I just don't get FF spacing!!

[RayD]

I see posts with people working out their spacing, adding the required extension, and hey presto round stars to the edges.  Why do I not seem to be able to do this on any of mine?  This is what I calculated for my Esprit 100 with my Atik 16200, so please correct me if I am wrong as I am so confused.

Required distance for dedicated Skywatcher FF - 52.5mm according to the FLO supplied information.  Therefore:

• Camera                               19.5
• Filter Wheel                         22.0
• Filter wheel adaptor                1.5
• Filter (Baader)                        0.7 (1/3 of 2mm glass)

Total equipment is:                        43.7

Right, so I ordered an 8mm extension, and then had a 0.8mm delrin spacer, which gives me my total of 52.5mm, which I duly connected, switched on, and..................

Well my stars were eggier than a really eggy thing, so after much frustration and trial and error, with zoom here, zoom there, and several checks with CCD inspector, I finally added 3.3mm of spacers on top of the 8mm extension which gave me a really good flat field, but comes out at at 11.3mm, giving me an overall spacing of exactly 55mm.

I just don't understand this, as it is, in my tiny brain, a mile away from the 52.5 noted by SW.  Is this a common thing and am I worrying over nothing, or have I got my numbers wrong?

[Alien 13]

I always thought that all FF used the standard 55 mm distance so that a DSLR like Canon 44 mm b/f with the 11 mm t ring worked in all cases.

Alan

[RayD]

Just now, Alien 13 said:

I always thought that all FF used the standard 55 mm distance so that a DSLR like Canon 44 mm b/f with the 11 mm t ring worked in all cases.

Alan

I'm not sure Alan.  I think the SW may be different as it does come with its own Cannon ring, so this may be thicker?  I was just going by the FLO information, which I checked and double checked, and it says 52.5.  My Hotech one and my ES one are both 55mm.

[daz]

Funnily enough 52 + 11 = 63 - which is actually the back-focus distance for the FF!

https://storage.googleapis.com/skywatcher-canada/upfiles/en_download_caty01390352363.pdf - page 7

I had to have a custom made one for my AtikOne

[RayD]

3 minutes ago, daz said:

Funnily enough 52 + 11 = 63 - which is actually the back-focus distance for the FF!

Ooh now that's really confused me.  I thought the adaptor ring supplied with the FF was 11mm which was why is was 52 left to make up.  Am I barking up the wrong tree?

[daz]

Not sure which adapter you mean... the one that allows the FF to attach to the draw-tube is not included in the calculation, and the one for the DSLR is well, for DSLRs!

BTW - you don't need to add the thickness of the filter in your calculations - you only to need to add the thicknesses of the housings. So, you have included the thickness of the filter wheel already

 

 

[Merlin66]

Unfortunately it sounds like you may be in the wrong tree....

The documentation certainly shows the 63mm spacing required......

[RayD]

40 minutes ago, daz said:

Not sure which adapter you mean... the one that allows the FF to attach to the draw-tube is not included in the calculation, and the one for the DSLR is well, for DSLRs!

BTW - you don't need to add the thickness of the filter in your calculations - you only to need to add the thicknesses of the housings. So, you have included the thickness of the filter wheel already

 

 

Ok thanks Daz.  The flattener from FLO came with an adaptor to drop to M48, and I have assumed this as being 11mm.

[RayD]

17 minutes ago, Merlin66 said:

Unfortunately it sounds like you may be in the wrong tree....

The documentation certainly shows the 63mm spacing required......

Yes I think this is why I'm getting confused.  my flattener from FLO came with a reducer which screws on the back of the flattener.  I think this is the 10.5mm (11mm nominal) which would then account for their suggested 52.5mm (from this reducer).  With this noted, my sums then add up all my spacing to 55mm, so even this 52.5 doesn't make sense.

I'm totally lost here 

[Waldemar]

The right spacing is rarely exactly the suggested one... it almost always needs trial and error to find the sweet spot.

When you do the adding up for the total backfocus distance to get as close as you can, you will have to add 1/3 of the filter's glass thickness.

The glass path is different (longer) then the air path... so for 3mm thick filters you add 1 mm to the measured length.

[RayD]

8 hours ago, Waldemar said:

The right spacing is rarely exactly the suggested one... it almost always needs trial and error to find the sweet spot.

When you do the adding up for the total backfocus distance to get as close as you can, you will have to add 1/3 of the filter's glass thickness.

The glass path is different (longer) then the air path... so for 3mm thick filters you add 1 mm to the measured length.

Thanks for that.  I have added 0.7mm (2mm Baader) with the guidance of FLO, who actually confirmed the information on their site was wrong, which was why I was getting in a pickle.  This has now been changed, but I think it was a SW issue as it is the same on other supplier's sites, so wrong there too.

I've got it pretty good now