Help with CCD to flattener spacings.

[focaldepth]

I want to add a CCD to my growing collection of gear and spacers/adaptors, cables etc.

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First a RANT.

Why do manufactures and suppliers not provide the simple basic measurements we all need!

FLO please take note.

All it would take is a couple of lines on each item description such as sensor to T thread = 19.9mm for example.

Or if it is more complicated and depends on other factors we need to know.

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Now that's off my chest here is one option (out of many) I am considering based on gear I have and gear I would like.

SW Equninox ED80

SW flattener

7.5mm T extension.

SX Filter Wheel (2")

SXVR H18

This is based of some distances I have obtained or guessed.

SW Flattener to sensor distance of 54mm. (Measured)

SX filter wheel path length of 29mm (inc T threads)

H18 sensor to face plate distance 17.5mm (Partial guess)

54-(29+17.5)=7.5

Hence the 7.5 spacer I proposed. (shim required?)

It is a case of garbage in garbage out.

Any help or suggestions welcome.

[shaunster]

which SW flattener is it?

I agree they should provide this information when they sell it!

I bought mine from Ian King and he supplied the correct spacers for me once I told him what equipment I was using, im sure FLO would do the same if you ask!

[Zakalwe]

If you have the flattened that is recommended for the Equinox, then the correct spacing from the shoulder of the FF to the sensor is 55mm, not 65mm.

[focaldepth]

My current flattener is the Sky Watcher one from FLO http://www.firstlightoptics.com/reducersflatteners/skywatcher-field-flattener.html.

To my inexperienced eyes it does a good job with my 7D DSLR, so I assume it will be good with a CCD.

You are right I could ask FLO (They have already answered other questions for me) but I would really like to consider several options knowing the spacings first.

I can't find anything on SX CCD sensor distances.

[shaunster]

Thats the flattener I have, its designed to be used 56mm from the sensor

[focaldepth]

for the Equinox, then the correct spacing from the shoulder of the FF to the sensor is 55mm, not 65mm

Thanks.

I would love to know how you know that, where do these numbers come from?

[focaldepth]

Just realised I incorrectly used 65mm above, I actually meant 54mm (canon 44m+10mm adaptor).

So now we have votes for 54, 55 and 56. Getting closer.

I still need the CCD sensor to face distance as well.

[ollypenrice]

While not true of absolutely all FFs the 55mm comes from the usual chip distance of DSLRs, though I think Canon's are 54? The FF makers aim to allow DSLRs to go straight on without spacers, just the T adapter.

Baader make a good range of spacers which I think are easiest to find on the Telescope Service website. I try to keep a selection 'in stock' for when they might be needed.

Olly

[focaldepth]

Thanks Olly, good call on Telescope Service, I will buy a selection to keep in my just in case box.

Sorry to harp on but still looking for some real CCD sensor distance data.

Anyone?

[Zakalwe]

Thanks.

I would love to know how you know that, where do these numbers come from?

I measured the distance from the shoulder of the FF to the focal plane of my Canon 50D. There is a small "witness" mark on the body of the camera, near the pop-up flash, which indicates the focal plane (and hence the surface of the sensor).

My QHY8L specifications gives the distance from the surface of the sensor to the shoulder of the camera body as 20mm, therefore I needed 35mm spacing. Bern at modern Astronomy supplied me with 2 x 17mm extension tubes, so I have ended up with a spacing of 54mm. The resulting images look OK....I guess if I was totally anal about it I could space it out another 1mm?

[shaunster]

All I will say is Ian King told me it was 56mm and he knows his stuff..

[mcrossley]

Things like filters and optical windows in the path will change the required distance slightly.

I found my Televue TRF-2008 which TV say "the spacing between flange and CCD is 56mm +/-2mm for best results" needs to be tuned within 0.5mm to get the absolute best out of it.

[Zakalwe]

Attached is a 240 guided second exposure, taken with an Equinox 80, Skywatcher field flattener, using 2 x 17mm extension tubes to give 54mm spacing. loaded into FITS Liberator, saved as .tif. Converted to .jpg in PS5, then resized to 50% of original size.

Judge for yourself.

[robbieince]

If you can allow for all glass surfaces behind the flattener it will be best.

I have been told that these extra plane elements (filters, optical windows etc) effectively add path length equivalent to 1/3 their actual thickness.

So you need that much less spacer.

Regards

Rob

[focaldepth]

Thanks all for your help.

I now have the definitive answer from Terry at Starlight Xpress, contracted indirectly via Yahoo Group.

Here is his reply;

Hi John,

I've just had a bit of an updating session and have added several new handbooks

to the web site. I have also included the back focal distance for the H18 at the

foot of the H18 page. It is 23mm, as Roger stated.

Sorry that the data was missing on the web pages!

Regards,

Terry

So it looks like most of the STarlight Xpress CCD are 17.5mm and The H18 is 23mm to allow space for the shutter.

So my calculation is now

54 (FF distance) - [29 (Filter Wheel) + 23 (H18)] = 2mm.

So some simple rings from Telescope Service will do the trick.

Look out FLO order on its way (4-5 week delivery though!)

If we get 6-7 weeks of clouds don't blame me.

[focaldepth]

Thanks Zakalwe, your picture is flattened enough for me.

[ollypenrice]

All I will say is Ian King told me it was 56mm and he knows his stuff..

Yes, indeed he does, and come to think of it Tak say 56mm for their flattener as well.

Olly

[focaldepth]

When I get the CCD in a month (and another month for the clouds to go) I will try a shot at 54mm and one at 56mm and bump this thread.

I have ordered a 2mm adjustable and some assorted plastic shims for this very reason.