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Mixed filter thicknesses and SW esprit 100 and asi2600mm

ZWO ASI2600MM Pro Equipment Filters Flattener
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Mark Crawford avatar
Mark Crawford avatar
Hi, Is anyone using a Sky-Watcher Esprit 100 with its field flattener and an Asi2600mm pro camera with a ZWO filter wheel with mixed 3mm and 2mm thicknesses of filters?

  Would i just be able to re autofocus on each filter without any noticeable focus problems? Or would i have to adjust the back focus when i change from a 2mm to a 3mm every time?  

I’ve been wanting to go from my 7nm filters to 3nm filters for a while now. Chromas are what I’m heading towards but could only afford one of them,  so i purchased an Antlia 3nm HA and an Antlia   3nm SII and a Chroma OIII in hopes of eventually replacing the Antlias with Chromas one at a time as finances see fit. I thought i would have something to work with in the mean time.

 In my haste i completely forgot about the difference in thickness and its implications on back focus. 

 I have read that scopes with shorter focal lengths and cameras with smaller sensors should not be as much a problem, but since I’m relatively new to this hobby, I’m not sure if my scope is considered a short focal length and is my sensor considered large or small.  

Sky’s are too awful now and for the forceable future to test so i was wondering if anyone in the AP community is using a similar combination?
John Hayes avatar
John Hayes avatar
The focus position will shift by about 330 microns between the 2 and 3 mm thick filters.  Unless you are running a very slow system (like around F/20) that's a very significant focus shift.

John
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Mark Crawford avatar
Mark Crawford avatar
John Hayes:
The focus position will shift by about 330 microns between the 2 and 3 mm thick filters.  Unless you are running a very slow system (like around F/20) that's a very significant focus shift.

John

Thanx John, 
Are you saying that even refocusing would not cut it? 
Are you saying even with refocusing i would have to adjust the back focus as well?
Charles Hagen avatar
Charles Hagen avatar
Refocusing would be sufficient to produce *an* image (albeit with some risks of not being able to necessarily always autofocus appropriately) but fundamentally your effective backfocal distance will also be changing and you may experience aberrations in one or both of the filter sets that would not be seen in the other. This isn't ideal and I cant say I'd recommend it. If it's all you have at the moment, there's no harm making what you can of it. Might just want to prioritize it as an upgrade.

Charlie
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Mark Crawford avatar
Mark Crawford avatar
Charles Hagen:
Refocusing would be sufficient to produce *an* image (albeit with some risks of not being able to necessarily always autofocus appropriately) but fundamentally your effective backfocal distance will also be changing and you may experience aberrations in one or both of the filter sets that would not be seen in the other. This isn't ideal and I cant say I'd recommend it. If it's all you have at the moment, there's no harm making what you can of it. Might just want to prioritize it as an upgrade.

Charlie

Thanx Charlie, 
thanx for your information. 
looks like im screwed.

Its probably a good idea to get a Antlia 3nm oIII filter so that all are matching thickness and keep my chroma oiii til i can afford  the other two chromas

Or do one night of Oiii at its proper back focus and the next nights with the Ha and SII at their proper back focus. 

just was hoping i didn't have to do that.
Charles Hagen avatar
Charles Hagen avatar
You should still give it a try with what you've got, don't let our speculation drive you to a purchase. It might be just fine and totally unnoticeable. Like I said, I doesn't cost you anything to try out the stuff you've already got. Worst case you have to buy an Antlia in the interim, best case you can stick with your prior plan and it's no biggie.
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Sergey Rakov avatar
Sergey Rakov avatar
Its probably a good idea to get a Antlia 3nm oIII filter so that all are matching thickness and keep my chroma oiii til i can afford  the other two chromas

Another option is to compare Chrome and Antlia head-on. My guess is after that choice will be simple )
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