Can this work? Reducer + field flattener at the same time?

[pipnina]

I'm making the move to a 130mm f6.6 triplet, but even though the flatteners seem very high quality and can bring it down to f5~ quite easily, I would preferably go a bit faster if I could! Seemingly the bigger refractors just don't go below f6 natively which is a bit strange since smaller optics can go to f1.4 and be very sharp, while having say a 75mm aperture.

I see flatteners like the Riccardi style, the 2.5" variant having s very good spot diagram and only 3% vignetting at full frame sensor sizes, but I only have an aps-c sized sensor and there doesn't seem to be a high quality flattener like that with a stronger than 0.75 reduction and no vignetting.

So this makes me wonder if instead of the M68 0.75x Riccardi, I went for the M68 1.0x Riccardi instead, but used a dedicated focal reducer as well to get say 0.66x magnification and get an f4.3 experience instead of an f5 experience?

My gut tells me it will hinder the setup in some way, by adding abberations or by causing reflections or messing with the corrected field flatness etc. But I could be wrong, would it work?

[vlaiv]

It won't work.

Not sure which one is M68 version of x0.75 Riccardi, but why don't you go with M63 / smaller version of the two? I know that there are M82 and M63. M82 can illuminate full frame, but M63 can't - it can do only 42mm diagonal, which makes it suitable for APS-C sized sensor (full correction and illumination over 30mm).

Question is - why do you want "faster" scope? Maybe you should aim for specific resolution on that scope rather than "speed"?

By the way - there is no optics that at F/1.4 that is sharp - at least not sharp in telescope terms. I'm yet to see lens that is diffraction limited, let alone with high Strehl ratio.

[pipnina]

29 minutes ago, vlaiv said:

It won't work.

Not sure which one is M68 version of x0.75 Riccardi, but why don't you go with M63 / smaller version of the two? I know that there are M82 and M63. M82 can illuminate full frame, but M63 can't - it can do only 42mm diagonal, which makes it suitable for APS-C sized sensor (full correction and illumination over 30mm).

Question is - why do you want "faster" scope? Maybe you should aim for specific resolution on that scope rather than "speed"?

By the way - there is no optics that at F/1.4 that is sharp - at least not sharp in telescope terms. I'm yet to see lens that is diffraction limited, let alone with high Strehl ratio.

Woops, yeah I was looking at m63 Vs m82

Maybe I'm just getting confused by the way those correctors are advertised, but a typical FF sensor is 42mm diagonal I think, and given as the M63 model lists only 3% vignetting at that radius it would seem very suitable for FF sensors to me, but that relies on the marketing material being trustworthy I suppose.

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p11122_Riccardi-0-75x-APO-Reducer-and-Flattener-with-M63x1-Thread.html

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p5125_APM-Riccardi-APO-Reducer-and-Corrector-0-75x---M82-connection.html

I do note though that the m63 only mentions the 42mm figure while the m82 actually advertises FF coverage. But only the m63 model includes a spot diagram and vignetting stats, so it's hard to see what I'd be gaining if the stat sheet for the smaller one already shows near perfect coverage...

 

As for superfast lenses. Oh yeah I suppose they aren't close to diffraction limited like most quality telescopes, but the quality ones do have very good optical qualities regardless such as almost 0 distortion, extremely consistent star sizes across the field etc, although at f1.4 the vignetting can be as strong as 70% as per the sigma 105mm pro art. But I think if the intention is to use the camera's native pixel scale for that lens, being diffraction limited would cause an f1.4 optic to be insanely oversampled. But if the desire is for speed and forgoing extra Res from drizzle, then the softer faster optic might be preferable as long as the image isn't oversampled I suppose? But yes I suppose it is true a telescope like the RASA will have much more contrast in a given spot diagram than even a very expensive f2 camera lens.

[vlaiv]

22 minutes ago, pipnina said:

As for superfast lenses. Oh yeah I suppose they aren't close to diffraction limited like most quality telescopes, but the quality ones do have very good optical qualities regardless such as almost 0 distortion, extremely consistent star sizes across the field etc, although at f1.4 the vignetting can be as strong as 70% as per the sigma 105mm pro art.

If you want to see how a lens performs - check out its MTF diagram - and pay attention to LPMM part (that is line pairs per mm - and convert that to pixel size - two pixels per line pair / one pixel per line and one per gap).

You'll be surprised what sort of pixel sizes match lens.

I have Samyang 85mm T1.5 (same as F/1.4 only "cinema" version without click stops on aperture ring).

Above is MTF wide open. Red line is 10 lpmm while grey is 30 lpmm.

30lpmm corresponds to pixels being 16.666um (1000um in mm / 30 = 33.3333 per line pair, half of that per pixel so 16.666um) and MTF is already at 70% and dropping with distance from the center.

Here is what stars look like (Artificial star) at center and at corner of 4/3 sensor (ASI1600):

Top to bottom is F/1.4, F/2, F/2.8 and F/4 and left is center while right is edge.

In first row center star is not showing this good because exposure was not long enough - but there is like massive halo from chromatic aberration.

This is actual star field at F/1.4 in R, G and B channel - you can see how much bloat there is in red compared to green and blue has halo around bright star.

Same image in color reduced to %50 size (pixel size 4.8um as this was taken with ASI178mmc):

Strange star color can be easily seen as well as halo around bright star.

While center of the field can be tidied up by stopping aperture to say F/3.2 or maybe F/4 - astigmatism+coma in outer field can't (they also show at F/4 only 11mm away from the center given that diagonal of ASI1600 is only 22mm).

But we digress.

I think that m63 Riccardi will be excellent FF/FR for that scope (and probably most other refractors).

 

[pipnina]

2 hours ago, vlaiv said:

If you want to see how a lens performs - check out its MTF diagram - and pay attention to LPMM part (that is line pairs per mm - and convert that to pixel size - two pixels per line pair / one pixel per line and one per gap).

You'll be surprised what sort of pixel sizes match lens.

I have Samyang 85mm T1.5 (same as F/1.4 only "cinema" version without click stops on aperture ring).

Above is MTF wide open. Red line is 10 lpmm while grey is 30 lpmm.

30lpmm corresponds to pixels being 16.666um (1000um in mm / 30 = 33.3333 per line pair, half of that per pixel so 16.666um) and MTF is already at 70% and dropping with distance from the center.

Here is what stars look like (Artificial star) at center and at corner of 4/3 sensor (ASI1600):

 

Top to bottom is F/1.4, F/2, F/2.8 and F/4 and left is center while right is edge.

In first row center star is not showing this good because exposure was not long enough - but there is like massive halo from chromatic aberration.

 

This is actual star field at F/1.4 in R, G and B channel - you can see how much bloat there is in red compared to green and blue has halo around bright star.

Same image in color reduced to %50 size (pixel size 4.8um as this was taken with ASI178mmc):

 

Strange star color can be easily seen as well as halo around bright star.

While center of the field can be tidied up by stopping aperture to say F/3.2 or maybe F/4 - astigmatism+coma in outer field can't (they also show at F/4 only 11mm away from the center given that diagonal of ASI1600 is only 22mm).

But we digress.

I think that m63 Riccardi will be excellent FF/FR for that scope (and probably most other refractors).

 

Indeed I have noticed my own camera lenses suffering from some questionable coma relatively close to center as well.

That said the sigma 105mm f1.4 pro art seems to claim what looks like a far higher MTF... At least in the center?, but I haven't found anyone using the lens for astro to try and judge the quality/sharpness in real life tests. Sigma claim it uses "FLD" (fluorite equivalent, supposedly) grade elements in these lenses which would imply it is very up market if telescope marketing translates here.

That said, the 70% vignette shows up very strongly and represents an insane almost 2 whole stop difference between the center and edges, I can only imagine that becoming visible in final corrected images where the center looks cleaner than everywhere else...

 

As for the riccardi, thinking about it, what would the real benefit be of the m82 version if the m63 can actually illuminate all but the very corners of a full frame sensor to 100%, and full frame is illuminated by 97% even at the very edges. Just for illumianting even *bigger* sensors like 36*36mm square sensors or items like the ZWO medium format? Or maybe it depends on the light cone and focuser, where some telescopes might vignette more easily on the smaller riccardi?

Either way, I guess 130mm triplet at f5 would still be better for speed than my current 200mm f4 newt on account of the lack of central obstruction combined with 33% off-center vignette, compared to no obstruction and no vignette. Certainly better than my old 130P-DS newt.

 

[Richard_]

What about the Starizona Apex? This is an 0.65 reducer which comes in two sizes, with the larger size being suitable for focal lengths >500mm. According to the manufacturer, it has an imaging circle of 30mm and they recommend use with sensor sizes up to APS-C. 

I don't have this reducer, but I was considering this before settling on the WO x0.8 reducer. Thought it might be worth looking into! 

https://www.firstlightoptics.com/reducersflatteners/starizona-apex-ed-065x-reducerflattener.html

https://starizona.com/products/apex-ed-65x-reducer-flattener-for-apo-refractors-and-rc-acf-telecopes

 

[pipnina]

12 minutes ago, Richard_ said:

What about the Starizona Apex? This is an 0.65 reducer which comes in two sizes, with the larger size being suitable for focal lengths >500mm. According to the manufacturer, it has an imaging circle of 30mm and they recommend use with sensor sizes up to APS-C. 

I don't have this reducer, but I was considering this before settling on the WO x0.8 reducer. Thought it might be worth looking into! 

https://www.firstlightoptics.com/reducersflatteners/starizona-apex-ed-065x-reducerflattener.html

https://starizona.com/products/apex-ed-65x-reducer-flattener-for-apo-refractors-and-rc-acf-telecopes

 

I looked at it, but some anecdotes suggested it wouldn't really satisfy an APS-C without considerable vignetting. I am struggling to find hard stats or a spot diagram to show me the level of quality and correction and illumination so I could compare it to the riccardi. As I understand the small riccardi and large starizona are similarly priced.