Atik 4000 Back Focus

[fatwoul]

The stated backfocus for the Atik 4000 is 17mm. Does anybody know if this is just the physical distance from the front of the camera to the sensor, or whether it accounts for the refractive index of the 3mm optical window?

[fatwoul]

See, the reason I ask is because on this page of their site:

http://www.atik-cameras.com/external/Website/Drawings/Atik4000mech.pdf

It shows the backfocus as 17mm. BUT the refractive index of typical optical window glass is such that 3mm of glass should add about 1mm of backfocus, which would account for this page on the same site:

http://www.atik-cameras.com/support/frequently-asked-questions

stating that the 4000's back focus is 18mm.

[ollypenrice]

Hmmm... I hadn't seen the 17mm figure and have always used the 18mm. However, with a variety of lenses, scopes and flatteners I've always found I needed to suck it and see... This means embroiling myself in an infernal assortment of adapters which lock themselves together and consume half a weekend as a result!! Not ranting, of course...

Olly

[fatwoul]

Thanks Olly. As has been suggested, I suspect that the true, practical backfocus is indeed 18mm, and that the 17mm shown in the technical drawing is just the physical distance from front of camera to sensor, with the 3mm window contributing an additional 1mm of backfocus. So maybe the diagram is misleading in referring to it as "backfocus" in the first place. I'm waiting to hear for sure from Steve Chambers.

I'm starting to think, however, that an adjustable adapter, with a locking ring on it, might be the way to go. That way the correct distance can be carefully determined, and moreover can be adjusted slightly if different filters are used in the wheel.

[dmahon]

I thought that you had to SUBTRACT 1mm because of the glass, not add it.

http://www.qsimaging.com/downloads/QSI-500-Series-Backfocus.pdf

[Stephen]

Baader Varilocs are great for this, I have one on each of my cameras on the OAG

[Davey-T]

What are the visible indications in images when the back focus is wrong

Davey-T

[fatwoul]

dmahon - is that not the opposite to what is suggested here: http://tech.dir.groups.yahoo.com/group/ccd-newastro/message/67993

[fatwoul]

Baader Varilocs are great for this, I have one on each of my cameras on the OAG

Indeed, Stephen. Do they do one in M54x075, or are they only available in T?

[fatwoul]

...and 17mm rather than 29?

[dmahon]

dmahon - is that not the opposite to what is suggested here: http://tech.dir.groups.yahoo.com/group/ccd-newastro/message/67993

I think they are saying the same thing.

Astrodon are talking about how much space you need to add from your reducer to your CCD. The glass means you can add 1mm.

Eg. If you should space to 55mm, now you must space to 56mm. If your camera is 45mm, you need an 11mm spacer.

QSI are talking about how much the glass camera affects the backfocus distance of their camera.

Eg. If the camera measures 45mm, with glass it only measures 44mm. So space out to 55-44mm (the same 11mm).

A piece of glass is the opposite of adding a spacer.

[Earl]

the 383 manual say 17mm aprox and has a 2mm window my reducer needs 56 +-4. ill be tinkering tonight

[Stephen]

Indeed, Stephen. Do they do one in M54x075, or are they only available in T?

Don't think so

[fatwoul]

OK, I think you helpful chaps might be getting me on my way.

So here's the question for someone better at this than me:

The telescope in question has a back focus of 56mm.

The EFW2 is 22mm thick, and contains Baader filters, which are 2mm of glass.

The 4000 (according to the mechanical drawing) has its sensor 17mm deep, but has an optical window 3mm thick.

Assuming all the glass has a refractive index of 1.5, what length adapter would be required to fill the gap between the telescope and the filterwheel?

(Correct answers are worth 100 points and my eternal gratitude. Oh, and a promise of a cup of tea if you're ever in Plymouth.)

[dmahon]

OK, I think you helpful chaps might be getting me on my way.

So here's the question for someone better at this than me:

The telescope in question has a back focus of 56mm.

The EFW2 is 22mm thick, and contains Baader filters, which are 2mm of glass.

The 4000 (according to the mechanical drawing) has its sensor 17mm deep, but has an optical window 3mm thick.

Assuming all the glass has a refractive index of 1.5, what length adapter would be required to fill the gap between the telescope and the filterwheel?

(Correct answers are worth 100 points and my eternal gratitude. Oh, and a promise of a cup of tea if you're ever in Plymouth.)

=56 - (17-(0.33*3)) - (22-(0.33*2))

=18.66

[fatwoul]

Crike. Glad I didn't order my adapter yet. It would have been way too short.

[dmahon]

In answer to the next question, I would probably go with something like this:

http://www.teleskop-express.de/shop/product_info.php/info/p596_Baader-variable-locking-T-2-Extension-12-16mm---33.html

http://www.teleskop-express.de/shop/product_info.php/info/p2261_T2-Extension-Tube-5mm.html

to get a variable 17-21mm adapter (buying them seperately may be cheaper on postage - it sometimes it, wierd).

Unless Wayne from Billetparts can knock you up something bespoke (I'm surprised FLO haven't already asked him for a set of spacers in varying lengths)

[fatwoul]

Thanks, but my next question was going to be "are you sure?" , because I'm ordering the adapter from Precise Parts. A variable one would be great, but unfortunately it needs to be M54, not T.

[dmahon]

As you are going for 56mm, I assume you have a focal reducer / flattener - most of them have a tolerance of a few mm either way anyway (so the 1.6mm attributable to the glass doesn't really matter).

If you want to be really perfect, you'll use a copy of CCD inspector and make tiny adjustments until it's spot on. Just because the maths is right, doesn't mean it will all work as it should!

[dmahon]

Would either of these help:

http://www.firstlightoptics.com/adaptors/atik-m54-to-m42-t-adapter.html

http://www.teleskop-express.de/shop/product_info.php/info/p203_Adaptor-M54-to-T2---low-profile.html

[fatwoul]

If you want to be really perfect, you'll use a copy of CCD inspector and make tiny adjustments until it's spot on. Just because the maths is right, doesn't mean it will all work as it should!

Of course, you're right. It just niggled me. I hoped I could get the adapter as correct as possible at the point of ordering it. The glass thickness probably won't matter much, but the reason I started to worry was because Precise Parts mention the thickness of filter glass as a contributing factor in their FAQ.

The adapters you linked could well be useful anyway, as I already have a lot of T-sized bits that I'd like to be able to attach to the new scope, but I reckon if I can get a wider adapter for this job, it'd be neater, cos the filterwheel has M54 on the front, too, so it would simply be an M54 male-male adapter of (in this case) 18.66mm length.

[fatwoul]

I've had a reply from Atik about the inconsistency, and they've "corrected" the mechanical drawing, to show a mechanical distance (although they call it optical backfocus, which I think is incorrect) of 16.5mm, and an optical window thickness of 4mm.

So now the maths is:

adapter length = 56 - (16.5-(4/3)) - (22-(2/3))

= 19.5mm

This adapter is getting bigger all the time...