Field Flatteners/Reducers

[Mark2022]

I recently bought an SVBony 503 70ED. There is a specific field flattener/reducer for it but it costs about 3x the price I can buy the 80ED flattener. The 70ED is an F6 scope and the 80ED an F7. SVBony have stated that you can only use the 70ED flattener with the 70ED scope etc BUT a chap on the  Facebook forum has the 80ED flattener with the  70ED scope and says it works absolutely fine. I have tried to communicate with SVBony on this but they seem to be shuffling their feet about it all. What I'd like to know (because I hear of people buying 3rd party flatteners for their scopes all the time) is: Do flatteners actually need to be specific to scopes or not? I read about 3rd party flatteners/reducers being offered all the time as long as your scope is of a certain focal ratio - for example "This flattener suits scopes in the F5 to F7 range".

Can anyone clarify this rather confusing issue?

Thanks.

[Clarkey]

As a general rule the specific flatteners should give slightly better result than the generic versions. However, I am using the generic Stellamira FF with my 115mm F7 scope and the results are absolutely fine (in my eyes). The SM flattener is £79 whereas the specific one is £130. In reality you won't really know until you try (I might be wrong on this - someone with a better idea of optics may say different). Personally, for a small ED doublet I would try a generic version if the cost difference is significant.

[callisto]

I'm using a OVL FF (£89) on my William Optics Z/star 73 III apo... work's very well 👍

[fozzybear]

I use also an OVL FF on SW 72ED no problems. Maybe ask FLO for their advice on either the FLO or stellaMira options?

[Mark2022]

20 hours ago, Clarkey said:

As a general rule the specific flatteners should give slightly better result than the generic versions. However, I am using the generic Stellamira FF with my 115mm F7 scope and the results are absolutely fine (in my eyes). The SM flattener is £79 whereas the specific one is £130. In reality you won't really know until you try (I might be wrong on this - someone with a better idea of optics may say different). Personally, for a small ED doublet I would try a generic version if the cost difference is significant.

Thanks Clarkey. I sometimes get the feeling that these Astro manufacturers just like to make money! 😉

[Mark2022]

18 hours ago, callisto said:

I'm using a OVL FF (£89) on my William Optics Z/star 73 III apo... work's very well 👍

Cheers Fozzy. I think I'll just purchase the SVBony 80ED version at £45.99. As the guy said, it works perfectly well.

[Mark2022]

18 hours ago, fozzybear said:

I use also an OVL FF on SW 72ED no problems. Maybe ask FLO for their advice on either the FLO or stellaMira options?

FLO are ok but I've asked questions of them before and one or two of their answers have led me to purchase something which didn't do the trick. I far prefer to poll users. Good to get that feedback though, Callisto. Cheers.

[LaurenceT]

8 minutes ago, Mark2022 said:

Cheers Fozzy. I think I'll just purchase the SVBony 80ED version at £45.99. As the guy said, it works perfectly well.

Where have you seen that one?

[vlaiv]

20 hours ago, Mark2022 said:

Can anyone clarify this rather confusing issue?

In principle, field flattener - flattens the field. It has no idea what sort of optics sits in front of it, but is design to handle only certain level of field curvature.

If element is pure field flattener - then it will work more or less ok with range of optics that produce field curvature that is ball park of what this flattener corrects. Tweaking the distance of field flattener to focal plane will tweak correction for a given telescope.

Some items passed off as field flatteners are correctors in general and are designed / matched to particular scope (they correct for more than curved field). As such - they produce the best results with said scope, but that does not mean that:

a) said field flattener can't be used effectively with different scope

b) different field flattener can be effectively used instead of that one with scope first FF was designed for

Issue is that it is very hard to make optical element that only corrects for one type of optical aberration. They all introduce some additional aberrations to some degree. Optical aberrations go into "positive" and "negative" direction - and ideally you want to match them between optical elements so that they cancel out - one brings in positive part and other brings in negative part.

This is why "matched" correctors work well - they are designed so that their residual other aberrations for the most part cancel out with those of matching scope.

Now if you use such matching corrector with different scope - a range of things can happen. That different scope might have same / similar aberrations as original scope in which case aberrations will still cancel out to some degree, or scope can have unrelated or even same aberrations as that corrector - in which case aberrations can "enhance" rather than cancel out.

Unless you know optical design of both scope and corrector - there is no way of telling how good a match they will be (unless they are designed to match each other). That is why it is basically trial and error of using generic field flatteners - you need to try with your optics, or take advice of someone who used that combination and found it works satisfactory.

[Clarkey]

1 hour ago, Mark2022 said:

I think I'll just purchase the SVBony 80ED version at £45.99

If you can get any FF for £46 just buy it. Bargain in astro terms.

[Mark2022]

20 hours ago, LaurenceT said:

Where have you seen that one?

It was on Ebay (new) but has since been removed (oddly since I asked the shop the question). However, I've bought one at £49.99 from Retevis on Ebay.

[Mark2022]

19 hours ago, Clarkey said:

If you can get any FF for £46 just buy it. Bargain in astro terms.

The offer disappeared so I bought for £49.99.

[Mark2022]

20 hours ago, vlaiv said:

In principle, field flattener - flattens the field. It has no idea what sort of optics sits in front of it, but is design to handle only certain level of field curvature.

If element is pure field flattener - then it will work more or less ok with range of optics that produce field curvature that is ball park of what this flattener corrects. Tweaking the distance of field flattener to focal plane will tweak correction for a given telescope.

Some items passed off as field flatteners are correctors in general and are designed / matched to particular scope (they correct for more than curved field). As such - they produce the best results with said scope, but that does not mean that:

a) said field flattener can't be used effectively with different scope

b) different field flattener can be effectively used instead of that one with scope first FF was designed for

Issue is that it is very hard to make optical element that only corrects for one type of optical aberration. They all introduce some additional aberrations to some degree. Optical aberrations go into "positive" and "negative" direction - and ideally you want to match them between optical elements so that they cancel out - one brings in positive part and other brings in negative part.

This is why "matched" correctors work well - they are designed so that their residual other aberrations for the most part cancel out with those of matching scope.

Now if you use such matching corrector with different scope - a range of things can happen. That different scope might have same / similar aberrations as original scope in which case aberrations will still cancel out to some degree, or scope can have unrelated or even same aberrations as that corrector - in which case aberrations can "enhance" rather than cancel out.

Unless you know optical design of both scope and corrector - there is no way of telling how good a match they will be (unless they are designed to match each other). That is why it is basically trial and error of using generic field flatteners - you need to try with your optics, or take advice of someone who used that combination and found it works satisfactory.

Thanks Vlaiv. Makes sense.

[mcrowle]

I was delighted to find that the Sky Watcher coma corrector, designed for fast Newtonians like my 12" f/4 also works very well with my SW Equinox 80mm ED refractor. I only wish I'd thought to try it earlier, rather like auto-guiding but that's another story!
 

Regards, Mike.

[Wulfstan]

Wondering how well the 80 ED flattener turned out on the SV 503 70ED.  Pretty much in the same boat here, the 80 ED Flattener is $89 and the 70 ED flattener is $189!

 

given that price difference I might just sell the 70 ED and buy the 80, but before I jump to extremes, I am  interested in hearing how this worked out?