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Correctly measuring backfocus for EdgeHD800 + 0.7x reducer

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#1 RazvanUnderStars

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Posted 15 July 2020 - 12:01 PM

I hope to get a confirmation on where to start measuring the backfocus distance for the combination mentioned in the title, an EdgeHD800 and the 0.7x reducer

 

The diagram in the reducer's manual suggests measuring from the plane A but Celestron has made errors in the documentation in the past with regards to imaging trains so I'm asking to be sure. Sometimes backfocus distances are measured from flanges (plane B in the image) and the difference in this case is 12mm.

 

bf.jpg

 

Thanks!

 

 

 

 



#2 Tapio

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Posted 15 July 2020 - 12:11 PM

Here's good info.

https://stargazerslo...ook-right-help/

Also images where you can see from star images if you are too close or too far.


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#3 sg6

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Posted 15 July 2020 - 12:16 PM

Should be B as that face is "fixed" the next batch of mechanical units could have say 2mm less threaded section, or 2mm more. So that is the one that has the possibility to change and as such render everything obsolete.

 

So B.

 

Also the measurement is from a "reference" face and A is not a face.



#4 james7ca

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Posted 15 July 2020 - 12:33 PM

Here is a good thread on determining the backfocus spacing on Celestron's EdgeHD series. This thread has discussions on the larger EdgeHDs but some of the information is useful even for the 8".

 

https://www.cloudyni...w/#entry9338752



#5 JPetruzzi

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Posted 15 July 2020 - 12:38 PM

I hope to get a confirmation on where to start measuring the backfocus distance for the combination mentioned in the title, an EdgeHD800 and the 0.7x reducer

 

The diagram in the reducer's manual suggests measuring from the plane A but Celestron has made errors in the documentation in the past with regards to imaging trains so I'm asking to be sure. Sometimes backfocus distances are measured from flanges (plane B in the image) and the difference in this case is 12mm.

 

attachicon.gifbf.jpg

 

Thanks!

It's "A".  If you plate solve with the cam 105mm from that surface you get the correct f/7 focal ratio (1400-1500 depending on how good your focus is) I've played with spacers and with shortening the distance because the reducer is optimized for APS-C chips but my 533MC-Pro has a smaller sensor, so I have more room to play. Started getting vignetting on my Edge HD8 at 111mm.  Keep it at 105 from position A, the documentation is right. smile.gif

 

(Also, if you happen to have a ZWO camera, the 55mm spacers they provide + the Celestron Edge T-adapter (with extension removed) adds up to exactly 105mm, perfect.)


Edited by JPetruzzi, 15 July 2020 - 12:43 PM.

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#6 RazvanUnderStars

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Posted 15 July 2020 - 12:45 PM

Tapio, thanks for the link, those diagram with the effect of too-close and too-far focus are very useful.

 

sg6, that was the reason I asked but the Celestron's doc shows the measurement starting from the very end of the threads (maybe because the surface of the lens is really close to that plane?)

 

james7ca, many thanks, I see others were wondering about the same. Interestingly, message #27 in that thread shows that in the more distant past, Celestron documented the measurement origin to be what I labelled "B", but later corrected it to be "A". Maybe this is why there is confusion about it.

 

Jim, thanks. Yes, I do have a ZWO (ASI294MC Pro). I have the Celestron OAG as well, though, which takes some space, so I'm doublechecking now that everything is right.


Edited by RazvanUnderStars, 15 July 2020 - 12:47 PM.


#7 JPetruzzi

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Posted 15 July 2020 - 12:58 PM

Tapio, thanks for the link, those diagram with the effect of too-close and too-far focus are very useful.

 

sg6, that was the reason I asked but the Celestron's doc shows the measurement starting from the very end of the threads (maybe because the surface of the lens is really close to that plane?)

 

james7ca, many thanks, I see others were wondering about the same. Interestingly, message #27 in that thread shows that in the more distant past, Celestron documented the measurement origin to be what I labelled "B", but later corrected it to be "A". Maybe this is why there is confusion about it.

 

Jim, thanks. Yes, I do have a ZWO (ASI294MC Pro). I have the Celestron OAG as well, though, which takes some space, so I'm doublechecking now that everything is right.

I'm planning on getting that OAG myself, I'm thinking it'll work best using t-threads to connect and not the sct adapter, should take the place of one of the ZWO spacers right after the celestron T adapter (and a filter drawer will eventually take the place of the other one) but I don't have any hands-on with it yet. :)



#8 Midnight Dan

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Posted 15 July 2020 - 01:22 PM

Definitely "A".

 

-Dan


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#9 Michael Covington

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Posted 15 July 2020 - 01:37 PM

The instructions for the reducer say 105 mm from "A" to sensor.  With their camera adapter installed as instructed, with a DSLR and a T-ring, the sensor is indeed 105 mm from "A".

And "A" is where the flange of the camera adapter makes contact.  It is not just a threaded section of uncertain length.


Edited by Michael Covington, 15 July 2020 - 01:37 PM.

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#10 markb

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Posted 15 July 2020 - 01:51 PM

Interesting discussion due to Celestron's failure to be clear, as abetted by Celestron having termed the threaded potion, inexplicably, the 'baffle lock nut', as well as in not using the expected depiction of visible threads in the drawings.

 

I have been approaching the point I needed this settled, myself, and had also found inconsistent answers/claims in recent weeks.

 

Another thread this week linked to a thread with the same conclusion, A, in the diagram at

 

https://www.cloudyni...starting-point/

 

JPetruzzi nailed it down, in my mind at least, with the plate solve.

 

It appears Celestron always measures from the rear flat of the threaded portion (I'll mostly skip the discussion of end of threads v flat; I do think it incomprehensible the last tip of the thread was intended), and mention the end of the 'baffle lock nut' in some sources (again, lending credence to the flat claim for the 0 datum point).


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#11 Midnight Dan

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Posted 15 July 2020 - 03:29 PM

Should be B as that face is "fixed" the next batch of mechanical units could have say 2mm less threaded section, or 2mm more. So that is the one that has the possibility to change and as such render everything obsolete.

 

So B.

 

Also the measurement is from a "reference" face and A is not a face.

Just want to correct some things mentioned here.

 

In most threaded fittings, like say extension tubes, the two parts are mated when the flanges touch.  The length of the threads are not too important because they can be shorter or longer and the parts will still mate at the same point.  This is what SG6's statements are based on.

 

However, the focal reducer has an SCT mount on both sides, and SCT mounts are exactly the opposite of what I just described.  The rear surface, A, is indeed a face.  An end view would show a wide flat mating surface at the end of those threads, not a thin threaded wall as you'd see in an extension tube.  And the threaded ring that screws onto those SCT threads is NOT designed to bottom out at the flange.  Instead, it is designed to draw the two surface together to mate at the end of the SCT threads.  In fact, some items designed to screw onto an SCT have a loose threaded ring just for that purpose.

 

On an SCT thread, the length of the female thread on the attachment are not important as long as they are long enough to draw the two surfaces together.  The important, locating surface is where the two surfaces meet, at the end of the focal reducer threads, not the flange at the bottom of the threads.

 

-Dan


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#12 RazvanUnderStars

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Posted 15 July 2020 - 03:40 PM

Thanks again, everyone. That means I'm close with the setup I've cobbled together. I put a 16.5 mm adapter from Starfield and a 4mm threaded spacer ring which I don't remember where I have it from. 

 

I measured the threads of the reducer with a digital caliper, they were 11.8mm. As markb points out, there is a difference (of, I think 1mm) between the end of reducer's lens and the end of the threads but I don't think Celestron wants us to measure anything from the actual optical surface. 

 

 

Between B and the end of the camera (C, in green) there are 110mm (there's a margin of error as the OAG is wider and gets in the way of the caliper; I did my best to avoid parallax errors while eyeballing it).

 

So: 110mm - 11.8 (reducer threads) + 6.5mm (between C and the sensor inside) = 104.7mm, close enough to the recommended 105mm. 

 

 

The only nagging feeling I have is that there must be something better than that 4mm ring. The 16.5mm adapter goes inside 2mm, the camera the other 2mm (so about 3 rotations, the threads are 0.75).

 

 

bf2.jpg



#13 markb

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Posted 15 July 2020 - 04:25 PM

The 55mm flange to film/sensor distance is an oldie, going back to film camera days, where the camera mount T adapter not only matched the 42mm thread to the bayonet/screws/etc of the body, it also varied the adapter thickness to ensure that the T lens flange ended up (hopefully) exactly 55mm from the film plane. That's presumably why the 55mm keeps popping up in spacing specs. If that helps figure out what you need, if post #5 doesn't.

 

ZWO should have a spec for the camera body flange to sensor distance.

 

Also, if you want to make small spacing changes, Baader has sets of delrin T spacers at reasonable cost, I am sure others do as well. There is also the Svbony adjustable T thread (42mmx.75mm) spacer. There was also a post on spacers this week with a link to the Svbony.

 

https://www.cloudyni...focal-reducers/

 

it's for EAA, and they can end up with unexpected spacer lengths.

 

One of the Baader spacers might help add or make the the missing .3mm. Amazon and the current Baader importer, AgenaAstro. 

 

The set's thinnest looks like .6.


Edited by markb, 15 July 2020 - 04:38 PM.

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#14 RazvanUnderStars

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Posted 15 July 2020 - 04:36 PM

Many thanks, I'll have a look.

 

Check JPetruzzi's post #5 above, it seems to answer your question.

 

The 55mm flange to film/sensor distance is an oldie, going back to film camera days, where the camera mount T adapter not only matched the 42mm thread to the bayonet/screws/etc of the body, it also varied the adapter thickness to ensure that the T lens flange ended up (hopefully) exactly 55mm from the film plane. That's presumably why the 55mm keeps popping up in spacing specs.

 

Also, if you want to make small spacing changes, Baader has sets of delrin T spacers at reasonable cost, I am sure others do as well. There is also the Svbony adjustable T thread (42mmx.75mm) spacer. There was also a post on spacers this week with a link to the Svbony.

 

https://www.cloudyni...focal-reducers/

 

it's for EAA, and they can end up with unexpected spacer lengths.



#15 james7ca

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Posted 15 July 2020 - 08:53 PM

AgenaAstro carries metal M42 spacers that go down to 0.1mm -- which seems impossibly thin, but here you go:

 

  https://agenaastro.c...-mm-s-set7.html

 

They are supposedly individually stamped with their thickness which makes me wonder how they can stamp something that is only 0.1mm thick (that's only 4 thousands of an inch, about the same as a standard piece of paper.)

 

Also, to the OP and other interested parties (OIP), note that some of the measurements in the manual for the components of the Celestron OAG are NOT correct. Not all of them are wrong, but if you go strictly by Celestron's manual you would likely end with a configuration that is the wrong length (you need to measure after you've fully assembled the OAG, spacers, and camera). It sounds like that is what the OP did (measured after assembly), but one should NOT just assume that the thickness stated in the manual are correct.

 

Finally, for those who may want to read further on some of these "errors" and who may be wanting to outfit one of the larger EdgeHDs (i.e. not the 8") here is a link that discusses some of these specific issues:

 

  https://www.cloudyni...-2#entry9998419


Edited by james7ca, 16 July 2020 - 05:47 AM.

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