypothesis that the amount of purple is increased by small amounts of color error in the scope.  Unless you have an atmospheric dispersion corrector, it is difficult to remove this component.

 

Derek

Edited by Derek Wong, 30 December 2016 - 04:56 AM.

[Marcsabb]

Well, I tried orlyandico suggestion and took several pictures of Rigel with a slr and and an astro camera, too. First, I made sure it was in focus with a Bahtinov mask, then I took several pictures at diffent exposure settings. All showed a prominent and uniform purple haze around the disk; extending the experiment to other stars it seems the halo is always present, but strongly subdued in all but the brighter stars.

 

This afternoon I also took again several shots of distant objects silhuetted against the bright sky but all the infocus objects showed no easily discernible aberration at the borders.

 

So I have a ED lens system that is good enough for landscape photography but that renders brights stars with an excessive purple violet fringe all around.

Edited by Marcsabb, 30 December 2016 - 05:16 PM.

[Peter Besenbruch]

Well, I tried orlyandico suggestion and took several pictures of Rigel with a slr and and an astro camera, too. First, I made sure it was in focus with a Bahtinov mask, then I took several pictures at diffent exposure settings. All showed a prominent and uniform purple haze around the disk; extending the experiment to other stars it seems the halo is always present, but strongly subdued in all but the brighter stars.

 

This afternoon I also took again several shots of distant objects silhuetted against the bright sky but all the infocus objects showed no easily discernible aberration at the borders.

 

So I have a ED lens system that is good enough for landscape photography but that renders brights stars with an excessive purple violet fringe all around.

It sounds like you may have an FPL-51 system. Please post some of the pictures.

[Marcsabb]

Here's a couple of shots. The first is Rigel and its huge purple halo visible; I did not even had to take a long pose at high ISO to make it evident.

 

And the second is a long exposure of the Orion nebula. Here the halo is visible also on the smaller stars because of the long exposure. I admit I've little experience when it comes to refractors, having owned only some achromatic small ones, but this to me does look a bit... excessive.

 

PS: I took these pictures without any filter whatsoever. I noticed that band pass filters for nebulas or light pollution reduce the halo consistently becaue they block, among other wavelegths, the higher portion of the spectrum, so violet light coming from the lens is greatly attenuated.

 

What do you say? Should I contact the vendor so he can ask for explanations directly to the manufacturer? Is this a defect unrelated to glass?

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Edited by Marcsabb, 31 December 2016 - 06:49 AM.

[chrisastro8]

DSLRs sensors do seem to emphasize lower wavelength halos around bright stars even with the latest optical designs from premium providers - I've read about some blue halo issues on the best designs corrected to 400nm, posted by others on forums over the years that have reset some user expectations. 

 

But you are seeing this at the eyepiece also and it shows as a vivid purple in your DSLR images.  Given that its a fast f/6 doublet I would have expected something like this but its a judgement call whether this is acceptable to you or not.  If it were a fluorite f/6 doublet, I'm not so sure what my expectation would have been since the Taks I'm aware of are deliberately designed to be slower.  Agema are introducing new Fluorite doublets that are also deliberately slower at f/8 also for presumably the same reason.

 

I really like your scope and would meet my own needs for reasonably priced, build quality, portability, low weight and overall good looks.

 

Thanks for posting about a scope that is a new one to me and hope that my ramblings in some way useful.

 

Chris

[chrisastro8]

One further thought - my Takahashi FS-60 is a fast f/6 Fluorite doublet and a DSLR (Canon 60Da) image of the double cluster does show some violet halos around the brighter stars.  Zoom in on the cropped image to see this.  I absolutely would have expected this Violet Halo in this test image along with field curvature as you move towards the edges.

 

Not as much halo as yours, but this is expected since its a much smaller optic at 60mm and Fluorite.

I can discern also this also at the eyepiece but I regard my FS-60 as the finest fast double and I absolutely love this scope which was the reason for me purchasing a large Tak.

 

Here's the cropped picture:

 

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[Marcsabb]

In the eyepieces it's even worse; In my UWANs it appears asymmetric like a puff on one side of the brightest stars. Some EP designs don't like fast scopes, but I had the occasion to test these on a fast newtonian and they don't exhibit erratic behaviour or odd distortions.

 

I took some shots also with the astro camera and the halo is still there. The only way to make it less annoying is to block light below 450nm. I'm not happy at all because this is not what I was expecting. Like I said, I'm no refractor expert but I've seen pictures taken with ED doublets and they certainly can do better than this. I'm not expecting fluorite performance but this looks hardly better than what an achromat can do.

[junomike]

In the eyepieces it's even worse; In my UWANs it appears asymmetric like a puff on one side of the brightest stars. Some EP designs don't like fast scopes, but I had the occasion to test these on a fast newtonian and they don't exhibit erratic behaviour or odd distortions.

 

I took some shots also with the astro camera and the halo is still there. The only way to make it less annoying is to block light below 450nm. I'm not happy at all because this is not what I was expecting. Like I said, I'm no refractor expert but I've seen pictures taken with ED doublets and they certainly can do better than this. I'm not expecting fluorite performance but this looks hardly better than what an achromat can do.

if the Image seen at the eyepiece is as bad or worse than shown in the Pics, than  this seems more like an Achromat than an Apo or even Semi-Apo.  I'm reluctant to believe it's anything close to FPL-53.  Even with 100mm and F6 this should be far better for CA control. 

Also, your pic of Rigel show almost a pinched optics effect (at least to me).

 

Mike

[Marcsabb]

The pinched effect on the first pic could be well a matter of a slight vibration when the picture was taken. This evening I won't be able to do any further test but I hope tomorrow the weather will remain fine so that I could work up more pictures for you to judge.

[Edwin]

I have to say that if you google a little bit on '100 mm f6' I only see hits on FPL51 doublets like the one of Olivon, Airy or Primalucelab; prices of these scopes match the price you paid.Often these lenses are being sold in telescopes under different labels and I don't know of any other FPL53 doublet offerings, besides the TS scope (that is discontinued?). Also that CA on the M42 photo is really horrible, but I'm no expert (visual only).

Edited by Edwin, 31 December 2016 - 01:01 PM.

[Peter Besenbruch]

In the eyepieces it's even worse; In my UWANs it appears asymmetric like a puff on one side of the brightest stars. Some EP designs don't like fast scopes, but I had the occasion to test these on a fast newtonian and they don't exhibit erratic behaviour or odd distortions.

If the star is in the center of the field, there should be no asymmetry of the false color.

​

 

I took some shots also with the astro camera and the halo is still there. The only way to make it less annoying is to block light below 450nm. I'm not happy at all because this is not what I was expecting. Like I said, I'm no refractor expert but I've seen pictures taken with ED doublets and they certainly can do better than this. I'm not expecting fluorite performance but this looks hardly better than what an achromat can do.

​I say the color correction in the images you posted above doesn't look very good. Yes, I would expect some color with a 100mm, f6, but not that much.

​

​I remember a discussion earlier on Cloudy Nights about a 130mm, f7 from Teleskop Service, and the type of glass used. Most vendors sold FPL51 scopes, while TS sold an identical looking scope and claimed FPL53. That led to a huge discussion that resolved nothing, but you should be aware that questions about bogus glass claims do exist.

​

​I mention this, because there was a discussion about an ED 100(2)mm, f6 a while back, and again, only TS claimed FPL53.

Edited by Peter Besenbruch, 31 December 2016 - 10:42 PM.