The Good, the Bad and the Ugly

[Demonperformer]

Last night was a night of experimentation, with mixed results.

Il buono ...
It was my first run with the ZS71 that I had bought earlier in the year. As per this thread, I have been struggling to work out which "correct" distance is the correct "correct" distance to have the 1600MM camera from the flattener. I decided that the only solution was to pick an option and try it to see what happened. Well, a 10mm extension tube + OAG + 3mm extension tube + nosepiece worked. In the bottom right hand corner, the stars were trailing in an 8>2 direction, which suggests something wasn't quite right, but I got the system to firstbase on focussing and the tube needs to be extended by almost exactly 1¾” to achieve t.
Happy with that result.

Il cattivo ...
My next exercise was to try to find the focus point of the guide camera, for which I am using a QHY5LII. The initial step was try it as a "main" camera (displaying the image in sharpcap) rather than as a "guide" camera, although still plugged into the OAG [if that makes sense]. As with the 1600MM, for initial focussing I decided to use the moon - a big bright object, that will be visible on the sensor even if focussing is miles out. I was not sure which edge of the display equated to the prism of the OAG, so moved the mount to put the moon in the middle of each side in turn and used exposure lengths of about 1s. Nothing. Not just no moon, absolutely zilch. Pushing the exposure time to silly amounts (~30s), I got the speckling of coloured dots revealing lots of bad pixels (I have captured a bad pixel map for when I get to use PHD2), but of astronomical objects, not a hint.

Things I know:
(1) The camera is detected in sharpcap and, a week or so ago, was producing an image of a star when replacing an eyepiece normally.
(2) When I look down the hole in the OAG (on its own) I can see an image of what is in front of it.
(3) My previous attempt at guiding was a total failure, but I'm now wondering if the "star" that I was attempting to guide on was, in fact, one of these bad pixels ... that would fit with what was happening last night and could explain a lot.

Things to try:
(1) Set up the scope with OAG, but no camera, in daylight and look through the hole in the top of the OAG. Can I see anything? If not, why not? What is causing the obstruction?
(2) Rearrange the pieces, attaching the OAG to the flattener and putting the 10mm extension tube between the OAG and the camera. My thinking is that the cone of light gets smaller as it gets to the camera and maybe I am missing it altogether - further away means a bigger cone of light, so less likely to miss it. The problem I see with this is that the distance from the prism to the camera would have to increase correspondingly to achieve focus, and that would leave the camera above the tube in which it sits, producing a whole new set of problems.
Not particularly happy with this result, but at least I may have solved the cause of my previous disastrous attempt at guiding and have a few options to pursue.

Which just leaves il brutto ...
My only comment is: remember my avatar before I changed it? Enough said!

Thanks.

[happy-kat]

I can't offer anything but interesting to read your approach and analysis of what you did and result.

[MattJenko]

Rather than the moon, shine a torch down the end and see where the prism in the OAG picks up the light and where it doesn't. It can be quite tight as to where in the light cone it will work. Even if you are protruding into the sensor light to begin with, at least you will know the edge limits.

[Demonperformer]

Right, in an attempt to avoid losing another decent night's imaging, I have performed a small test:

(1) Set up scope + 15mm ring + OAG & look through hole in top of OAG
(i) With lens cap on, dark
(ii) with lens cap off, can see out of the window
: OAG set up the right way round and light is passing through the system

(2) Attach camera (QHY5Lii) to laptop and start sharpcap
(i) Cover chip, image on screen goes black
(ii) Uncover chip, image on screen turns white
: Sharpcap can see camera and responds the the signal generated by it

(3) Extend scope focusser to 1¾” (the distance at which the 1600MM focussed) & put camera in OAG
(i) Screen shows white with dark shadows
: Camera can see through the OAG, but is out of focus

(4) Move camera as much as possible within the OAG head to see if I can achieve focus
(i) Cannot achieve focus with this configuration
: Need to change configuration

(5) Take some measurements
(i) Distance from centre of prism to back-edge of OAG ~6.5mm
(ii) Distance from back-edge of OAG to 1600MM chip ~ 17.5mm (total = 24mm)
(iii) Distance from centre of prism to top of OAG ~ 45mm (i.e. 11mm too much)
: Moving the OAG to the opposite side of the 15mm extension ring will bring me much closer to focus

(6) Move 15mm ring
(i) Distance from centre of prism to 1600MM chip now 28+17.5 = 45.5mm
(ii) Distance from centre of prism to top of OAG  still ~ 45mm
: Still unable to bring image to focus

At the risk of being hounded-off the forum for quoting Barry Manilow (or even just for knowing that I'm quoting Barry Manilow) - where do i go from here?

Thanks.

[Demonperformer]

Just noticed ... to avoid any confusion, in the first post where I have put "10mm extension tube", it should read "15mm extension tube" ... so I guess that also qualifies me for il stupido ...

[Demonperformer]

Amendment to (6) above, where I say the distance is now 28+17.5:

Where did I get 28? Well, I measured it, but (as the keen-minded among you will realise) 6.5+15 does not equal 28. This figure includes the 3mm extension ring that was already on that side of the OAG (6.5 + 3 + 15 = 24.5) but also the threaded part of the 15mm ring, which should not be included as this disappears into the 17.5mm of the camera.

This may explain why I continue to not be able to achieve focus, but does little to help me get past the problem, so any suggestions still welcome.

Thanks.

[MattJenko]

Have you tried removing the front extension off the ASI1600 itself to reduce the sensor distance on that down to 6.5mm for more options? It is a female/female connector, so reverses the T-thread, which may cause panic.

[Demonperformer]

That could be a possibility. But it would not gain all that much. There would be two possibilities:

Either, it would require an 8mm tube of internal thread, enough for the thread on the end of the camera + the filter holder + the thread on the 3mm ring (which has to go on the camera-side of the OAG as that is the only place it will go and still use the whole of the thread to which it is attaching), so would only gain me 3mm.

Or, it would require a similar tube of 6mm and moving the filter holder to the threaded tube on the other side of the OAG, which would gain me 5mm, but would produce quite significant vignetting.

I suppose one final option would be to find a FW/OAG combination that was 40mm wide and had threads that fitted on both the FR & the camera without nosepiece. But even something like this requires a distance of 17mm from the FW to the chip, so would require the nosepiece to be in place.

Thanks.

 

[Demonperformer]

Having slept on this problem, I am now considering it from another angle.

My assumption in OP was that, because I got an image that was half-way reasonable on the small screen, I had got the total distance precisely right. But, we know from hiroshiman's correspondence with James at FLO "Spacing wise with flatteners and the like the distance is usually given from the thread on the flattener, so if the lenses are inside by a few cm it doesn't matter" (here), so the total gap between the flange on the FF/FR and the chip needs to be 55-57mm. So, looking again at this, the total distance from front-edge to chip is 54mm, with a back-edge to chip distance of 17mm which, allowing for the external thread disappearing inside the nosepiece of the camera, just about matches the 1600MM. So, this would just need some sort of spacer from 1-3mm (thinking some sort of 'washer' type thing, rather than an extension ring - my 3mm ring adds more than that if the thread onto which it is being screwed is longer than 3mm) slipped over the FF/FR thread to push the FW that distance further away, to provide the correct spacing. And the OAG position should be perfect, as it is in one piece with the FW. This would mean that I would not need to worry about having to swap filters using the holder provided with the 1600MM camera.

I think this might be the way to go.

[Gina]

Getting all this sort of thing right is a real pain in the a**    I've been at it myself as you will no doubt have seen.  I thought I had it right with ASI1600MM-C plus SX Mini FW+OAG and field flattener on my Esprit 80ED but I don't seem to be there yet.  I've left it for now while I take widefield images with camera lenses - which I have at last got sorted out, using ZWO FW mini with camera screwed directly onto without F-F adapter.

[Demonperformer]

It's not only distances, it's thread-sizes: M48, M42, T, T2, ... AARRGGHHHH!!!

[Gina]

Absolutely!!

[Demonperformer]

... & M28.5 & C & CS ...

[sloz1664]

Best time to set up an oag is in daylight. What you have to consider is the spacing distance to both the ZWO and the QHY sensors have to be the same.

Therefore, as shown in the sketch X + Y = back focus distance. Calculate what your spacings should be. focus your ZWO on a distant object and take images, when focused make a note of mm scale on your ZS71 focuser. Now do the same with the QHY. Adjust your focus until focused and check the mm scale on your ZS71 focuser. Check the focuser readings and if there is noticeable difference you will need to recalculate the spacings. If it's a minimal amount you may be able to achieve focus within the adjustment of the oag. It is so much easier to complete this task during the day. 

 

Steve

[Gina]

Yes, I set up in daylight too - have a very handy tree on a distant hill about a mile away.

[Demonperformer]

Yes, that was what I was doing yesterday, discovering that it was not possible to position the QHY in the OAG, with the focusser at the position that brought the 1600MM into focus, so that it too achieved focus.

Not quite sure how "X+Y=back focus distance" is derived, though.

 

[Demonperformer]

10 hours ago, Demonperformer said:

So, this would just need some sort of spacer from 1-3mm (thinking some sort of 'washer' type thing, rather than an extension ring - my 3mm ring adds more than that if the thread onto which it is being screwed is longer than 3mm) slipped over the FF/FR thread to push the FW that distance further away, to provide the correct spacing.

I guess this could be what I am talking about.

[sloz1664]

3 hours ago, Demonperformer said:

I guess this could be what I am talking about.

Hi DP

Yep I have a pack of the delrin spacers. Totally invaluable for the sub mm spacings required for back focus.

 

Steve

[Alien 13]

I noticed that FLO has just started talking about offering specific spacers for common scope,filterwheel/camera combinations, has to be a good move.

Alan

[Demonperformer]

Agreed, but I don't see this combination getting one anytime soon. Actually, it was looking at their Esprit/Atik spacer that put me onto their delrin spacer rings. I had been googling for "delrin washers" and getting items that were 7mm thick, and even I knew that wasn't what I wanted ...

[Demonperformer]

I have pulled the trigger on the FW/OAG combined.

If I am ever going to get beyond the "screw a filter into the nosepiece" stage [worse - the "unscrew the camera, unscrew the filter, replace with next filter and put the camera back on in the same orientation" stage], I will need a filterwheel and OAG to fit into the space available and the OAG I have got will not allow a FW to be fitted. Therefore, this is where I am going to end up anyway, so I may as well do it now, rather than struggle with what I've got and then do it later. Also getting the pack of delrin spacer rings. With the potential to add upto 15mm in 0.2mm increments (true, not every combination), if I can't get the spacing right then I might as well sell-up, and so the extra £10 seemed to be worth it.

[Gina]

Yes, I use those Delrin spacers too - very useful for fine adjustments.

19 hours ago, Demonperformer said:

Not quite sure how "X+Y=back focus distance" is derived, though.

X and Y are the back focus for guide camera and main camera respectively when added to other distances. ie. X+Z wants to equal Y+Z where Z is the rest of the optical path to the reference plane of the field flattener.  Hope that makes more sense   The diagram says it actually.  X must equal Y.

[Demonperformer]

OK, so that would mean

X+Z = Y+Z = backfocus distance, from which we would get X=Y. This is what I was trying to explain (probably not very well) with

 

On 12/10/2016 at 11:22, Demonperformer said:

(i) Distance from centre of prism to 1600MM chip now 28+17.5 = 45.5mm
(ii) Distance from centre of prism to top of OAG  still ~ 45mm

although coming at it from the other end: (i) is the distance that achieves focus on the 1600 [X], so that has to equal (ii) [Y], which happens above the top of the OAG, so although fine in theory, it can't be used in practice.

Thanks.