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56.2mm backfocus

Takahashi FSQ-85EDX Refractors Flattener
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ZoboZodiac avatar
ZoboZodiac avatar
Hi all, I recently started using an FSQ-85 with the edp flattener and I’m just trying to figure out what people do with the back focus. I see in the manual that the back focus is supposed to be 56.2mm and this is 1.2mm more than the standard 55mm.
I’m pairing a ZWO 2600mc duo with it, but at the moment I’m just using 55mm as they’re the adapters which came with it.

What do people usually do with this back focus? It seems most of the Takahashi scopes use 56.2mm but I can’t find many examples of what spacers people actually use in practice
Mariusz Golebiewski avatar
Mariusz Golebiewski avatar
Hi,
Something like this can be useful: Baader T-2 adjustment/spacer-rings
Bruce Donzanti avatar
Bruce Donzanti avatar
Backfocus on the FSQ-85 is extremely sensitive.  I also added 1/3 of my filter thickness to the 56.2mm and used thin spacers like the ones Mariusz mentions above.  There are others I like a little better that you can purchase from Agena Astro.   Take your time to get the backfocus correct.  You won't regret it.
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ZoboZodiac avatar
ZoboZodiac avatar
Thank you both, I’ll have a look at getting some of those spacer rings. I’m sure it’s a non issue, but because they take away a little bit of thread have you ever noticed things being a bit less secure with them on? I’m assuming it’s not an issue in practice
Willem Jan Drijfhout avatar
Willem Jan Drijfhout avatar
Thank you both, I’ll have a look at getting some of those spacer rings. I’m sure it’s a non issue, but because they take away a little bit of thread have you ever noticed things being a bit less secure with them on? I’m assuming it’s not an issue in practice

That's a good point and it greatly depends on the extension tubes you're using. For example, I use both Baader M68 and M48 tubes. The M68 have a very generous length of thread, easy to add 1-1.5mm of spacer ring. The M48 tubes have much less thread, and I would not add more than 0.5mm to them.
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Willem Jan Drijfhout avatar
Willem Jan Drijfhout avatar
Hi all, I recently started using an FSQ-85 with the edp flattener and I’m just trying to figure out what people do with the back focus. I see in the manual that the back focus is supposed to be 56.2mm and this is 1.2mm more than the standard 55mm.
I’m pairing a ZWO 2600mc duo with it, but at the moment I’m just using 55mm as they’re the adapters which came with it.

What do people usually do with this back focus? It seems most of the Takahashi scopes use 56.2mm but I can’t find many examples of what spacers people actually use in practice

Takahashi-scopes all come with system charts, which show which adapters/spacers you need for which configuration. In almost all of them, there is an option that gets you to a DSLR mount, for example Canon EF. On the camera-side, manufacturers such as ZWO almost always have an adapter in their portfolio that give you the same for example EF mount, including the standard EF sensor distance (44mm). So it will give you the back focus distance that you need, with the added benefit of an easy-to-use bayonet-coupling to put the camera on. I have done this for a long time, and served me well, until I started adding tilt-correctors etc.
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