Imaging fracs not really designed for imaging...

[nmoushon]

I've noticed lately that imaging scopes aren't really suited for imaging. Well, besides the fast astrograph that cost you hand or a leg or the F/4 Newts. So I guess I'm taking about Refractors and RCs. I'm not very knowledgable when it comes to the optical design on telescopes so if I'm horribly off basis here I'm sorry.

But when I see refractors and RCs that are touted as great imaging scopes their native f/ratios are never suited very well for imaging. Refractor are better suited than the RCs but not by much. I tend to see fracs native ratio between f5.5-f7 and with RCs they tend to be F/8 or F/9. The only way this makes sense to me is that is all a marketing scheme to get you to buy more equipement so that you can use the for imaging. i.e. A reducer.

Take my ED80 Equinox for example. Its at a native f/6.25. Not the best for imaging. I would think it would be just as easy to make it at least F/5 as it would be to make it F/6.25 or faster at F/4 even. With the RC why can't they make the faster? Not necessarily f/4 but why not f/6? I know its not because of the wanted long FL b/c a reducer do just that anyways.

With that said I do tend to over simplify things sometimes and this might be one of them. So is it a marketing scheme to get more money or does the design becomes so much more difficult that the cost to make it isn't worth it or is there something else I'm missing?

[mytola]

I agree that some scopes that are marketed as imaging scopes really are unsuited. But there are physical reasons for the fact that companies don't make cheap fast scope for imaging I think. As I've understood for refractors, the faster the lens system, the more difficult it is to control the aberrations...you need more complex systems with more glass, which means more elements that can get out of collimation with the optical axis, thus more precise manufacturing and QC needed...and expensive low dispersion glass is needed to avoid ca.

Someone correct me if I'm wrong, but for catadioptric scopes like RC or SCT, I think it's a matter of the size of the central obstruction...a faster scope needs a bigger secondary mirror, so you need to make a trade-off. Then some include a reducer, but then you have all the problems coming with a refracting system again.

[ollypenrice]

As far as refractors go it's pretty simple. True flatfield fast astrographic refractors do exist but you have to pay for them. The less you want to pay the slower they get, and/or the worse corrected they get and/or the smaller flat field they offer.

I don't know what limits RCs to slow F ratios. It may be the central obstruction but also the fact that both mirrors are hyperbolic and making them ever faster, while being compatible, may get to be a nightmare.

For me fast and cheap don't go together. It really is that simple. For those prepared to faff around sorting them out, there are cheap fast imaging Newts but I won't be buying one. It just isn't my thing. I like imaging, not fiddling.

Olly

[nmoushon]

Ok so if making a replica of my ED80 but faster (at F5 instead F6.25) doesn't exist only because the maker doesn't want to go through the trouble of making sure its corrected? Is making sure they are corrected really that difficult? I'm sure for someone like me that knows very little it would be a nightmare but for the pros I would think it can't be that hard. Especially with all the computer and machines doing most the work and calculations now a days. Unless I'm utterly wrong and making them fast and corrected is extremely dificult.

Take your FSQ106 for instant. Native F/5 is fast no doubt. But why did Tak include a reducer to bring it down to F/3.6 instead of just make it F/3.6 to begin with? I have a suspicous feeling the price of a Tak is not because the process to make equivalent telescopes is so hard and expensive but because they are the one of the few that take the time take make something to that perfection. Not saying anything bad about them mind you. Just I dont understand why reducers are needed if you can just build a scope from the beginning to the exact specs needed.

[perfrej]

The reducer is bound to decrease image quality as it adds a bunch of extra glass and associated defects. Maybe the price and difficulty of making a true f/3.6 scope of that size is just too high - or it may be a downright impossible task.

/per

[ronin]

Optically neither a spherical lens, spherical mirror or parabolic mirror are designed for imaging.

The whole lot are a compromise that will result in a non-ideal image.

[mytola]

Wonder when/if we'll see diffractive optics in telescopes...

[JamesF]

Isn't this largely because the lower end "imaging" scopes have to be something of a compromise to cover as many people's needs as the manufacturer deems reasonable to make production worthwhile?  Not everyone wants to capture massive fields of view in one hit.  Some may be willing to compromise a bit of imaging time in return for greater image scale.  People starting out in DSO imaging (who presumably make up a fair proportion of the buyers at that end of the market) may find it easier to cope with optics that are a little less demanding.

I believe one of the reasons RC designs tend to be slower is just the physics of optics.  From memory, where you have multiple optical components working together the effective focal length (and thus the focal ratio) varies as the product of that of the individual components so it's easy to reach a "slow" design very quickly.

James

[opticalpath]

..... Especially with all the computer and machines doing most the work and calculations now a days. Unless I'm utterly wrong and making them fast and corrected is extremely dificult.

 

With few exceptions, all scope designs have a focal plane that is curved, not flat.  Not good for imaging with modern larger camera sensors, and it gets much worse at fast f/ratios.  The fast 'flat-field' designs incorporate extra optical elements to correct this and other aberrations that get worse at fast f/ratios.  The extra elements add significantly to the cost and complicate the design; you see something similar in fast photo lenses. 

 

So it's not that it can't be done, just that it's very expensive to do it well, like in the Tak. FSQs. 

 

Adrian

[ollypenrice]

Ok so if making a replica of my ED80 but faster (at F5 instead F6.25) doesn't exist only because the maker doesn't want to go through the trouble of making sure its corrected? Is making sure they are corrected really that difficult? I'm sure for someone like me that knows very little it would be a nightmare but for the pros I would think it can't be that hard. Especially with all the computer and machines doing most the work and calculations now a days. Unless I'm utterly wrong and making them fast and corrected is extremely dificult.

Take your FSQ106 for instant. Native F/5 is fast no doubt. But why did Tak include a reducer to bring it down to F/3.6 instead of just make it F/3.6 to begin with? I have a suspicous feeling the price of a Tak is not because the process to make equivalent telescopes is so hard and expensive but because they are the one of the few that take the time take make something to that perfection. Not saying anything bad about them mind you. Just I dont understand why reducers are needed if you can just build a scope from the beginning to the exact specs needed.

It's not that they don't want to take the trouble to make fast corrected lenses. Rather, they suspect that you wouldn't want to pay for them! Despite the available technology fast optics really are expensive to make, to assemble and to fit out with a mechanical infrastructure capable of allowing them to perform. This is why the fast cheap imaging reflectors on the market are best regarded as kits. Their cheap mechanical parts will not usually work from the box.

Your ED80 is a doublet. There is no way in the world you can hope to get a doublet much faster than that. Adding a third lens helps, but it helps the price as well, and no 'front element only' refractor can give a flat field. A rear element is always needed.

There are several reasons, I think, why Takahashi didn't make the FSQ at F3.6 native. Firstly it might be very difficult with a standard 4 element Petzval design. I don't know, but maybe an additional reducing lens would still be necessary. Secondly a selling point of the FSQ106 is its utterly enormous corrected flat field, originally designed to cover medium film. As CCD sensors get bigger (one SGL member already uses a 36x36mm chip) this will be increasingly important. However, the 88mm circle at F5 is cropped viciously down to 42mm (or 44, can't remember) at F3.6. That's not quite enough for my chip, never mind the 36x36. Thirdly, most owners like the choice. Wider but less resolved field or smaller, more highly resolved one. And, maybe, not everyone wants to be trying to focus at F3.6 and keep focused all night.

Olly

[pete_l]

Ok so if making a replica of my ED80 but faster (at F5 instead F6.25) doesn't exist only because the maker doesn't want to go through the trouble of making sure its corrected? Is making sure they are corrected really that difficult?

Yes, basically - on an industrial scale,  it is.

To put it simply:

• Fast
• Good
• Cheap

pick 2

[steppenwolf]

Rather, they suspect that you wouldn't want to pay for them! Despite the available technology fast optics really are expensive to make, to assemble and to fit out with a mechanical infrastructure capable of allowing them to perform.

I rather suspect that this is the crux of the matter - yes, computer-aided design can come up with exotic lens shapes and computer controlled lens polishing can turn the design into reality but this is just the start. Holding this lot together in the harsh environment of the outdoors when imaging and keeping it all in place and aligned through various temperature changes is the real challenge and this adds a further layer of complexity and cost. This mechanical underpinning becomes more critical with shorter focal lengths where the focus sweet spot reduces dramatically so it is no wonder that there are so many reports of issues in this realm!

The reason for the unbelievable success of the SW ED80 as an imaging instrument lies in its relatively slow focal ratio of f7.5 and the fact that it uses only two optical elements - there's not much to go wrong although they still require a field flattener to be of any use for imaging. Unfortunately, as you progress in imaging the call of better correction of the various aberrations becomes stronger but the cost increases exponentially which is why there is such a good market for a true APO with a short focal length at a reasonable price and therein lies the problem - we want the technology but don't want to pay for it! Something has to give and it is usually the mechanicals rather than the optical elements themselves that fall short. All 'normal' refractors, including true 3 element APOs suffer from field curvature so we always need to factor in the cost of a field flattener but we don't always want to reduce the field of view at the same time. It is disingenuous of the manufacturers to describe these competitively priced instruments as astrographs without pointing out clearly that they still require correction of field curvature to be of any use!! The option of reducing the field of view should be just that - an option!

[Peter Drew]

Whatever happened to the Schmidt camera, typically F2 or faster? 

[nmoushon]

Thanks for all the explanations. I guess I didn't really understand just how demanding it was to get everything in order to make an imaging refractor and then making it fast just makes it even more demading. Maybe in the next decade or so we'll see technology advance enough that buying a Tak FSQ wont put you in the dog house for the year. But until then it is what it is. Not that its bad as it is.

[JamesF]

Whatever happened to the Schmidt camera, typically F2 or faster? 

Isn't Fastar/Hyperstar effectively a Schmidt camera?

James

[ollypenrice]

Isn't Fastar/Hyperstar effectively a Schmidt camera?

James

Well, I believe a Schmidt camera used a curved photgraphic plate well down below the corrector plate. The Hypestar uses a lens close to the chip at the top of the the tube. There is a resemblance, though. (Besides being pigs to get right, that is!)

Olly

[Earl]

The advantage of TAK's without reducers is there is no spacing to be worried about.

Did I ever say I hate spacing?

[Davey-T]

Having recently been in the market for a new refractor I found there are F/7 triplet "imaging" refractors that  are unable to bring some 2" eyepiece , diagonal combinations to focus, so in trying to give the best of both worlds they end up failing in both camps.

So then having spent £1500-£2000 on a scope  you have to resign yourself to spending another £200-£250 on a reducer/ flattener

Dave

[opticalpath]

.... It is disingenuous of the manufacturers to describe these competitively priced instruments as astrographs without pointing out clearly that they still require correction of field curvature to be of any use!! The option of reducing the field of view should be just that - an option!

Second that! Wildly optimistic labelling of uncorrected scopes as 'astrograph' makes me sigh.

Other marketing hypes that set me off include:

- describing systems as 'coma free' but failing to mention the other problem aberrations that remain uncorrected, such as field curvature or astigmatism.

- showing theoretical spot diagrams as they would look on the curved focal plane .... not as they would look projected on to a real-world flat camera sensor.

Adrian

[ollypenrice]

...which is why I'm a bit picky when budget manufactuers claim to be taking on the big boys when they are not within a country mile of doing so. You can end up being called a snob when in fact you're talking about simple realities like whether or not this instrument will give a clean, evenly illuminated image on a flat CCD chip bigger than a clearly specified size.

Olly

[kirkster501]

A great discussion.

Back to the OP.  I bring my RC8 scope down from F8 to F5.6 with a telecompresser.  Its still a narrow FoV though.

DSO imaging at F8 and slower in the UK with our light polluted skies and rubbish weather requires the patience of a saint.  It will end in frustration.

[nmoushon]

The discussion is fine by me. Its actually part of why I put up the OP in the first place. I understand now that getting optics good enough to image is hard and to get them fast makes it harder but I still can't get around companies labling products as astrographs or the perfect imaging scope when in reality its not.

I still don't understand why they can't make the reducer/flattener a permanent part of the scopes optical train instead of an add on. You can't image without them anyways. Well you can just not to the correct standards. Would shift more weight to the center of the scope as well as put less weigh on the focuser.

[kirkster501]

I still don't understand why they can't make the reducer/flattener a permanent part of the scopes optical train instead of an add on.

Size of the potnetial market.  Such a fast scope may not sell and be worthwhile.  I am sure they have done their market research.  I'd prefer a premium refractor to be F5 andfgive me an option of a wider field by using the - expensive - reducer.

[nmoushon]

I agree when it comes to scopes like the Tak FSQs but take my ED80 for example. You can't image w/o the R/FF because its slow-ish and doesnt have a flat field so why not have it built in? This is where I think its a marketing ploy to just make more money. I can't really blame a company to want to make money but still.

[Gina]

...which is why I'm a bit picky when budget manufactuers claim to be taking on the big boys when they are not within a country mile of doing so. You can end up being called a snob when in fact you're talking about simple realities like whether or not this instrument will give a clean, evenly illuminated image on a flat CCD chip bigger than a clearly specified size.

Olly

I take your point Olly   I'll see how the Esprit 80ED performs but my long term wish of a baby Q seems to have drawn nearer