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Spacing Between Field Flattener & ASI1600MM

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#1 Brisby2

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Posted 15 February 2020 - 11:11 AM

I went to test out my new asi camera (finally, a clear night!) last night, but I believe the spacing is off because of the fact that I couldn't achieve sharp focus at all... stars only managed to look like a blob in 1 second exposures (I made sure the L filter was in front of it). I'm pretty sure the spacing between my FF and the sensor should be 55mm, but please correct me if I'm wrong. Anyways, I mostly need help trying to figure out what spacers I need to make this work?

 

Here's my imaging train:

 

ImagingTrain.jpeg

 

I have the FF in the focuser, which is attached to a 21mm spacer, and then threaded into the ZWO 7-position filter wheel, and then the M42 10mm extension that the nose of the camera is threaded into.


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#2 sg6

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Posted 15 February 2020 - 11:38 AM

Just guess/estimate:
21mm spacer + Filter Wheel @20mm (???) +10mm + 6.5 for the 1600 makes 57.5mm

So 2.5mm too much.

 

A LOT depends on the 20mm for the filter wheel. 20mm is about the only value I can locate for it, and looking at the images of it then it could be less. No obviously specified value to what it adds in terms of separation. Just gives the main disk assembly thickness, but there seem to be machined insets.

 

As to 55mm that seems to be the normal. However which flattener is it?

 

ES scopes seem a bit of a nightmare with regards to flatteners. Just seems no one can identify an ideal item, and that includes ES.

 

Off to check for the 3rd or 4th time the fliter wheel contribution.

Is it the standard filter wheel, not the mini ?

 

Basically need more information on the attached bits.


Edited by sg6, 15 February 2020 - 11:44 AM.

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#3 Brisby2

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Posted 15 February 2020 - 11:48 AM

Just guess/estimate:
21mm spacer + Filter Wheel @20mm (???) +10mm + 6.5 for the 1600 makes 57.5mm

So 2.5mm too much.

 

A LOT depends on the 20mm for the filter wheel. 20mm is about the only value I can locate for it, and looking at the images of it then it could be less. No obviously specified value to what it adds in terms of separation. Just gives the main disk assembly thickness, but there seem to be machined insets.

 

As to 55mm that seems to be the normal. However which flattener is it?

 

ES scopes seem a bit of a nightmare with regards to flatteners. Just seems no one can identify an ideal item, and that includes ES.

 

Off to check for the 3rd or 4th time the fliter wheel contribution.

Is it the standard filter wheel, not the mini ?

 

Basically need more information on the attached bits.

The flattener I have is Explore Scientific as well... I honestly am not 100% sure what the width of the filter wheel is, it's the standard 7-position one that fits 36mm unmounted filters. I just measured it with a steel ruler to be 2cm, so I think your 20mm estimate is correct.

 

This is probably a dumb question, but can 2.5mm throw off focus to that extreme of a degree?


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#4 DSOs4Me

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Posted 15 February 2020 - 11:57 AM

Usually if you are +/- .5 to 1mm you are ok. Try to get within that measurement.


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#5 dswtan

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Posted 15 February 2020 - 12:12 PM

The regular (at least) ZWO filter wheels are 20mm:

https://astronomy-im...ng-1024x871.jpg

 

This may actually be more helpful to you:
https://astronomy-im...tions-55mm.html



#6 sg6

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Posted 15 February 2020 - 12:34 PM

Concerning 2.5mm out I would have expected focus in the center somewhere along the focuser travel, the edges would be out so not a flat field but at some stage a sharp center.

 

ES had flatteners and some worked some didn't. One worked on their triplet and not on their doublet, and/or the other way round.

 

Seems from elsewhere that people try the SV flattener, and still some say OK and others not so OK - maybe difference ES scopes in use.

 

I don't image much, actually very rare, but I am glad I bought WO stuff, their latest adjustable flatteners makes it all very easy.

 

Is there a reference for the ES Flattener. I have the vague idea ES pulled some as they just didn't work.

 

Just to add to the numbers - if it is the 2" flattener that has 55mm engraved on it, then ES say the distance is 55mm (as you would expect) then say +/-2mm. So could be 53mm to 57mm.

 

Further "mess" - if you read the ES page on the flattener it seems many have found that 45mm is the best distance.

 

Sounds rather extreme but could you try a 45mm separation ?


Edited by sg6, 15 February 2020 - 12:48 PM.


#7 Brisby2

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Posted 16 February 2020 - 01:13 PM

Concerning 2.5mm out I would have expected focus in the center somewhere along the focuser travel, the edges would be out so not a flat field but at some stage a sharp center.

 

ES had flatteners and some worked some didn't. One worked on their triplet and not on their doublet, and/or the other way round.

 

Seems from elsewhere that people try the SV flattener, and still some say OK and others not so OK - maybe difference ES scopes in use.

 

I don't image much, actually very rare, but I am glad I bought WO stuff, their latest adjustable flatteners makes it all very easy.

 

Is there a reference for the ES Flattener. I have the vague idea ES pulled some as they just didn't work.

 

Just to add to the numbers - if it is the 2" flattener that has 55mm engraved on it, then ES say the distance is 55mm (as you would expect) then say +/-2mm. So could be 53mm to 57mm.

 

Further "mess" - if you read the ES page on the flattener it seems many have found that 45mm is the best distance.

 

Sounds rather extreme but could you try a 45mm separation ?

Once the clouds clear up this week, I'll probably play around with the spacing and test 45mm... I just can't fathom that 2.5mm would create such a big difference in focus.



#8 ezwheels

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Posted 16 February 2020 - 01:45 PM

I went to test out my new asi camera (finally, a clear night!) last night, but I believe the spacing is off because of the fact that I couldn't achieve sharp focus at all... stars only managed to look like a blob in 1 second exposures (I made sure the L filter was in front of it). I'm pretty sure the spacing between my FF and the sensor should be 55mm, but please correct me if I'm wrong. Anyways, I mostly need help trying to figure out what spacers I need to make this work?

 

Here's my imaging train:

 

attachicon.gifImagingTrain.jpeg

 

I have the FF in the focuser, which is attached to a 21mm spacer, and then threaded into the ZWO 7-position filter wheel, and then the M42 10mm extension that the nose of the camera is threaded into.

 

I can't see where it is, but depending on exactly where the FF is in your image above, it looks like you may have way too much space between the FF and the camera. 

 

Once the clouds clear up this week, I'll probably play around with the spacing and test 45mm... I just can't fathom that 2.5mm would create such a big difference in focus.

 

Generally speaking, the faster the scope the bigger the difference. My FSQ106 has a critical focus zone (CFZ) of about 50 microns. That is .05mm!shocked.gif


Edited by ezwheels, 16 February 2020 - 03:58 PM.


#9 Der_Pit

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Posted 16 February 2020 - 02:41 PM

If the distance is wrong, then primarily the outer stars will suffer and be deformed.  On the optical axis you should still get a decent star.  If not then it's probably some other problem.  Can you actually focus through the focus, i.e., have stars getting smaller, then larger again?

If not, then the problem is likely the part between the flattener and the telescope, which is either too short or too long to allow you to reach proper focus within the range of the focuser...  Try removing (or adding) some distance pieces there.

 

As for the accuracy:  It has to be better, the shorter the backfocus is.  55mm BF ones usually should be within 1mm.  But as mentioned, that affects the stars in the corners.

Also, the distance should be a bit larger, as you have to take the filters into account.  They will increase the needed (physical) distance by a third of their thickness.  So if you have 3mm thick filters, the proper distance would be 56mm.

So if the 57.5mm above is correct, you're not too far off, and definitely should see sharp stars in the center....



#10 Brisby2

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Posted 16 February 2020 - 04:46 PM

I can't see where it is, but depending on exactly where the FF is in your image above, it looks like you may have way too much space between the FF and the camera. 

 

 

Generally speaking, the faster the scope the bigger the difference. My FSQ106 has a critical focus zone (CFZ) of about 50 microns. That is .05mm!shocked.gif

I think the spacing between my FF and the sensor is somewhere around 57.5mm-60mm... The field flattener in the image is right inside where the first locking pin is closest to the camera. I guess I'll just have to keep experimenting to see where the "sweet spot" is.



#11 ezwheels

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Posted 16 February 2020 - 06:48 PM

I think the spacing between my FF and the sensor is somewhere around 57.5mm-60mm... The field flattener in the image is right inside where the first locking pin is closest to the camera. I guess I'll just have to keep experimenting to see where the "sweet spot" is.

Ok, that number is for getting the flattest field with the FF. Since it is difficult to see where the focuser drawtube ends and the compression ring begins, I am still wondering about all the space that appears to be eaten up by the extensions between the focuser and the 2" compression ring.

 

Sorry for editing your image. Don't sue.

post-310740-0-44468200-1581782752.jpg



#12 Brisby2

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Posted 16 February 2020 - 07:43 PM

Ok, that number is for getting the flattest field with the FF. Since it is difficult to see where the focuser drawtube ends and the compression ring begins, I am still wondering about all the space that appears to be eaten up by the extensions between the focuser and the 2" compression ring.

 

Sorry for editing your image. Don't sue.

attachicon.gifpost-310740-0-44468200-1581782752.jpg

The thing marked is an extension tube. I haven't really tested it without it (it worked fine with my DSLR and field flattener), but it measures to be around 108mm from the threaded part on the end of the focuser drawtube to the edge where the compression ring is. If needed, I can take off a portion of it to where it only measures to be 50mm, but I'd have to see if I can even reach focus with that.



#13 ezwheels

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Posted 16 February 2020 - 09:22 PM

Yeah, I would worry about reaching focus first. As others have mentioned, you should always be able to get at least the center of the frame into focus. Once there, you adjust the spacing between the FF and sensor to get the edge stars as close to the focus of the center stars. Each time you change this spacing you will be refocusing by adjusting the focuser knob, but it should not be more than a mm in or out for the delta in drawtube position. 

 

This is copy/pasted from the Astro-Physics website and works really well:

 

"Determining what spacing change is needed:

Step 1: focus exactly on a star in the center of the frame. Note the exact focus position. Step 2: focus on a star at the corner of the frame and note the exact focuser position.
If the focuser moves out (away from the scope) in Step 2, then the field flattener is overcompensating and the focal plane of the camera must move inward - shorten the distance from flattener to the camera.
If opposite occurs, then lengthen the distance.
This can be done in steps of 1 - 2 mm at a time until the field is flat. This is the only way to get precise flat field."


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#14 astronate

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Posted 17 February 2020 - 11:12 AM

The thing marked is an extension tube. I haven't really tested it without it (it worked fine with my DSLR and field flattener), but it measures to be around 108mm from the threaded part on the end of the focuser drawtube to the edge where the compression ring is. If needed, I can take off a portion of it to where it only measures to be 50mm, but I'd have to see if I can even reach focus with that.

 

Definitely try this!  Depending on which flattener/reducer I use on my ES127, I use either one or two extension tubes.

 

Once you can get focus in the center of the field, be prepared to try 45mm backfocus from the ES Flattener/reducer.  Some require 45mm and some 55mm depending upon model and version.

 

Good luck!

-Nathan


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#15 Brisby2

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Posted 17 February 2020 - 10:20 PM

I'm thinking this one may be the 55mm spacing one (it's not engraved anywhere on it), but it could be 45... what would you guys suggest in terms of spacers to where maybe I could try out both spacing options?

 

Here's what I have:

 

Spacers.jpg

 

Top row from left to right - T-adapter, 1.25" nosepiece, 1.25" nosepiece

 

Bottom row from left to right - ES Field Flattener, 21mm Spacer, some kind of threaded ring?

 

Not shown is also another 10mm spacer that I believe is attached to the camera/filter wheel in that picture.

 



#16 TareqPhoto

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Posted 18 February 2020 - 11:50 AM

I hope i can get that spacing correct once i start to use flatteners/correctors/reducers.



#17 Der_Pit

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Posted 19 February 2020 - 08:47 AM

Usually you remove the 11mm one from the camera and screw the camera directly into the filter wheel, to get the distance as small as possible (to reduce vignetting), though that is not really an issue as you have 36mm filters.

 

If there is absolutely no info on the flattener (serial number etc.) then you'll indeed have to try both distances.  Quite helpful for such experiments are variable spacers like the Baader Varilocks (small, large).

 

Then focus on some star in the center, and watch the rim.  The image below (copied from some other thread here) should give you some info in which direction you're off...

 

flattener_cheatsheet.png


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