Confusion about image circles (tak fsq106 vs. others) - Forums

Topic starter

Apr 11, 2022

Maybe someone can sort out my confusion about the different sizes of image circles in relation to the f-ratio.
While studying the specs of a Takahashi fsq106 with its huge stated image circle of 88mm (let's not discuss how usable that would be all the way to the edges) I was asking myself if the surface brightness on the chip for a given object (or speed) would be lower than other refractors with the same f-ratio of F5.0, same focal length of 530mm (or same aperture respectively) and an image circle of let's assume only 44mm.
Or are the image circles for all refractors (telescopes) with a given aperture and focal length always the same and hence such a telescope doesn't exist? Then my question would become obsolete of course…
If such a scope existed and the brightness/speed of both scopes were the same (which should be assumed since they both have the same f-number) then my only explanation would be that some of the initially produced light cone is being constricted by baffles or somewhere else in the OTA.
Otherwise if such a scope with 44mm image circle would focus all the light entering its lens into the smaller image circle the image should be brighter in my understanding (in this example it should result in four times the brightness of the tak).

So what is your understanding? What am I missing?

Thanks, Michael

Engaging

andrea tasselli

Frequent contributor

Apr 11, 2022

Stated "usaeable" image circle diameter has nothing to do with brightness of the same. It (the image circle) only depends on the optical and mechanical construction of the scope in question so that light will fall even at the edge of that circle. Specific brightness only depends on the f-ratio, at least for the central paraxial ray bundle.

Michael E.

Topic starter

Apr 12, 2022

Thanks for your explanation!So my understanding is as follows:
All refractors (don’t want to mix in the effect of secondary mirrors of reflectors) with the same aperture and focal length should theoretically produce an image circle of the same diameter (assuming a pezval design doesn’t change things significantly) if there were no constrictions in the tube. Then it’s the manufacturers decision to see how much of the image circle is useable and constricts it accordingly. Does that make sense?

andrea tasselli

Frequent contributor

Apr 12, 2022

It does

Rouz Astro

Apr 12, 2022

All 106mm definitely wont produce an 88mm mage circle.

Most 106mm refractors will struggle to perfectly illuminate a full frame chip at 43mm with spots at the corners almost equal to the center.

in fact, I doubt the FSQ106 can produce sharp star at the edge of anything close to 88mm - 44mm off axis.

Tak site only shows spots up to 22mm off axis.

There are no numbers but I suspect modern small CMOS pixels we pick up aberrations at the edge of circles much smaller than 88mm.

https://www.takahashiamerica.com/takahashi-fsq-106edx4-quadruplet-refractor-telesco.html

Wei-Hao Wang

Apr 13, 2022

For the same aperture and F-ration, the illumination on the focal plane depends mostly on the sizes of the internal components like the focuser, light baffles in the OTA, flattener/reducer, etc. Clearly a scope of a 2" focuser can't produce a 4" illumination, right?

Helpful Concise

Rouz Astro

Apr 13, 2022

Wei-Hao Wang:
For the same aperture and F-ration, the illumination on the focal plane depends mostly on the sizes of the internal components like the focuser, light baffles in the OTA, flattener/reducer, etc. Clearly a scope of a 2" focuser can't produce a 4" illumination, right?

A 2 inch focuser usually works up to a 4/3 chip maybe an APS-C.

The faster the F ratio are larger the components need to be.

For example, my telescope has a 70mm circle, the focuser is 90mm at the opening.

I use a 43mm chip with almost no vignetting.