Help with spacing.

[Adamar98]

Hi,

I recently purchased a skywatcher Esprit 80 ed with the paired field flattener. I intend to use my SXV H9 with the scope but I'm a little unsure about the spacing.

The back focus with the field flattener is 54.9mm.

Here's what I've got.

Filter wheel = 29mm

Chip depth = 17.5mm

Filter = 1mm (I'm sure I've read that you add 1mm for the filter?)

Total = 47.5mm

That leaves me needing a spacer of 7.4mm.

Does anybody know where or how I can achive this? And am I right about adding the 1mm for the filter? Any help would be greatly appreciated.

Cheers

Adam

[ronin]

For the filter I think it is add 1/3 of the filter thickness.

It relates to the refractive index of the material and for glass this is generally in the 1.5 or 1.6 area.

[Steve 1962]

Hi Adam

I have a similar issue to you and I use a standard 5mm spacer and some Delrin spacer rings from FLO to make up the odd millimetres.

Steve

[lensman57]

Hi,

I recently purchased a skywatcher Esprit 80 ed with the paired field flattener. I intend to use my SXV H9 with the scope but I'm a little unsure about the spacing.

The back focus with the field flattener is 54.9mm.

Here's what I've got.

Filter wheel = 29mm

Chip depth = 17.5mm

Filter = 1mm (I'm sure I've read that you add 1mm for the filter?)

Total = 47.5mm

That leaves me needing a spacer of 7.4mm.

Does anybody know where or how I can achive this? And am I right about adding the 1mm for the filter? Any help would be greatly appreciated.

Cheers

Adam

1/3 of the filter thickness is a good starting point, however with a slow F7.5 scope and a mere  0.85 reduction you can be as much as a milimeter or so out without noticeable degradation. When you get to F5 and slower distancing becomes  more exact and at F4 an almost an impossible task.

A.G

[brantuk]

You might get away with one of these: http://www.365astronomy.com/t2-extension-tube-6mm-optical-length-p-3782.html

If not there's bound to be a size you can use - their range is pretty extensive. Hth

[wxsatuser]

Baader do a 7.5mm t2 thats pretty close,

http://www.firstlightoptics.com/adaptors/baader-t2-extension-tube.html

For filters add the following.

The formula is     t * (n-1)/n

t is filter thickness

n is refractive index about 1.5 normally

I believe....

Astrodon are 3mm thick.......add 1mm

Baader are 2mm with some at 3mm thick.....add 0.7 or 1mm

Astronomik are 1mm thick.....add  0.33mm

[Adamar98]

Cheers for the replies.

I use the baader 2mm filters. So that gives me a spacing of 7.7mm.

Using 6mm spacer and the Delrin spacer rings I should be able to get it spot on.

Thanks for all your advice.

Adam

[Adamar98]

Hi again,

So I'm struggling to make up 7.7mm. I can get either 7.6mm or 7.8mm.

Is the 0.1mm that critical?

Will it make that much of a difference to my images?

Cheers

Adam

[Jose Auger]

I have the same telescope (besides the Esprit 120). If you contact Precise Parts (http://www.preciseparts.com/ppmain/index.html) they can help you with the spacing and build one for you. They even have a "spacer builder" that guides you through the whole process. I got my spacers from them and they are a beautiful piece of engineering.