NGC2174 (Monkey head nebula) in Ha

[opticalpath]

There's no way he's got 63.4mm spacing with that setup surely. I've sent him an email, hope he responds.

I think maybe he has, Sara. Looking at the spec. of the components in his setup:

- #17392 NextGEN mount .......... 1.12 inches

- QSI 520wsc optical back focus with T-mount .......... 1.4 inches

Total: 2.52 inches = 64mm

Adrian

[swag72]

Yes I've just spent ages working that out!!!

So how can he have 64mm spacing and the focuser racked out to over 40mm, when I don't have enough inward travel? We have taken the focuser apart this evening, but there's nothing we can do with it. It looks like he has a stock focuser on there too.

Will it make a difference where I have the spacing? For example, if I lose 12mm between the camera and the filterwheel, it needs to go on between the filter wheel and reducer at the telescope end, but will that move the focus point?

[opticalpath]

It's as you said, Sara: the spacing that matters is just the distance between the camera sensor and the reference point on the reducer (usually the end of the reducer body closest to the camera). It doesn't matter what order the the spacers/ filter wheel go in between those points, although it's usual to put the filters as close as possible to the camera sensor to reduce the risk of vignetting.

It's a puzzle.

Is it possible that the spacing is more than you think? I only ask because I have made exactly this mistake by measuring to the wrong part of the reducer, or getting the distance from the front of the camera body to the sensor wrong! Just something to double check. It's the only thing I can think of if what the other guy says is true. Do you happen to know the focal length of the Optec reducer? If we have that, we could calculate how far the point of focus should move inwards (from normal infinity focus position) for any given spacing. That would tell us - at least approximately - what should be possible.

Adrian

[swag72]

OK, I have checked and double checked the spacing and ................ I'm currently 12mm over. I thought it was only about 5mm.

So, that will give me potentially an extra 12mm inward travel, yes? It may have been my measuring afterall!!!

I think I just need to sit tight until the adaptor arrives, then see from there. I am expecting the adaptor to lose me about 13-15mm - If it doesn't lose that much, then I know where I can get some space from now.

As long as the 12mm extra that I have now gives me an extra 12mm inward travel ...... does it work like that? Or am I clutching at staws?

[opticalpath]

<< So, that will give me potentially an extra 12mm inward travel, yes? >>

No! It's not a linear relationship. The distance that the focus point moves inwards is:

a - ( b x a ) / ( b - a )

where a = distance of CCD from FR and b = focal length of FR

So you can see that when the separation distance (a) starts to get close to the focal length ( of the FR, the focus point moves inwards by very large amounts compared to small changes in spacing between CCD and FR. When a=b, the in focus movement becomes infinite!

So it's quite possible that a 12 mm redction in separation could result in a much larger shift outwards in the point of focus. You could be OK!

What camera are you using, and is the FR adapter you're waiting for sized specifically for it?

Adrian

[swag72]

Blimey, I could be making some progress then!

The camera is the Atik 460EX. Regarding the FR adaptor - I have the Optec M48 adaptor. So I have had made an M48 - SCT (so that will attach directly into my filter wheel) - At the moment I have a 15mm or so adaptor in there, and this one will be about 2-4mm.

How do you work out the focal length of the focal reducer? It would be good to do the maths and know if it really will work!!

[opticalpath]

You can find the formulas in various places ...... like here:

http://www.wilmslowastro.com/software/formulae.htm#FR

Plugging in a spacing distance of 63.4mm, to give a reduction factor of 0.7x gives a focal length of 211mm for the reducer. So your spacing is well away from the critical distance where things go off the scale quickly. It means that if your spacing is 12mm too big, your point of focus will be 15mm further in than it would be at the correct spacing, if I calculated that right. 15mm too much and the focus would move inwards 19mm. That's not enough to explain the 40mm racked out focus you mentioned earlier though, so I'm still don't think this is the whole story. You're definitely measuring from the camera sensor to the right place on the reducer? If I've done the sums right, the spacing would have to be well out (90mm instead of 64) to get a 40mm change in focal position.

As you said, prob. best just to wait for the correct adapter and see how it goes.

Adrian

[swag72]

I am measuring to the edge of the reducer, so I think that end is fine. With regards the camera, it has a 13mm back focus, so I am measuring to the front edge of the camera and adding 13mm.

While I agree that the racked out focuser of the link is odd compared to mine, when I tried the AT67T the focused was close to minimum, I had about 10 mm spare.

I look forward to trying with the right spacing!

[opticalpath]

I had the same experience with the Astro Physics CCDT67 x0.67 reducer - not much in focus adjustment left to spare

It's hard to tell from the Optec web pages but they talk about their custom adapters being attached to a mounting plate with 3 screws. I got the impression that they were measuring up to that flange, rather than to the extreme end of the unit. If it's not absolutely clear cut from the documentation you got with it, it might be worth emailing Optec directly for confirmation of the optimum spacing, and the place on the body that it should be measured from. Especially since they talk about wanting 1 mm accuracy for optimum results.

Hope it works out well.

Adrian