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F10 apo's?

R26 oldtimer

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dweller25

dweller25

Posted
Posted
14 minutes ago, R26 oldtimer said:

Well..., as the title says, why aren't there any f/10 or even slower triplet apo's or ED doublets? Have apochromatic scopes turned to telephoto lenses, so need to be fast, or slow achromats are just as good?

I' m getting my pop-corn....

:happy1:

Probably because :-

1. You can get the same colour correction out of a shorter f/ratio which is lighter, shorter and therefore easier to use.

2. The trend is more towards fast focal ratios for imaging.

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DaveS

DaveS

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Posted

Just so.

The driving force for current triplet apos is for short, fast scopes, whether for imaging or visual. Skywatcher do 100mm f/ 9 ED doublets, while CFF offer longer fl versions of some of their models, at a price.

As I recall the original and legendary Cooke Photovisual Triplet design often went to f/15.

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ollypenrice

ollypenrice

Posted
Posted
1 minute ago, Dave1 said:

They no good then Olly?

Do they even exist? The person behind this thing was also behind the 'Cape Newise' which sold in some quantity despite having a number of dire flaws. Peoples' fingers and wallets were burned but the person in question proved (in my view) to be quite incapable of grasping the relatively simple idea that this was his fault and not the fault of his subcontractors. The point is that the consumer does not buy from the subcontractor. The buck stops with the person who makes the sale. Regarding the Zerochromat, the designer said on here that he could upscale them to much larger apertures (beyond anything he had actually manufactured at that time.) Now you are perfectly at liberty to believe his assertion that this would be entirely straightforward - or to take my view that making a big one first before talking too loudly about it would be good policy.

Olly

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Stu

Stu

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Posted

I've always just been frustrated by the fact that there are no review units in trustworthy hands. In theory they could be great, but that's all it seems to be currently.

Olly, I actually saw a Cape Newise 8" scope at AstroFest being used to demo a somewhat agricultural mount :) . I very nearly spent my hard earned cash on one of these, so glad I didn't!

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Davey-T

Davey-T

Posted
Posted
9 minutes ago, Stu said:

I've always just been frustrated by the fact that there are no review units in trustworthy hands. In theory they could be great, but that's all it seems to be currently.

Olly, I actually saw a Cape Newise 8" scope at AstroFest being used to demo a somewhat agricultural mount :) . I very nearly spent my hard earned cash on one of these, so glad I didn't!

I considered a Cape Newise after reading a glowing review in S@N magazine but plumped for a 10"SCT :rolleyes2:

Dave

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mikeDnight

mikeDnight

Posted
Posted
40 minutes ago, Stu said:

I've always just been frustrated by the fact that there are no review units in trustworthy hands. In theory they could be great, but that's all it seems to be currently.

Olly, I actually saw a Cape Newise 8" scope at AstroFest being used to demo a somewhat agricultural mount :) . I very nearly spent my hard earned cash on one of these, so glad I didn't!

I'm trying to tame my comments, so I'll say I'm so glad you didn't fall for the Cape Newise Stu. Two of the things were on loan to my local astronomy club and the optics were just monstrous! 

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mikeDnight

mikeDnight

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Posted

If someone is looking for a long focal length apo, they could do far worse than the Takahashi FC100DL at F9. If its absolutely necessary to go longer, then one of the FC100DC or DF F7.4 with the amazing Tak 1.6X extender Q attached will give you an optically perfect F11.8. Or if you're really bent on long FL, then the FC100DL with Tak 1.6X extender Q will give you F14.4. 

If larger aperture is needed then the Takahashi TSA or TOA, or an older FS 128 or 152 with their respective extender Q's attached will do the job perfectly. 

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25585

25585

Posted
Posted
40 minutes ago, mikeDnight said:

If someone is looking for a long focal length apo, they could do far worse than the Takahashi FC100DL at F9. If its absolutely necessary to go longer, then one of the FC100DC or DF F7.4 with the amazing Tak 1.6X extender Q attached will give you an optically perfect F11.8. Or if you're really bent on long FL, then the FC100DL with Tak 1.6X extender Q will give you F14.4. 

If larger aperture is needed then the Takahashi TSA or TOA, or an older FS 128 or 152 with their respective extender Q's attached will do the job perfectly. 

Now there is temptation. ? ?

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R26 oldtimer

R26 oldtimer

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Thank you all for your replies!

Actually there is the option of stepping down a big 5"-7" F7-9 apo to >F10, but that would be a bit "expensive" and purposeless. Also, although Taks are great, they are a bit on expensive side. Again you can go down the  extender - barlow  way, but I don't think it's better than a natively designed longer focal length scope.

Since I found that a focal ratio of 1000-1200 mm works very well for all-round visual (at least for me), and after seeing the very well priced SW 72ED, I thought that an 80-100mm doublet ED  F11-F12 at that reasonable price would be great. It looks (hopefully) that mass production of quality ED doublets will sooner or later substitute normal achromatic doublets

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