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Deep Spacing


Shibby

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I'm finally going to be joining the darker side of the dark side and, having saved for some time, plan to buy a mono CCD in the not too distant future... It's been a long time since I've bought any kit - something that needs to be forgotten about for a while if you get married, as weddings are darn expensive! :)

I think I've settled on the Atik 428ex. The FOV hit in comparison to my DLSR is gonna be a difficult pill to swallow, but if it truly is as sensitive and noise-free as claimed, then faster mosaics are at least a possibility.

So my questions mostly are on spacers. I know that my corrector (SW CC) requires 55mm and the camera has 13mm (does this include the thread?)

Am I correct in thinking I'll need an adapter with M48x0.75 female : M42x0.75/T2 male threads on it? Such as this one: http://www.altairastro.com/product.php?productid=16569

Apparently, this one adds 1mm, which would leave me a deficit of 41mm. I've probably made some incorrect assumptions here? But if not, should I get a 40mm extension? (e.g. http://www.rothervalleyoptics.co.uk/baader-40mm-t2-extension-tube_d5193.html) Would I get away with that?

Apart from the adapters, is there anything else I need to consider when buying a CCD?

The thought of spending more in one go than I've ever spent in total on this hobby makes me feel a bit queasy! Any encouragement wouldn't go amiss ;)

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The cam has a T2 female thread and the chip is 13.5mm from the face of the cam (there is a mechanical drawing on the Atik website) says it comes with 1.25" adapter so you just need a T2 extension tube of the correct length. 40mm sounds about right as the 1.25" adapter won't add much.

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Thanks Freddie. I won't be using the 1.25" adapter, so seems like the M48 adapter + 40mm extension will get me to 54.5mm - hopefully close enough!

Alas, the suspension on my car went TWANG this morning, so looks like I'll be waiting another Month :-(

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Having received both the Altair adapter + Baader extension, I'm having trouble with the T2 threads. After a couple of turns, the adapter becomes stiff and will not turn further. The problem seems to be the adapter because I've tried both items with other t-threads I have; the Baader extension is fine, but the M48 adapter has the same problem :(

Any one else have this trouble with T threads? I don't want to force it too much to avoid damaging the threads...

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I received my shiny new 428ex yesterday and thankfully there weren't many clouds in the box! :)

However, after carefully selecting & buying what I thought were the correct spacers (see above), it turns out not. I could not achieve focus at all with my coma corrector. With the 40mm extension, I did not have enough inward travel; it seems like there was a good 15-20mm difference to the usual focus position with my DSLR. The position should be the same if the backfocus is at the same 55mm, should it not?

I thought I'd try without the extension, just to see what happens. Of course, not enough outward travel! In the end, I had to use the 1.25" adapter and no filters (my existing ones are 2" and screw onto the corrector.)

So, anyone got any ideas what went wrong? How was it so far out?

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  • 3 weeks later...

Surely somebody out there must use a CCD with a coma corrector??

I'm now using a 20-30mm extension instead of the 40mm extension and I'm achieving focus, but I've no idea if the spacing is correct or what the actual effect of incorrect spacing is..?

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Lewis, I used to use mine with the 150P and CC.

All you need to do is get your basic measurement, then subtract the T-thread to chip distance for your camera - which in your case is 13.5mm. The mechanical drawing is here:

http://www.atik-cameras.com/external/Website/Drawings/series4_mech.pdf

This leaves you with a 41.5mm gap to fill between the end of the nosepiece and the shoulder of the coma corrector. The effect of incorrect spacing should be coma and not quite perfect focus, but you should should be getting something - even if youre 5mm out.

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This leaves you with a 41.5mm gap to fill between the end of the nosepiece and the shoulder of the coma corrector. The effect of incorrect spacing should be coma and not quite perfect focus, but you should should be getting something - even if youre 5mm out.

Thanks Rob, yeah that's what I thought. So, I tried an M48 adapter (1mm) plus a 40mm t2 extension. This should have gotten me within 0.5mm, however I couldn't even reach focus so it must have been way off - confusing, no? :icon_scratch:

Attached is a picture of the 40mm extension in place. There is one slight issue that might be hindering - the thread on the extension tube is a little longer than the one on the camera. Even so, this can't add more than a mm or 2... The other end is ok (the thread you can still see is the outside thread on the corrector).

post-5051-0-08228900-1377090397_thumb.jp

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I had the same issue with my camera, the extension threads are always too long camera side! Bear in mind this may cause a little binding if you use a shorter extension - so its best to have a pair of "soft" pliers on standby. What I did was to wrap electrical tape around the pincers so they dont dig into the extension when you grip it.

To make measuring easier, remove the stop collar on the CC and measure from the shoulder that is behind that. In this situation, electronic calipers are worth their weight in gold. But I cant see any reason why it shouldnt come to focus, with my CC and Newt I had room to focus with at least 10mm headroom left on the focuser from what I recall.... most confusing!

Assuming the exposed threads camera side are adding 2mm, and if you havent measured from behind the stop collar - that would be another 2-3mm. Which would make you about 3.5mm over, it all needs to be measured really (to two decimal places, just to be sure!).

Edit: Just had an idea. If its still playing up - even though you have it measured correctly, you have the option of sinking the CC into the drawtube (I can with the baader) by using an M48 extension tube, and then stepping down to T2, that will give you some in focus back. Im just going to check - in case ive imagined it! Pretty sure you can though.

Edit II: Yep, can confirm that my CC can be sunk into the drawtube, try it with yours.

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Now that is interesting - and would help me out with the focuser tube protrusion into the OTA, too... However, I don't think the collar is removable on the skywatcher corrector, but I'll certainly investigate that later!

As for the spacing, the quoted 55mm must be misleading if it's not from the available thread on the corrector. I have no real way of being sure if this is the case or not! All I can tell you is that with the 20-30mm extension set somewhere near 25-30mm, the focus position looks similar to that with my DSLR.

I will measure the distance the best I can, though. I'll call on friends to try and source some calipers :)

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The collar should be removeable, otherwise it couldnt be used with dobs - which would be a bit daft. If you cant take it off, measure the thickness of the collar then add that to the distance between the collar and the camera nosepiece. As far as im aware the focus distance for most correctors is always from the shoulder, and not the threads (as the shoulder is where the actual optics begin/end inside the corrector).

Calipers are pretty cheap from Maplins, definitely worth getting. I used a ruler for the first year, but it wasnt good for measuring since youre relying on eying up the markers to where they should be.

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