William Optics Megrez 88 - A review - Part 3

[centroid]

Ok, M31 second session with the WO Megrez 88.

As I said last night, when I looked at the subs, the red fringing wasn't evident, and as you can see from the image below, it isn't.

However, we are now seeing some blue fringing on the bright stars, and I've zoomed in and cropped a section of the main image, to make it easier to see this.

I am wondering if the colour, and degree, of fringing is related to the focus point, and as such hyper critical in its setting. Focus was intially achieved with a Hartmann Mask, using 1x1 binning and a very small framed area around the star. Finally, it was checked for best FWHM.

Overall, the image is much cleaner than the first in terms of CA, and if I had applied a star reduction technique, as, I and others generally do, then some may say that they would find this acceptable.

Unfortunately, the ED80 is a 'hard act' to follow in terms of colour correction, especially at the price.

It may well be that William Optics have pushed their generally excellent optics, a 'bridge too far' with the Megrez 88, in an effort to produce a 'fast' short focal length refractor at an affordable price. Compare the price of the f/5.6 Megrez 88 (£539) to the f/5.5 TMB-92 (£1300) for example.

However, as I said before I'm expert in optical glass and telescope design, can only report what I find, and make 'best guess' assumptions.

As I see it, there is a definite need for a reasonably priced 90mm refractor offering something a bit faster that the ubiquitous ED80, at least certainly in my observatory.

I have to say, that I feel slightly deflated, as I had high hopes for this telescope, and in most all areas, it has, and still does impress me. Its superbly engineered, and has optics that deliver a 'pin sharp' image to the eye.

If Steve is happy for to do this, and the skies permit, I'd still like to run some more imaging sessions with it. Peraaps in the forlorn hope that it will perform, when provided with the right conditions.

Both last night, and the previous night, the conditions were pretty rough, and under normal circumstances, I wouldn't have bothered to image.

As before, no processing has been applied to the image, and it is comprised of the same number of stacked subs as the first one, and with the same 120 sec exposure time.

Dave

[centroid]

Here's another image, taken in the M45 area during the first session, zoomed and cropped to show the CA.

It is noisy, as there are only 5 subs of 60 secs in the 'stack', due to the cloud arriving. However, its good enough to show the CA.

Dave

[FLO]

If Steve is happy for me to do this, and the skies permit, I'd still like to run some more imaging sessions with it.

You can test it any way you like David but keep in mind that the Megrez 88 is a fast doublet so it is inevitable that it will show some CA (I have yet to find a doublet that doesn't). The question is, for its price and spec, is it more or less than you would expect?

[centroid]

Thanks Steve, and I do most certainly accept that it is indeed a fast doublet, as I said earlier, maybe this one is just a 'bridge too far, albeit a bold attempt by WO to address a market need.

I'm only guessing, as I've not used one, but I suspect that the Megrez 90, with it's 621mm focal length and f/6.9 focal ratio, would exhibit far less CA than the 88 appears to do.

Is the glass used in the 88, type 53 as in the 90?, WO don't say, and so should we suspect that it isn't. Is the £91 diferrence between the two really related only to a 2mm increase in objective size and an extra 123mm of aluminium tube?. :?

At this stage in my assessment of the Megrez 88, I am inclined to say that for the visual observer, the Megrez 88 is an excellent scope for the price. As I said in Part 1, visually the amount of CA is so minimal, as to be completely accepatble IMO.

I guess its a matter of 'horses for courses' really, and that perhaps a virtually CA free, fast refractor for the imager, at just over £500, is not a realistic expectation.

Now if WO had put a third element in the 88, and added another £200 to the price, I'd be 'banging on your door' for one.

Anyway, joking aside, I am delighted to have been given the chance to try the Megrez 88, and I can't think of any other astro equipment retailer, that would have made such an offer. I can only say that the service, customer support and helpfulness of First Light Optics, is of the highest order, and may the company long continue to succeed.

I'm sure my sentiments will be supported by other members of the forum, especially those that have been following this review.

Dave

[GazOC]

WO usually specify when they use 53 and just say "ED element" when its not.

[FLO]

... I suspect that the Megrez 90, with it's 621mm focal length and f/6.9 focal ratio, would exhibit far less CA than the 88 appears to do.

Yes, the Megrez 90 is well corrected but at f6.9 it is slower. It also has an optically matched reducer flattener that brings its f-ratio down to f5.5

Is the glass used in the 88, type 53 as in the 90?, WO don't say...

Officially, we don't know but I wouldn't be surprised it it was. Don't read too much into their not disclosing glass type, some other premium manufacturers have the same policy.

Is the £91 difference between the two really related only to a 2mm increase in objective size and an extra 123mm of aluminium tube?. :?

The new M90 also has the 2.5" Linear Power focuser.

Now if WO had put a third element in the 88, and added another £200 to the price, I'd be 'banging on your door' for one.

A number of manufacturers have tried to bring an 80-90mm triplet onto the market below £1,000. They mostly fail. It is considerably more difficult to produce a good triplet than a doublet so below £1,000 you will be better to look for a suitable doublet.

Hope that helps

[GazOC]

Thats the thing with WO though Steve, its not their "policy" when they do use FPL-53.

[Whippy]

Have to say, it's been great reading the reviews Dave. But I think your hope of having a fast CA free refractor at around £500 is a lovely but unrealistic hope especially in these uncertain times. TBH I can see these 3"-4" starting to reach saturation point anyway and a manufacturer is really going to have to bring out something special to make themselves stand out from the crowd.

Tony..

[MartinB]

Guess you can focus the red or you can focus the blue but not both! I doubt there is anything wrong with the manufacture of the scope, it's the nature of light that is causing the problem. F5.6 for a doublet made by anyone, with any glass is going to show significant CA. A great fast scope for narrow band imaging though!

[FLO]

Thats the thing with WO though Steve, its not their "policy" when they do use FPL-53.

:silent:

[centroid]

A great fast scope for narrow band imaging though!

I don't think there's any doubt about that Martin, and or visual use.

As I said, for the price its a beautifully built and presented scope.

So, taking a step back to where I started in the search for a scope to replace my trusty Skywatcher ED80, with a wider FOV and a bit more 'speed', while retaining the excellent CA rejection of the ED80.

Without doubt, this can easily be achieved, at a price. The TMB-92 f5.5 Tiplet Apo, will do all of this, and some!!, but at a cost of £1300. Hardly fair to compare that with the Megrez 88 at £539.

Then stretching the budget even further to £1470, there is the Takahashi Sky-90 f/5.6 and focal length.

Interestingly, just like the Megrez 88, this is also a doublet!!. So, as a doublet, with the same f/5.6 focal ratio, would this display the same degree of CA as the Megrez ???. :? . £1470's worth of CA, OUCH!! . Somehow, I doubt if this would be the case .

So, is this then answer to my quest, or something that looks good on paper, but disappointing in use?.

The Meade Series 5000 ED80 TRIPLET!!, with an f/6 focal ratio and a focal length of 480mm.

Now I calculate this to give a FOV of 64x48 arc mins when used with the SXVF-H9C, compared to the 55x41 arc minutes of the ED80. So, I have my wider FOV.

The Meade is f/6 compared to the ED80's f/7.5. So I have a bit more 'speed', albeit not as fast as the f/5.6 Megrez 88.

Meade state that their scope uses Japanese FCD-1 ED Glass. Good or bad ?, I'm not qualified to say.

Ok, the focuser used on the meade looks very much like the focuser on my ED80, so not in the same league as the focuser on the Megrez. Then, I've never had any issues with the ED80's focuser, albeit I do 'drive it' with a motor focuser.

Finally, at £450, is the Meade superb value for money, or too good to be true??.

BTW, no 'Meade sniping' by the anti Meade brigade please. I've been using Meade for 6 years, and have no complaints. So, ignore the brand name, if you don't like it, and 'pitch' your comments at a technical level.

Dave

[GazOC]

I was going to suggest a Meade Triplet Dave but didn't want to hijack your thread. The glass used is the Hoya equivilent to Oharas FPL-51, the scope itself has been getting very good reviews amongst the CloudyNight refractor crowd. I think with those specs (80mm f6 and a triplet) you could reasonably hope for an effectively colour free refractor.

[Whippy]

Just a thought Dave, but have you thought about the WO ZS66? It'll give you a FOV of 59x79 arcmins. I've only managed to use mine once on the Pacman neb in Ha and it didn't do a bad job, plus from what I've read, the CA isn't an issue either .

Tony..

[centroid]

Gaz, thanks for that, I'll have a look at what's being said on Cloudy Nights.

Tony, in my quest for a bit more light gathering power, the 66 would be a step in the wrong direction from 80.

Ok, for mono imagers, but us OSC guys, need all the light we can get, as the Bayer Matrix Filter does reduce the light getting the CCD.

Dave

[centroid]

Oh yes, BTW gaz, I've been using Hoya filters on my SLRs for more years than I care to remember, and no complaints there either.

Dave

[centroid]

Had a telephone conversation with an accomplished imager, and quite a knowledgeable guy (non SGL), and he had experienced using the Megrez 88 with a DSLR, without having any issues with CA.

His was view was, that a true asto CCD camera, like my SXVF-H9C, was likely to be sensitive to light in the 'near UV', which could explain why I'm see the CA fringing.

He said that DSLR's are not sensitive in this part of spectrum.

If this is the case, then it would seem that the Megrez 88, would be fine for imaging with a DSLR, as the CA wouldn't be evident.

So, does this make the 88, a great scope for visual, narrow band imaging, and colour imaging with a DSLR. If so, then given it's finish and build qaulity, I guess that's not a bad portfolio for just over £500.

Dave

[Whippy]

Perhaps a UV cut filter might be worth a try Dave?

Tony..

[GazOC]

Wouldn't the fringing be blue if it were UV?

[centroid]

If you look at the image above Gaz, you'll see that it is blue, this time around. I think that the colour of CA colour presented, is dependent on the where the focus point is set. As Martin said, you can foucus out the red or the blue, but not both.

Optics is a very complex subject, and I certainly don't profess to be an expert in this field. As I said earlier, I can only report what I see, and make an educated guess as to the reasons why.

A comment from a review on the Cloudy Nights forum, said of the Megrez 88, that some red fringing was evident, and that achieving best focus for least CA of any colour, was tricky. This pretty much supports what I'm seeing.

Tony, most all CCD's are sensitive to the near infra red, and the use of an IR blocking filter is pretty much accepted as standard procedure. As my H9C always has an IR filter in place, I really don't want to stack another filter inline, to correct the shortcoming of a scope's optics. Good think though.

Dave

[GazOC]

Ah, I see know Dave. I was going from the Andromeda image.

[MartinB]

There is only one way for you to go Dave, anything less will just leave you restless. Save the pennies hang on the the ED80 and 90mm of Sky could by yours. Then you will be one happy bunny

[GazOC]

Theres also a very interesting 100mm f6.5 FPL-53 triplet on the way shortly.

[MartinB]

Theres also a very interesting 100mm f6.5 FPL-53 triplet on the way shortly.

CARRY ON!!!.......

[Whippy]

90mm of Sky

That's about all I've seen for the last few weeks....

Tony..

[GazOC]

Its a carbon fibre 106mm f6.5 53 based triplet by Astronomy Technologies (AstroTech) and will be sold by Astronomics in the USA from next month, not sure about the European dealers but IIRC Markus Ludes was testing them with a view to selling them.