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David Hardie

Quick Q About Focus Length

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Hi folks. I'm setting up my ZWO 1600 with my Edge HD and filter wheel.

The length required is 133mm.

The question I have is how critical is the 133? With included spacers I can make 132 haha. Or is there a better way to go, plus or minus the figure.  Or could it be a case that once set at 133 it may need adjustment anyway?

New to this so all advice welcome+

Cheers. David

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I don't think you're talking about focal length here unless there's a typo. You're unreduced scope has an FL of 2000mm becoming 1400mm with the 0.7mm reducer. Where does this figure of 133mm come from? Without the reducer you just focus and shoot.  With the reducer you need to respect the 'chip distance' between reducer and chip. Maybe this is your 133mm? If so you'll need to add about 1mm for filter effects but with smaller chips very high precision isn't usually needed. But first let's clarify the 133mm.

Olly

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1 hour ago, ollypenrice said:

I don't think you're talking about focal length here unless there's a typo. You're unreduced scope has an FL of 2000mm becoming 1400mm with the 0.7mm reducer. Where does this figure of 133mm come from? Without the reducer you just focus and shoot.  With the reducer you need to respect the 'chip distance' between reducer and chip. Maybe this is your 133mm? If so you'll need to add about 1mm for filter effects but with smaller chips very high precision isn't usually needed. But first let's clarify the 133mm.

Olly

Thanks mate yes I didn't word it very well it's the camera to Scope back focus measurements. It's supposed to be 133 mm from the scope back thread to the camera sensor. I've been able to make up adaptors like the ones shown here but just wondered how critical the size is. Maybe it's just a guide and possibly extra shims etc are needed? If so I'll try to put a Baader adjustable Vari adaptor in the chain for future adjustments?

20190621_191639.jpg

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OK, this isn't a chip distance so I very much doubt that it will be all that critical. SCTs can cope with a considerable range of backfocus but are optimal at a certain distance. I'd be amazed if this were critical but I'd ask those imaging with the scope. If you don't get a reply I'd just suck it and see. I'd certainly do that in your shoes.

Olly

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20 minutes ago, ollypenrice said:

OK, this isn't a chip distance so I very much doubt that it will be all that critical. SCTs can cope with a considerable range of backfocus but are optimal at a certain distance. I'd be amazed if this were critical but I'd ask those imaging with the scope. If you don't get a reply I'd just suck it and see. I'd certainly do that in your shoes.

Olly

Thanks Olly much appreciated!! I think it's easy to fall into the trap of thinking it has to be within 0.001 or something!!

I'll give it a try +- 2 mm or so!

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