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What adjustment do I need to ZS73 Reducer?


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The 1.8mm sets the normal back focus for a DSLR at around 55mm for Canon with T ring, so add up your accessories and subtract them from 55mm and adjust accordingly not forgetting to account for the 1.8mm

Dave

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The FF back focus distance is 56.8mm. A DSLR has a front plate to sensor distance of 44mm, The DSLR T-ring adapter has a thickness of 11mm so the 1.8mm added on the FF adjustment totals 56.8mm.

The 1600 has a front plate to sensor distance of 6.5mm (this distance is also called back-focus hence the confusion that arises.) The filter wheel is most likely 20mm so your total distance at the moment is 6.5 + 20 + 9 = 35.5mm. To reach 56.8mm you need to add an extra 21.3mm. The FF adjustment may only go up to 15mm, so a 10 or 15mm extension T-ring needs to be added and the remainder of the 21.3 set on the FF adjustment.

Alan

Edited by symmetal
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Thanks guys, that’s very informative! Will measure it all later, just having a wee beer in the garden gazebo, got to make the most of this summer !

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2 hours ago, symmetal said:

The FF back focus distance is 56.8mm. A DSLR has a front plate to sensor distance of 44mm, The DSLR T-ring adapter has a thickness of 11mm so the 1.8mm added on the FF adjustment totals 56.8mm.

The 1600 has a front plate to sensor distance of 6.5mm (this distance is also called back-focus hence the confusion that arises.) The filter wheel is most likely 20mm so your total distance at the moment is 6.5 + 20 + 9 = 35.5mm. To reach 56.8mm you need to add an extra 21.3mm. The FF adjustment may only go up to 15mm, so a 10 or 15mm extension T-ring needs to be added and the remainder of the 19.3 set on the FF adjustment.

Alan

Are you saying the FF might need to be set differently depending on if you have  DSLR or astro camera? I've just got a mono camera and wondering if I should set my Flat73 differently. The WO instructions feel a bit unclear.

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1 hour ago, Shimonu said:

Are you saying the FF might need to be set differently depending on if you have  DSLR or astro camera? I've just got a mono camera and wondering if I should set my Flat73 differently. The WO instructions feel a bit unclear.

The quoted FF back focus distance is the same, no matter what type of camera is used, it's the distance where the FF will present a flat focus field for telescope/FF combination. The camera sensor needs to be placed at that distance. (Note the effect of filters etc. on this distance below.) The FF instructions example for setting up the FF back focus tend to assume it's a DSLR hence the FF adjustment quoted being the extra required on top of the DSLR with T-Adapter 'back focus' of 55mm to achieve the required FF back focus distance of 56.8mm.

Astro cameras have a smaller front plate to sensor distance, also confusingly termed 'back focus', to allow filter wheels and/or tilt adjusters etc to be included in the camera train and ensure the total optical distance of these items is less than the FF back focus. The difference then being made up with T2 spacers of the appropriate length. Having adjustable field flatteners means these extra T2 spacers can be standard lengths, and the FF adjustment can conveniently make up the difference to get the exact FF distance.

In the reply to @blinky I forgot to mention that if filters, or any extra glass elements, are present between the FF rear and the sensor, approximately one third the total thickness of the glass elements should be added to the quoted FF back focus distance for calculations. The sensor protection window also needs to be included as a glass element so for example, if a 2mm thick filter and a 1.5mm thick sensor window are present around (2.0 + 1.5) / 3 or 1.2mm needs to be added to the 56.8mm FF back focus. In this case the FF back focus used for calculations should be 58.0mm. This is due to the difference in refractive index between air and glass causing the light path to be refracted when passing through the glass/air boundaries.

For a smaller sensor this difference may not make any noticeable image difference, but for larger sensors it needs to be taken into account to avoid distorted stars towards the corners of the image.

Alan

Edited by symmetal
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Dawned on me earlier, I used the field flattener before and my ccd/filter wheel etc were setup to be the same as a DSLR, I.e. 55mm so adding the 1.8mm is what I need to do. I’ve never taken into account the thickness of filters etc but assume since the ASI1600 is a relatively small sensor it hopefully makes no discernible diffference.

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Here we go - not perfect, I assume I have a little tilt somewhere?  I wonder if I can get it acceptable in all corners by making the good one a little worse to make the bad corner better....  I do have a tilt adjuster but could never figure out how you are meant to adjust whilst its connected to the scope, is this possible?

vega.jpg

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@Davey-T

@symmetal

Can you confirm this is correct..... I measured from the front face of the camera case to the front of the OAG and i get 42mm (see the attached pictures for all these measurements) as I have a spacer in place already to give me the correct focus point for the guide cam. Add to that 6.5mm for the code and you get 48.5mm. 56.8 - 48.5 = 8.3mm which i have set on the reducer. I suppose this does not take into account the cover slip on the ccd or the filter depths, just away to see if I can find them now but does the above calculation sound good?

20210725_105441.jpg

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20210724_234711.jpg

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You've measured from the front of the OAG to the front of the camera as your main measurement. It should be from the rear flat plate of the FF to the front of the camera. There looks to be a gap of 3mm or so between the FF rear and the OAG front due to the threaded section on the front of the OAG and a bit of exposed thread on the FF rear section as the OAG thread is not very deep. This is part of your FF spacing distance so needs to be included in your main measurement.

Add the 6.5mm camera 'back focus' to this main measurement to get your final physical spacing (1). Add your 1mm or so filter/window distance correction to the 56.8mm specified for the FF to get the actual back focus distance spacing needed (2). The value (2) - (1) is then the setting you want to set on the FF adjustment.

There does look to be some tilt on your test image with the top left OK and the other 3 corners showing noticeable spread of the stars. You may find after resetting the spacing distance as I just mentioned that the 3 bad corners are better and the top left is worse due to tilt. See how you go with another test image. Correcting tilt is another whole can of worms.

Alan

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OK folks - how does this look?  I think I might need to add a little more extension but t my eye it looks better - the tilt seems a bit improved as well, or will that become evident again once I add more space/extension? 

Altair30s_corners.jpg

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Bottom left still looks the worst - silly question, but do I determine what corner of the camera is bottom left to adjust tilt?  I do have a tilt adjuster but Im not sure I can use it, due to the adjustment screws hitting off the filter wheel and knocking it out of tilt!

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Your latest image does look like the corners are better though there is trailing in the centre stars so your alignment or tracking was out during the exposure which has affected all the image giving trails in one direction making it look worse. The bottom left though looks worse as you say.

Determining what corner of the image corresponds to a physical point on the camera body requires some mental gymnastics. It's best to start with the long image axis being parallel to RA direction. When the camera is pointing roughly to the South the camera writing on the rear is either the right way up or upside down depending on which side of the meridian you are. Choose the side of the Meridian where the image you capture is the same orientation as if you were looking at the target visually. This is for a refractor without a diagonal as usually used for imaging.

However some capture/display software may flip the image vertically for display. (reflection about the horizontal axis) I believe it's whether they adhere strictly or not to the fits protocol where coordinate 0,0 is at the bottom left for a graphical image type display. Camera image coordinate 0,0 is at the top left. Astroart and Fits Liberator do flip the image while SGP which I use doesn't. I suspect most don't flip the image but you need to check.

Also the orientation of the writing on the camera is no guarantee of what is the image top. I have several Zwo cameras, and for the same image orientation some models have the writing the right way up while others have it upside down. I can't remember which way up the 1600 writing is.

If you always have your camera in a fixed rotation position for imaging you can try adjusting tilt with the tilt corrector. If you often rotate the camera orientation then you need to determine where in the image train the tilt occurs and fix it there, as the tilt corrector may then only fix it in one orientation. It often is a sloppy focuser which flops to one side when imaging near the Meridian as the scope is then on its side and the focuser tension screws which are now on the side don't inhibit up/down focuser movement. Check this is all tight with no movement before proceeding with tilt correction. If in all orientations of the scope, including both sides of the Meridian the images look the same with the star errors in the same corners you can use the tilt corrector.

Having taken a test image in a southerly direction, see if the image matches the view in Stellarium which is what you would see visually. If it is, or is rotated 180 then your capture program/viewer hasn't flipped it which is good. Choose the side of the Meridian for testing where the image is visually the right way up.

Now the bottom left of the image corresponds to the top right of the camera looking from behind the camera at the target. The image is naturally rotated 180 degrees through the imaging train, for a refractor without a diagonal.

As you've found the low cost tilt correctors are awkward to use with the screws, often inaccessible without dismantling. On one of mine I've managed to put the required FF spacing spacer between the tilt corrector and the filter wheel just allowing access to the screws with an allen key. These push/pull screw tilt adjusters are awkward to use and it's difficult to achieve a repeatable screw setting.

I've ended up using one of these Gerd Neumann M48 tilt adjusters on one of my setups as they are much easier to adjust with just three screws on the outside rim of the adjuster which enables fine repeatable adjustment as the plates are pushed together by a strong spring pressure so the two screw push/pull arrangement isn't needed. However they are rather pricey particularly the larger one.

This lengthy diatribe shows that tilt adjustment isn't straightforward and may lead to frustration. 😬

Alan

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Thanks Alan - I strongly suspect I will adjust the flattener till I get to a point where I can live with it - Im not a pixel peeper!  I also realise that this is not the worlds most expensive scope or camera and as such Im realistic in what I can achieve...  Also, I think its the camera that has the tilt as it did follow it from top right to bottom left when I rotated the cam.

Thanks for all your assistance with this, it has been really helpful

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No problem blinky. 🙂 If you think it's just the camera and you can get the tilt adjuster fitted in the imaging chain the quick way is to just give a large tilt adjustment on one of the screws and see the effect on the image. Likewise repeat with the other two screws in turn on their own. You hopefully will find one, makes a good improvement in the necessary corner and you can then fine tune it if you can.  When you've tightened up the grub screw on the section you've adjusted the large screw should end up flat with the tilt adjuster face so shouldn't push against the filter wheel case.

It would be handy if there was the option of getting these adjusters with the screws on the other side so that they can be more accessible in some situations.

Alan

Edited by symmetal
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  • 1 month later...

I emailed William Optics about this and they send an image of the focuser and said to tighten the screws.  When I checked, one of them did need tightened, only about 3/4 of a turn, the others were tight but I gave them a little 16th of a turn to tighten them and its now looking a lot better 🙂

image.png.a236c10abec539c82bcf46bc424e26e2.png

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It's dificult to say but I've tended to find star shapes that aren't just elongated pointing towards the centre, would indicate the spacing is too great. There may be tilt as well as the top stars show a different orientation to the bottom ones. I would make a fairly large FF adjustment at first of around 3mm or so to get an idea of what shape stars are when it's definately too far or too near and go from there.

Alan

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Cool, pretty much what I thought - I think I need to go a mm or so further in but I like the idea of taking it out further to see what shapes they look like when its knowingly too far out

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