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swag72

Would you be happy with this guiding graph?

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OK, so out imaging tonight, but just not hapy with things right now. I can not for the life of me get nice round stars. I am only taking 240s subs as well. So my initial question is by looking at the PHD graph, would you be happy with it?

I think it looks good as a guiding graph, which makes me think that the guiding isn't the problem. I will post a sub up tomorrow, but this PC can't cope with that!!

So, if the guiding is OK, what is causing odd shaped stars towards the corners. They're not egg shaped exactly. more like saturn, where you can see slight 'ears'.

SW reducer being used on a 1000D.

post-18339-133877651089_thumb.jpg

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Images added of the complete data and another of the top right of the image. My stars aren't properly round and I'm lost now to explain why. I hope someone can help me!

post-18339-133877651096_thumb.jpg

post-18339-133877651103_thumb.jpg

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In answer to your question, yes. If my guiding produced a graph that flat I would be very happy, mine usually looks like the Pyrenees.

As for the non-round stars, you possibly need a field flattener, maybe? The stars at the edge of my images are usually egg shaped even when the center is sharp.

Others, more knowledgeable, may have more insight.

M....

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I obviously can't comment on the graph, but it looks similar to a few "good guiding" graphs I've seen (which is nothing like mine!).

If it's not a flexure issue(?), might the camera not be square in the focuser/reducer(?). I could be way off, but I'm afraid that's about my limit at the moment...

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As suggested above, if not already, I would see if you require a flattener for that scope. Central stars look tight and round, out to edges they look a tad stretched but not bad by any means!!

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I am using the SW 0.8x flattener / reducer already, so that is sorted! Regarding everything sitting square - I did notice a few outings ago that I kept getting oof stars on the right of the image, regardless of camera orientation. Upon inspection, I noticed that if the focuser was really tightened in the tube, then the final turn knocked it slightly of square. That is all sorted now.

I take great care with the camera to ensure that it's all square - Would I not be right in thinking that if something wasn't sqaure the stars would actually be oof, whereas they are all looking fairly tight I think.

Really can't think of anything more!

Edited by swag72

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Sara,

The guide graph looks good to me.

So if the guider is "locked on" OK, then it must either be field rotation (poor polar alignment), differential flexing or something in the field flattener set-up.

I can't think of any other possible suspects at this stage......

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Thanks Ken - I will spend time tonight with PHD and test my polar alignment. It should be OK as the scope hasn't moved since I last did it. Thankfully, I can leave it all set up and just cover it over!

I will a picture of the business end of the scope later and see whether anyone can see any obvious flexure possibilities.

You say something in the field flattener set up - Do you mean the field flattener itself?

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I'm not familiar with your flattener, but I know from painful experience that the spacing can be/ is usually very, very critical.

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Hi Sara - Whilst you're looking down the spacing route, perhaps you can post a couple of "raw" subs so that we can have a look at them and compare the two, maybe one from close to the start of the run and another from close to the end...? Also is there any "drift" across the frames when on an imaging run? I understand that that can be an indication of flexure insidiously creeping in somewhere...

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This is a bit of a mystery as the guiding graph is good and the following plot on you image shows a VERY flat field and sensor tilt was measured as being very small, not enough to show this error! Can be see full size crops of each corner please? WE need to exclude differential flexure from the equation.

post-13675-133877651173_thumb.jpg

Edited by steppenwolf

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Sara I wonder if the stars in the bottom left of the image are the same shape as those in the top right. If so then I would suggest that flexure is the problem. My old guide scope was seated in adjustable rings and even with a good graph I had odd shaped stars on short exposures. Once I changed to a fixed guider bolted down the problem went away.

Also, and this is a long shot, but even though the graph looks good I've found that it can be misleading sometimes mostly if your calibration step isn't right. Could you post your PHD settings?

It also might be worth taking some subs without the flattener to rule it out.

Mark

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Here is the link to the rull res raw image of the above.

Link through rapidshare. From here you should be able to view the corners of the image.

Thanks for the ideas so far, that looks like a flat field Steve to be sure. I'm off to take some pictures of the business end. Not sure about the comments regarding spacing and the reducer - I don't really get it, if the spacing was wrong I'd not be able to achieve focus would I? My bahtinov mask suggests that my focus is fine - And I've still got considerable play both ways on the focuser, so it's not like I've run out of focus space (If that makes sense!)

Edited by swag72

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The spacing people are referring to is the sensor to FR mounting flange and it is critical - this is not a focus issue.

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I see - Well I have had to add the FLO adaptor to the reducer, this one, as the moonlite is a 2" focuser and the SW reducer only fits into the SW focuser.

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Apologies now for the mass of pictures!!

1) Camera with FLO adaptor connected to the reducer. I would estimate the extra length to be between 1-2mm as it screws into the SW reducer

2) Adaptor detached

3) Camera attached to the focuser - All square

4) Cables coming from the guider - all tied in with a cable tidy.

5) The guider bracket connected tightly to the moonlite

6) The cable tidy is then attached via the scope rings.

If there is anything in these images that are shouting at you as a potential problem then do say! I am new to trying to keep these cables tidy and out of the way and really don't know what may or may not cause flexure.

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post-18339-133877651196_thumb.jpg

post-18339-133877651204_thumb.jpg

post-18339-133877651211_thumb.jpg

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That adapter for the EP holder is fine as although it will physically affect the focus position, it has no impact on the sensor to FR flange measurement so all is well there. Just to keep the focus in context, this is not an issue as you would expect to adjust the focus of the telescope to allow for the different focuser anyway let alone this tiny adapter - focus, not a problem in your set-up.

I am impressed that you are getting such good guiding with the Finder-guider as I have never been keen on this idea but have to say that your graph has altered my opinion of this guiding method.

Differential flexure may possibly be a factor here - I have just this week started to run the guider and imaging cables separately so that the weight of one set of cables cannot act on the other camera but this is a tiny point.

As soon as I have time, I'll try and have a look at your full frame image.

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Thanks for the ideas so far, that looks like a flat field Steve to be sure.

It is a fraction over-corrected but I'd be very happy with that if it were mine.

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Looking at your picture of the finder guider...is it using the original rubber O ring and 2 threaded screws and the spring pin? This might introduce some flex.

The spacing of the reducer/flatener can only be right, using the camera adaptor as you are, this will not be an issue.

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Ive had something like this before, a perfect graph but still odd shaped stars.

What i did was to double check the PA, but your cabling (although its neat) is probably putting a bit of strain on the finderguider. What I do is completely seperate the QHY cables from the rest and tape them to the OTA so theres no slack, drag or movement in the guider cables. Also, i packed my finder out with masking tape to really stiffen it up. Hope that helps :)

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@ Martin - Yes the SW bracket is using the bits you say - I will look into beefing that up somehow.

@ Uranium - My cables look so tidy!! I will take the guider ones out as you suggest!!

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OK, out tonight with the Ha filter doing 20 min subs. I've still not got nice stars,

So what have I done?

Detached the guiding cables from the others and the bracket is solid. Checked my polar aignment last night and its spot on. Did drift alignment last night, a sight tweak that was all.

No cables are snagging or tight or pulling - What else can it be?

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... back to looking at PHD settings (maybe...?) :rolleyes: or is the graph still as good as the one in your first thread...?

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I just KNEW you were going to say that.... :rolleyes::). Clutching at straws here, but has ANYTHING changed since your last "good" imaging session...?

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