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Now I'm confused and bewildered (80mm 'Fracs)


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From the new Astronomy and Nature site (looking really good guys and gals BTW, must get in there one of these days) there sprang a general lust for the Sentinel semi-Apo 'scope, which looks very nice. I also noticed that they offer the 90mm Achro somewhat cheaper and then I went off on a trail of allegedly similar 80mm ish Apo 'scopes.

From the £200 Sentinel I went to the £300 ED80, to the £400 Revelation, the £1100 Megrez and the £1400 TV76. Being a "bang for the buck" sort of guy, this spread of prices, and one assumes there are mechanical / optical differences which justify the spread, how different can an APO be? Granted the Sentinel only claims Semi-Apo, but it seems like one cracking deal. Do I go for the Sentinel and have change for a car, the Revelation plus an EQ6 Synscan or the TV76?

The range seems huge.

Any comments from the initiates?

Captain Chaos

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It's the old 80/20 rule. The first 80% of performance costs you 20% of the price... the last 20% performance costs you 80% of the price. It depends how much more APO you want. The TV is a world apart from the sentinel. But is it £1000 better? Personal preference. I went mid way and brought the ED100 for £500.

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The Sentinel for money can not be beat. As a visual observer I cannot justify the cost between semi & Apo. The semi hard sows CA, and to be honest the VR-1 sorts that out on the moon!!

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I'm thinking that my skies and skills* will prevent me from using the extra APOness of the more exotic toys, not to mention somebody preventing me from spending that kind of dosh, so I think I'll be going for the Sentinel initially. The ED100 is too close to my 120mm Helios plus I'm thinking of getting another mount. This click, wait three minutes whilst walking about tripping over things then click again and repeat for half an hour is loosing it's appeal. I could use another mount to look at things with the SC3 while the other 'scope is looking at things with the DSLR. Might even risk my eyes looking down the hole.

* The wind, wobbles and lack of good tracking / focus that I get now will hide any minor improvements, I'm sure. Might be pier time, but I need to find somewhere that a) can see the right bit of sky, and :D won't be in the way / get run over.

Captain Chaos

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Good question CC,

The Sentinel, Revelation, WO, etc, all appear to come from the same Taiwanese stable. As far as I can tell, they are equal in performance though the extras – diagonal/case/eyepiece – supplied do vary so I guess you must make your decision based on that, its price and supplier. The sentinel version appears to be the favourite on SGL though there are one or two Revelation owners (myself included) who like the supplied 2” diagonal.

As for the APO v Semi APO part of the question: This subject tends to attract strong opinions (discussion of the subject has been banned on at least one forum). Personally, I think the problem is that there is no official standard for APO performance. This has allowed manufacturers and distributors to come up with their own. Not surprisingly, the Roll Royce brands insist upon a standard that only their ultra-expensive telescopes can meet. Other manufacturers are more ‘practical’.

Experience tells us that price is the most reliable indicator of optical performance. But Gordon is right, it gets progressively more difficult/expensive to squeeze the last ounce of quality/performance out of an already good optic so, inevitably, the law of diminishing returns applies.

In short: Semi APOs like the Sentinel are very good and offer the most ‘bang for buck’. To take it to the next level - TeleVue, TMB and the like - is very expensive so you need to ask yourself...

Am I really bothered by a bit of false colour?

Hope that helps :D

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Currently I'm not permitted to spend any more, this is a planning thing for now. I've been looking at getting a second HEQ5 and a 'scope as there seem to be savings to be made on the deal. To buy an ED80 and HEQ5 seperately costs a lot more than buying them together. Shame the Sentinel isn't offerd with the HEQ5.

Captain Chaos

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I'm very pleased with my ED80 for imaging. Most peoples brains can handle the CA on non apos and "process it out" (ie ignore it). However it causes much more serious problems when imaging including mono because it is not possible to bring the colours into equal focus. Therefore when imaging CA remains a problem even with mono cameras. To what extent this justifies moving from an ED80 to a televue I have no idea. If you are spending a grand on a mount and are using a high tech camera and extra £100 on the ED80 compared with the Sentinel would seem pretty reasonable though.

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A cut and paste from an earlier message then:

Apochromatic means "no colour", i dont think there are any lenses out there that produce no false colour only mirrors can claim that. Therefore it's down to how much colour your willing to live with. Apo is just a marketing buzzword that allows people to charge more money for higher quality optics. IMO

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personally I dont think that "semi without colour" is a valid description. I appreciate the sentiment they are trying to portray, however it only confuses people. I understand though that it's difficult to measure quite how much "without colour" a particular lens has. Theres nothing like a strehl number for apo-ness. Maybe someone should invent one....

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