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SW ED80 + Reducer elongated stars problem


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Hi all,

I have a problem with elongated stars on the ends of my images with an ED80 and the appropriate reducer from Skywatcher.
I looked in threads like Sara's about the same problem (different camera though) but did not get anything out of it about fixing the problem.

Gear in question and distances (according to manufacturers specifications) are:

SBIG ST8300m(17.5mm)->
SXFW filterwheel(29mm)->
FLO M48 to T2 adapter(10mm)
For a grand total of 56.5mm

The specifications for the Skywatcher .85x Reducer/Flattener for ED80 claim it needs to be 55mm from the shoulder of the reducer to the camera sensor.
But from what i have gathered from various threads in here when using filters, one should add 1mm to the total spacing, so in theory i should be fine. (+0.5mm off from reducer specification).
But as the images following exhibit, that is not the case and i am getting elongated stars on all corners.
Anyone can advise on steps to establish where the elongation comes from based on the images? I can provide more fits if required.

Aberration inspector corners crop and magnified of single 600sec frame
post-39392-0-39879300-1438696744.jpg

post-39392-0-83738400-1438696758_thumb.j

Single 600sec HA frame

post-39392-0-50406100-1438696767_thumb.j

22 x 600sec frames stacked

post-39392-0-56527600-1438696823_thumb.j

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I cant comment directly on the SW FF/FR but experience with mine suggests that the spacing needs to be sub milimeter perfect,  my guess would be an increase in spacing so its worth experimenting with cut up milk bottles/cans etc if that fixes it FLO can supply thin spacers http://www.firstlightoptics.com/adapters/baader-t2-delrin-spacer-ring-set.html.

Of course if the reverse is true (less spacing) then its a bigger problem.

Alan

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Hi Silios,

The SW FF/FR is very tight when used with a large sensor camera.

Typically it is 55mm +/- 0.5mm for a 35mm sensor... this distance must include any allowance/adjustments for filters etc.

For camera's with small sensors (say around 6.5mm) then the allowance can be greater at around +/- 3mm max, since they don't see the extreme edge of field.

At 56.5mm with your large sensor you will be well outside the limits and get the resulting field curvature.

It looks like you need to loose 1mm somewhere in the spacing... perhaps thinner adaptors on the filter wheel.

Hope this helps.

Best regards.

Sandy. :grin:

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There is an easy way to blag the 80ED into giving you better stars with the reducer.

When focusing, dont use a centrally placed star - move it out to about 2/3rds to the corner - then focus it. That will balance the whole field for you. Been there (with my 8300 camera), done it, and it works!

PS: You might also be better off rotating the camera by 90deg, the 8300 and 80ED can do the whole Cygnus loop in just two frames - for example:

9668988592_07c73e3a40_c.jpg

Feel free to pixel peep the corners and see for yourself :)

Fullsize: https://farm4.staticflickr.com/3807/9668988592_1cdfcdf021_o.jpg

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Thank you all for the suggestions! (awesome picture by the way Uranium235!)

My calipers say that the FLO M48 to T2 adapter is actually adding 3mm to the distance instead of the specification of 10mm (maybe i understand "adds 10mm to optical path" wrongly )

So I have been trying a couple of spacers (+7mm ,+12mm, +16mm ) and results are still the same, elongated stars at the end.

I was trying to get too far away so as to see the effects of the attached document (which i think i got sometime in the past from a thread in this forum) and something peculiar is happening : when i add too much space, say 66mm i STILL get stars on the edges that are elongated towards the center of the image if i have good focus.

When i'm just a bit of out of focus then the stars assume the form in the "too far away" shape #2 in the document.

Its mind boggling (and frustrating :D)

If anyone has any more ideas or use to have the same setup, id love to hear them!

Star shapes _ FOCAL REDUCER.docx

Edited by silios
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It seems that you have quite a fat filter wheel (mine is 21mm), and unless you are using a zero distance adaption between the FW and camera the problem is that extensions sometimes add more than they are quoted because more often than not, extenstions do not fit flush to surfaces (thread is too long).

You need to be aiming for an overall ditance of 56mm from shoulder to chip (to take into account the filters and CCD window), so subtract 17.5mm and youre looking at ~38.5mm of distance to fill (ie: between the shoulder of the FF/FR and end of the nosepiece of the camera). This is where its handy to have an array of extensions available so you can mix it up a bit. Try not to fall into thinking A+B+C adaptor will make X distance, fit it all together and use the calipers to measure the distance.

And again, use the 2/3 focusing technique to balance the field (it really does work!). To be honest, you could fiddle with its forever and you still wont get it nailed on a chip that large because even with a DSLR using exact spacers (ie: the M48-EOS bayonet) which are purpose built - you still wont get perfect stars in the corners (well, I didnt anyway!). Get it close, then focus accordingly.

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Yes the filter wheel is a bit fatty but good quality (and a bit out of spec as well in regards to dimensions spec says 29mm callipers say 29.5mm) .

You have a point about the measurements and i'm not sure if it threads fully into the 8300.

I will redo the calculations and then measure as you say when everything is threaded tonight.

Lets hope Sandy, Alan and my initial calculations, are off a bit and I don't wind up needing to "loose" mm's :D

I did try the 2/3rds focusing last night and it does work indeed! :)

Thank you all, once again

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Depending on the filter you are using you add about 1/3 of the thickness of the filter glass to the optical path. Your star shapes suggest that your sensor is too close  so you need to experiment with increasing the distance by 0.5mm at a time or invest some serious $ in CCD inspector software. I think the trial is free and fully functional for limited period.

Regards,

A.G

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I'm on it!

started last night to make spacers from 1mm plexiglass to determine the correct distance (or get close to it).

I discovered though that the scope is suffering from a terrible amount of focuser tilt and i think i need to address that problem before continuing with the spacer!

It could be that this is why all my efforts to find the correct spacing where always off the mark.

The stock focuser is not coping well with all that gear hanging from the back of it.

post-39392-0-58155300-1438852813_thumb.j

Edited by silios
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I'm on it!

started last night to make spacers from 1mm plexiglass to determine the correct distance (or get close to it).

I discovered though that the scope is suffering from a terrible amount of focuser tilt and i think i need to address that problem before continuing with the spacer!

It could be that this is why all my efforts to find the correct spacing where always off the mark.

The stock focuser is not coping well with all that gear hanging from the back of it.

attachicon.gifed80_focuser_tilt.jpg

So much for a Synta made astrograph.

A.G

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Yikes! My focuser was never that bad!

Have you screwed the reducer directly to the focuser tube, or used the push-fit adaptor?

That is with the baader click lock -> FLO 2" inch adapter -> FFR but the subs from when the FFR was threaded directly to the tube, have roughly the same tilt according to inspector.

Also i rotated the scope in the rings 90 deg and upside down and the tilt is always on the same place, which i think means that its not just droop - slop in the tube.

Maybe the focuser is misaligned or could be the primary cell is off? its not adjustable though so i hope its not the primary..

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Doesn't the very basic stock focuser allow crude collimation simply via the way it fits into the tube and is then held by radial screws through oversized holes? I don't have one in front of me so this is from memory. You loosen the radial screws, move the focuser by hand and retighten. An abominable task, I must admit. Best done using the ballbearing artifial star test. (Glue ballbearing to flocked black card, set it up 30 metres from the scope, illuminate it with a decent beam and observe it. It will give an excellent 'point source' to work with.)

Olly

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Indeed it has 3 radial screws but I'm not sure they have enough play to fix the amount of error. (not that I know how much that error is :D)

Thanks for the awesome trick Olly, i will get on it asap, cause its hard to carry all the operations in the dark :) plus getting to sleep in 5 a.m for days in a row, without results, does not add to the moral in order to work next day!

Edited by silios
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Indeed it has 3 radial screws but I'm not sure they have enough play to fix the amount of error. (not that I know how much that error is :D)

Thanks for the awesome trick Olly, i will get on it asap, cause its hard to carry all the operations in the dark :)

I learned it form TeleVue over the phone but it's widely used by optical engineers. Dany Cardoen has one set up in his workshop and he has mirrors at Paranal. Good enough for most of us!

Olly

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Before you fiddle with the focuser, remove the clicklock and pushfit adaptor and just screw the reducer onto the focuser tube - its by far the most rigid and reliable method of attaching your imaging train.

It was first in the list of tests, because you mentioned it previously :)

Took 10 test shots and unfortunately it had the same tilt.

Had a look with a Cheshire and a laser and it looks way off.

I have replaced the three flimsy Phillips screws with some nice hex replacements and it looks we are getting somewhere on the tilt front.

Photos to follow soon :D

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I never heard anything of the short as well for ED80's.

Had a chat with the guys at the local shop where i bought it in the first place and they where sceptical as well :)

Good news is I think its quite ok now, and can proceed with the optimum distance tests!

post-39392-0-89415600-1438901091_thumb.j

post-39392-0-28062000-1438901049_thumb.j

Edited by silios
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I think there is some progress on the reducer spacing front.

Did a quick (and noisy) 6 panel mosaic and stars look allot better on the edges.

The mosaic has some gaps though :D

post-39392-0-69274800-1438988087_thumb.j

If anyone disagrees do say so :p ,

cause I got a momentary glimpse of the sun midday through the telescope (while slewing to a far off antenna for tests)  and vision is still a bit blurry!

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You missed a bit! :D

Time for a little victory dance methinks.... case closed!

True that!

Sandy, Alan, A.G, Olly & Rob :

I have learned quite allot of tricks (ball bearing artificial star, 2/3rds focusing, ccd inspector, using brains :) )

Thank you all for the input and ideas.

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