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Old 06-05-2020, 12:52 PM
stephen2615 (Stephen)
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Backfocus out by about 2.4 mm

Greetings,

I have a reducer/flattener that requires 54.8 mm back focus. My new QHY 16200A CFW7 with an OAG cannot make that distance at it accounts for 54.5 mm. The flattener has a thread of about 5.7 mm. The adapter that attaches to the front of the OAG is 3 mm thick. That means there is going to be a discrepancy of 2.7 mm too long on the flattener but about 0.3 mm can be made up with the 54.5 mm imaging train distance.

So assuming you have not nodded off with those numbers, would a 2.4 mm discrepancy be a disaster for a flattener? Should I just ditch the OAG and get a better guide scope?

Cheers

Stephen
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Old 06-05-2020, 01:09 PM
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Atmos (Colin)
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2.4mm is quite a lot under many circumstances, especially with a reducer involved but the best way to tell would be to try it. If you’re happy with the way the corners look you can keep it where it is but if you’re not you at least know why.
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Old 06-05-2020, 01:39 PM
glend (Glen)
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You need to remember "The Rule of Thirds" is relation to focal length changes as a result of adding glass in front of your camera sensor. If you have filters in the path they will effect the focal length. How thick are your filters? Is there a glass cover over the camera sensor compartment? Forget the cover glass on the sensor itself, but many cameras have sensor comparnents where they may have gas filling, dessicant, etc. That glass counts.
For example, say your filters are each 2mm thick, and you have 1mm cover glass on the camera compartment, that totals 3mm, under The Rule of Thirds, you will need to add 1mm of spacing.
Secondly, most Flatteners have a range in which they can work, try it and see.

My personal preference has always been to use a guidescope, it just makes life easier; unless your dealing with long focal lengths and you can pick a star with the prism. If your starting out, stick with guide scopes imho.
What sort of imaging scope is this you are using?

Last edited by glend; 06-05-2020 at 01:55 PM.
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Old 06-05-2020, 01:42 PM
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The_bluester (Paul)
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It would take some custom machining but is the M48 threaded camera side of the reducer removable with a locking ring behind it that retains the glass elements inside the reducer? I am assuming you have one of the QHY OAG assemblies where you can bolt it direct to the front of the camera or filter wheel, and/or bolt on adapter plates to fit it to everything else?

If that was the case and the diameters of the fixing screws and thread of the reducer housing were suitable you could get an adapter plate made which bolted to the OAG on one side and screwed in to the female threads in the reducer in the other.

I assume you have this reducer?

https://www.astronomyalive.com.au/pr...cer-flattener/
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Old 06-05-2020, 03:21 PM
stephen2615 (Stephen)
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[QUOTE=glend]

I am confident on using an OAG as I have been for sometime. It took a while to get both the imaging camera (currently a QHY 163C and a ZWO OAG) and the guiding camera in focus but once set, it works well just as long as I don't look at the OAG without a smile on my face. I read that back focus has to be meticulous so it will be interesting to see what does happen with this. I primarily want to do narrowband targeting as well as broadband LRGB depending on the circumstance. One person said to crop the images to remove any rubbish from the edges but when forking out so much money, I would like to get good data across the sub.

My Optolong filters are still being manufactured or shipped, not quite sure of their thickness. I can't see the sensor in the camera but there is something there and it might be some sort of cover for it. The 16200A is all a bit high tech compared to CMOS cameras.

Last edited by stephen2615; 06-05-2020 at 03:42 PM. Reason: Bit more info on gear
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Old 06-05-2020, 03:27 PM
stephen2615 (Stephen)
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[QUOTE=The_bluester]

You are correct about the reducer vendor and he suggested I machine off the 2.4 mm. QHY are sending me an adaptor that attached onto the OAG and has a M48 interface (it comes with a M54 and M42 adaptors). Machining off 2.4 mm of the flattener is an option if I can figure out how to remove it. I will see what happens. QHY said that 55 mm back focus is not a standard for astro stuff. Both my flatteners demand it. Go figure.
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Old 06-05-2020, 05:07 PM
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The_bluester (Paul)
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I am going a different way. I have a QHY OAG coming to put on to my SVX80, which also has an M48 thread on the end of the flattener. That was 55mm backfocus as well but the actual lenses are quite a bit further up the tube than that 55mm, so I had them machine it to add a couple of mm of backfocus to the whole assembly. At the moment I will be using the QHY supplied M54 plate with an M54 to M48 step ring.
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Old 10-05-2020, 10:12 AM
stephen2615 (Stephen)
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[QUOTE=The_bluester]

I wish I had the ability to adjust the back focus with my main scope. The WO Z61 has an adjustable flattener. It is currently set to the "mandatory" 12.9 mm but I can easily wind it back to 10.4 mm which would give me the 55 mm back focus.

Just out of curiosity, what does the placement of the lens have to do with back focus? If the vendor says 55 mm, then wouldn't that be because of the setup with the flattener.
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  #9  
Old 10-05-2020, 07:39 PM
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The_bluester (Paul)
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The 55mm in the case of the flattener with my scope is from the shoulder on the male threads on the camera side for their standard setup, but in this one the lens set is quite deeply set down inside that point so they are able to accommodate backfocus anywhere up to about 100mm by means of changing the depth of the rear tube of the flattener, in my case they made it in three screw together sections.

The Celestron 0.63 reducer/corrector for my SCT is the opposite where you have to watch out not to put fingerprints on the camera side lens as it is literally at the end of the threads!
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