Colimation issues or not, this happens everytime.

[Astroscot2]

Steve, can you test with just the camera and no AOG / filter wheel, do you have extension tubes to take your backfocus between 62.5 - 63.5mm? 70mm will give you out of focus stars for sure, the backfocus distance is critical, I know this from my own tests. Your DSLR is a slightly bigger chip which the corrector wont cover the corners. At least testing with the QH9 you can answer the question on the orthogonality.

[Catanonia]

Steve, can you test with just the camera and no AOG / filter wheel, do you have extension tubes to take your backfocus between 62.5 - 63.5mm? 70mm will give you out of focus stars for sure, the backfocus distance is critical, I know this from my own tests. Your DSLR is a slightly bigger chip which the corrector wont cover the corners. At least testing with the QH9 you can answer the question on the orthogonality.

The QHY9 is all square and correct as (including the CCD) as is the focuser.

I understand that 70mm (my setup with a 5mm extension) will give not optimum results, but wanted to see if it would make a difference.

I am going to try without the TS OAG so should be able to test at, 56mm, or 61mm or 66mm using the spacers I have to see if it makes any difference.

I have also moved the prism in the OAG to the very edge incase this is what is causing the issues. I can't see it being that but who knows. The prism is parallel to the longest edge of the CCD and always has been as this is how I like it.

[Tim]

Steve, to test star shapes you want a star field as bright and large stars as possible and take very short exposures, 1 sec or so.

I usually choose a reasonably bright star, not Vega etc and then move it around the chip to test the same star in various locations.

If you get the same result with the 1000D then you know its the reducer. That would be my first step?

One thing you can check inside, are the comet stars in the same corner as the OAG?

[Catanonia]

Steve, to test star shapes you want a star field as bright and large stars as possible and take very short exposures, 1 sec or so.

I usually choose a reasonably bright star, not Vega etc and then move it around the chip to test the same star in various locations.

If you get the same result with the 1000D then you know its the reducer. That would be my first step?

One thing you can check inside, are the comet stars in the same corner as the OAG?

Thanks Tim, will try that moving the star around, but pretty sure that when it hits this corner / quadrant, it will show the comet trails.

The OAG prism runs along the longest length of the CCD and parallel ie top or bottom. Not sure which way is up with the CCD.

If it were interacting, then I would expect the effect across the whole image or at least one half and not just one corner.

I am beginning to wonder about the claims of 65mm to the CCD - Reducer distance. Wondering whether this is glass to glass or to the front T2 thread on the corrector. That is like 5 mill inside the corrector. I am bang on 65mm to the front of the corrector with no wiggle room to reduce this unless I loose the TS OAG Hopefully removing the TS OAG will show this up if it is the case once I space it out accordingly.

[Tim]

Measure from the flat faces of the units Steve.

You'll need to get the spacing right, but if it was that doing the stars in, they'd be the same all over I would have thought.

Hmm, you know the three thumbscrews that hold the TS OAG onto the plate which fits into the camera, do you have them all evenly tightened? I've noticed on mine you can offset the camera a little by overtightening one of the screws.

[Catanonia]

Thanks Tim, will check the thumb screws and ensure it is straight. I did check and they can off centre as you said but they look ok.

Ok now to add to the confusion.

The Reducer on the TS website is said to have a chip to distance of 65mm. I thought I had 65mm, but actually after VERY carefull measurement and stripping of the train I actually have 59mm.

So I put in a 5mm spacer to give me 64mm as close to the recommended distance and the brief image I got was of horrid stars. But there was no guiding, mount not set etc etc. Then the clouds and rain came in.

So hoping for a clear spell tomorrow to continue the tests.

I have seen advice that if you can drop 5mm you can get more coverage, but perhaps 6mm less was too much and hence the problems.

Hopefully 64mm will do the trick.

[Tim]

And the filters buy you a mm or so dont forget!

[Catanonia]

And the filters buy you a mm or so dont forget!

inside the wheel so don't count

[ollypenrice]

I seem to have arrived a bit late at this one and while reading through it I instantly thought, Chip not orthogonal. If the distortion follows the orientation of the camera then it can surely only be something which turns with the camera?

Have you tried taking an exposure without a filter in the line? It won't make a great pic but it would eliminate filter tilt as a possibility.

To be honest I still suspect the chip orthogonality. Could it have moved since you checked it? I'd double check.

The DSLR test will be the key one I think. No filters and a 'second opinion' on orthogonality. If the tilt is generated by the the filters or chip then you'd have got away with it at slower f ratios as you'll be aware, of course.

Olly

[Tim]

Quote:

Originally Posted by Tim

And the filters buy you a mm or so dont forget!

inside the wheel so don't count

The actual filter glass has a "focal length" which is different to just no filter. Finding accurate info on the details has eluded me so far, but I think the common practice is to allow 1mm extra or so for a filter. Somebody will have proper details I guess.

[Catanonia]

ah gotcha Tim, well if I add a mm to 59mm + 5mm spacer then that would be bang on 65mm.

Olly, I have checked and double checked the CCD and it is square and level. Check with digital depth gauge.

Weather was pants last night, and looking tonight as well. Not even a chance of a 10 min clear patch to test.

[Psychobilly]

The actual filter glass has a "focal length" which is different to just no filter. Finding accurate info on the details has eluded me so far, but I think the common practice is to allow 1mm extra or so for a filter. Somebody will have proper details I guess.

Ken (Merlin66) will tell you exactly how much the glass thickness alters the fl... I seem to recall it's around 1/3 of the thickness of the glass

[Catanonia]

Ken (Merlin66) will tell you exactly how much the glass thickness alters the fl... I seem to recall it's around 1/3 of the thickness of the glass

Not much then about 0.33333 mm

[harry page]

Hi

Just to confirm the filter thing:D

It is indeed about 1/3 the thickness of the filter i.e my 3mm thick IDAS filter shifts the focal point out by 1 mm

What this means in practice is if you say have a physical spacing of 65mm and insert the IDAS filter in line my spacing is now practicaly 64mm so I would need to add 1mm to bring it back to the desired 65mm.

Slightley confusing I know , but this reducer is extremley fussy

Harry

[Tim]

Which is tricky..... but Baader do Delrin spacers which can help, and also make threads easier to loosen. I think FLO were going to stock them.

[Psychobilly]

In the past I have cut shims out of acetate or better still polyester drafting film sheet using a cutting compass... just remember to cut the OD before the ID ...

Peter...

[Catanonia]

Rain, rain and more rain with high clouds stopped testing last night. Not looking good for tonight either

[Tim]

When you did the double cluster shot, did all the filters have the same comet tails?

[Catanonia]

When you did the double cluster shot, did all the filters have the same comet tails?

Just checked and yes they did in the same place. The filters are parfocal btw.

Comon weather, got good hopes for my latest colimation and several options to try out.

[martin_h]

PM sent....

[Catanonia]

PM sent....

and replied. Thanks mate, a generous offer

[martin_h]

and replied. Thanks mate, a generous offer

Another package to open, just like Christmas....oh wait!

[Catanonia]

ok update time.

Some rough testing last night, not perfect but very rough

The stated back focus of the reducer is 65mm (just awaiting TS to confirm this)

At 65mm I get bad coma in the stars in the corners. Very bad trailing.

At 59mm, I get my best results with the comet trails.

Using some logic, the sweet spot should be in the middle.

I have been basing my findings on the following diagrams on the TS website and I am afraid I have fallen foul of the old exaggerated truths on information / websites

According to this diagram on the TS website

That should cover my 8300 chip in the QHY9

However careful searching on the ASA website, I found the following document

http://www.astrosysteme.at/images/Corr_2Zoll_Reducer.pdf

Reading this document (see the attached diagram) it actually states to get this circle to cover the 8300 chip you actually need to reduce the back focus by 3mm to get an extra 5mm gain in FOV (Someone did mention this earlier on to achieve more FOV, but I didn't realise you HAD to do this for the 8300 chip)

So put basically, according to the literature, I need to go from 59mm to 62mm to get a good image.

Misleading in the very least, all the information on the website says 65mmm will cover the 8300 chip, in fact this is wrong !!!!!!

So now trying to figure a way of adding on an extra 3mm. I know from testing that an extra 5mm is terrible, so it is a fine balancing act here.

Looking at the ASA diagram it starts to make some sense.

65mm back focus and you fall into the black inner circle in the diagram and that is why I get bad coma in the corners as the 8300 chip is falling outside this circle.

59mm then I am outside the 62mm dotted circle and hence the problems in the image and bad curvature.

Sweet spot is 62mm the dotted line that I need to achieve.

[martin_h]

You should be able to squeeze 3mm out of those spacers...when they arrive..

[Catanonia]

You should be able to squeeze 3mm out of those spacers...when they arrive..

In the meantime I have fashioned a spacer out of an old ring binder cover that give me pretty close to the +3mm I need.

Hoping to test tonight before we go out to dinner under clear skies Been waiting for 3 nights and tonight when we are going out to dinner with friends, it will be clear