Focal Length

[Graeme1858]

My 9.25" (235mm) F10 SCT has a focal length of 2350mm. I put an F6.3 focal reducer on it which should, according to the maths, reduce my focal length to 1480mm.

I use ASTAP to Plate Solve and it's excellent, really quick. However, it reports that my focal length should be set to 1317mm.

My camera back focus is bang on. Nothing else has changed. My ASI294MC used to do the same thing. The Plate Solve still works, so it's not too much of a problem but it would be good to know what's happening.

Astap FL1370mm.JPG (19.69 KiB) Viewed 1284 times

Thanks

Graeme

[JayTee]

I have noticed the same thing using Plate Solve 2. It makes me wonder also. But, I look at the frame and if it includes everything I want, well then I chalk it up to a mechanical curiosity. In my case, the only thing in the optical path is a field flattener that should not alter the FL at all. Hmmm!

I just did a plate solve in All Sky Plate Solver from one of last night's images and it computes an FL of 708mm. The scope is listed at 714mm, so I guess that isn't such a big deal.

[Graeme1858]

Yes, the plate solve still works so it's possibly not a problem.

But 1480 to 1317 is a huge difference and it means that I have to enter the settings in NINA incorrectly to match or the plate solving doesn't work. So that means I can't be certain of the displayed field of view in the Framing tab. Also, I use Stellerium as the planetarium in NINA so I need to set the ocular settings incorrectly for that to display correctly too.

My main issue is that I have a 13% error in my focal length figures that I can't explain and there could be implications that I've not thought of!

Graeme

[KathyNS]

SCTs have variable focal length, since they focus by altering the focal length of the system. 2350 mm is just the nominal focal length, what it was designed around. It will probably give you the clearest image if you arrange the back spacing so that the focal point is actually at 2350 mm. But measuring different focal lengths by plate solving is normal for SCTs.

[Graeme1858]

Ah, yes, that makes sense.

So would a 1480mm to 1317mm focal length reduction be considered normal and to be expected?

Also, is it not the case that all telescopes adjust their focus by adjusting their focal length? And they come to focus at their design focal length. So my 2350mm telescope will come to focus somewhere either side of 2350mm depending on the optics attached. My focal reducer should reduce that by x 0.63 to 1480.5mm. But it actually reduces it by x 0.56 to 1317mm.

I've got the 105mm documented back focus measurement and my stars are perfectly round across the field. So I won't be changing that.

So perhaps I'll use 1317 in Stellerium and NINA and stop worrying about it and enjoy the wider field of view!

Graeme

[Bigzmey]

Also, is it not the case that all telescopes adjust their focus by adjusting their focal length? And they come to focus at their design focal length.

Other designs (Newts, fracs) have fixed focal length and focus by moving EP or camera. It is only in some folded designs (SCT, Maks) one moves the prim mirror to focus and thus changes FL. But even there you have an option of attaching external focuser. In such case one can lock the primary at the fixed FL, but not necessary "native" FL since you need to accommodate light path added by focuser.

[KathyNS]

Also, is it not the case that all telescopes adjust their focus by adjusting their focal length?

No. Most telescopes have a fixed focal length that never changes. Those telescopes focus by moving the eyepiece until its focal point coincides with the telescope's focal point.

Cassegrain designs (SCTs, Maks, and possibly some others) focus by keeping the eyepiece stationary and changing the focal length until the focal point coincides with the eyepiece's focal point.

[Graeme1858]

Thanks for the clarification. All these years of using reflectors and I never really thought much about the difference between focal length and length of light path!

Anyway, getting back on topic, is it normal for a 2350mm focal length catadioptric telescope with a 6.3mm focal reducer to have a focal length of 1317mm? Or is there something wrong with my telescope?

Graeme

[yobbo89]

you should be able to adjust your focal length with the distance from ccd/cmos to barlow aswell, i always let fire capture calc the target fl and relabel the image after for a quick documentation when imaging planets. i don't know where i have seen this ,but i recall someone might of had a calculation for it ie 1mm spacing = 13mm adjusted real fl

[SkyHiker]

Thanks for the clarification. All these years of using reflectors and I never really thought much about the difference between focal length and length of light path!

Anyway, getting back on topic, is it normal for a 2350mm focal length catadioptric telescope with a 6.3mm focal reducer to have a focal length of 1317mm? Or is there something wrong with my telescope?

Graeme

This CN thread has a table for a C11 and focal reducer, HTH.

[Graeme1858]

you should be able to adjust your focal length with the distance from ccd/cmos to barlow aswell, i always let fire capture calc the target fl and relabel the image after for a quick documentation when imaging planets. i don't know where i have seen this ,but i recall someone might of had a calculation for it ie 1mm spacing = 13mm adjusted real fl

Thanks for that Rob. But, that would need an additional 12mm of spacer to bring the fl up to 1480. Or maybe 12 x 0.63, but either way this would put me well over the 105mm recommended back focus and this would start happening.

[Graeme1858]

Well it seems to me, all things considered, in this thread and Henk's thread, (thanks Henk), that there's nothing wrong with my telescope and nobody knows why my focal length is 1317 not 1480 given my native focal length, focal reducer and camera backfocus.

So I'll set up Stellerium and NINA to 1317fl and get on with it!

Thanks for everyone's input.

Graeme

[SkyHiker]

Well it seems to me, all things considered, in this thread and Henk's thread, (thanks Henk), that there's nothing wrong with my telescope and nobody knows why my focal length is 1317 not 1480 given my native focal length, focal reducer and camera backfocus.

I think Kathy had it right. Anytime you focus with an SCT, for instance to add the focal reducer, you change the focal length of the combined primary and secondary mirror system. It is not that difficult to analyze with thin lens formulas, equating the SCT to a Barlowed primary objective lens. The analysis shows that you cannot simply apply the focal reduction factor to the original (non-reduced) focal length. I attached the calculations if you are interested.

I had been looking into the Barlow formulas when I wanted to image Saturn by putting a Barlow into my 12" Newt astrograph (using more than 20 cm of rings to get focus). I figured I could try adding a focal reducer to that idea and solve your problem. Hopefully I did if there are no bugs.

[Don Pensack]

Ah, yes, that makes sense.

So would a 1480mm to 1317mm focal length reduction be considered normal and to be expected?

Also, is it not the case that all telescopes adjust their focus by adjusting their focal length? And they come to focus at their design focal length. So my 2350mm telescope will come to focus somewhere either side of 2350mm depending on the optics attached. My focal reducer should reduce that by x 0.63 to 1480.5mm. But it actually reduces it by x 0.56 to 1317mm.

I've got the 105mm documented back focus measurement and my stars are perfectly round across the field. So I won't be changing that.

So perhaps I'll use 1317 in Stellerium and NINA and stop worrying about it and enjoy the wider field of view!

Graeme

Uh, no. As the mirrors have their spacings increased or reduced by moving the primary mirror, this changes the focal length of the scope--for every 10mm added to the back length over 100mm (aprox), the focal length of the scope increases about 31mm on the 8" SCT.
It'll be slightly different on the 9.25" since the primary mirror is slower and the secondary magnifies less.
Focal reducers are like anti-Barlows--with increasing distance from the lens, the reduction increases, the opposite of a Barlow.
Hence, an f/6.3 reducer will only yield a 37% reduction at one distance back from the lens (I don't know what that is, but it's about that with a 1.25" diagonal and increases its reduction to 45-50% with most 2" diagonals.
Perhaps your camera's chip sits a good distance back from the lens.

With most telescopes (refractors, newtonians, etc.), the focal length of the scope is fixed and varying the position of accessories does not change the focal length.