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Effective FOV with f/6.3 Reducer

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#1 MarMax

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Posted 15 August 2020 - 02:30 PM

This question relates to eyepiece projection photography (cell phone). I'm finally starting to get some "not terrible" images and would like to know how a spreadsheet calculation relates to the field. Pics so far are in my gallery.

 

From what I've read the maximum FOV for 2" is 1.04". When you use the f/6.3 reducer this maximum FOV is now supposedly 1.65". Assuming that backfocus is correct for a true f/6.3, is the effective FOV really 1.65"?

 

I have the inexpensive Meade 56mm 52 degree plossl and according to my EP spreadsheet it should provide the 1.65" FOV. And my Agena 38mm 70 degree should provide a 1.51" FOV, with the f/6.3 reducer.

 

Will I actually get these FOV's with the cell phone camera?

 

I'm partially asking this to help in using Stellarium or Telescopius to plan my imaging. Since they provide FOV as you zoom in and out it's very handy to pick the best lens for each object. My EP choices at this point are only two, an 18mm 82 degree and the 38mm 70 degree. With the [f/6.3] I have four options, 18mm, [28.6mm], 38mm and [60.3mm].

 

It all looks pretty straight forward in the spreadsheet but I have no idea if this is same as or close to what happens in the field.


Edited by MarMax, 15 August 2020 - 02:37 PM.


#2 MarMax

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Posted 16 August 2020 - 05:14 PM

Please excuse the ["] vs. [°] error throughout the post. Was a bit late when I wrote it and had intended to search and replace these and forgot.



#3 MarMax

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Posted 18 August 2020 - 01:57 AM

I can see why no one has replied . . . pretty dumb question now that I've actually played around with various combinations. It's pretty much pointless to use the f/6.3 with 2" EPs. The only way to maximize my FOV is to go f/10 with very low mag EP's. I don't have anything nice but my inexpensive 56mm plossl is about as much as it's possible to achieve in terms of FOV with my scope. And if I want to go top shelf then a 41 Pantopic is probably the ticket.

 

As has been stated in many posts on the subject, you are limited to a 24-27mm field stop before vignetting shuts you down. I should have spent more time searching before asking a ridiculous question.

 

I guess the only benefit is a faster scope with less exposure time required. With equivalent ISO the f/10 requires about 2.5x in exposure time. This can be helpful with the Alt-Az mount since it's not a stellar tracker. An f/10 exposure of 10 seconds can be done in 4 seconds at f/6.3.

 

The next f/6.3 vignetting test will be with the 40mm plossl.


Edited by MarMax, 18 August 2020 - 11:02 AM.


#4 MarMax

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Posted 19 August 2020 - 07:57 PM

It's actually quite hilarious having a conversation with myself. So today I just pointed the scope at a distant tree using the f/6.3 setup and tested different EPs to see which ones will work without vignetting. Here are the results:

 

Celestron 40mm plossl (1.25", 43° FOV, 27.9mm FS) - some central obstruction present

 

Meade 32mm plossl (1.25", 50° FOV, 27.9mm FS)

 

Celestron 25mm Kellner (1.25", 55° FOV, 26.2 FS)

 

22T4 ( 2", 82° FOV, 31.5mm FS) - a very slight hint of vignetting at the very outer edge

 

ES 18mm (2", 82° FOV, 35.8 FS) 

 

So the 24-27mm published field stop (FS) range will definitely work and for my current setup it looks like I'm good up to say 30mm. I do have my backfocus for the f/6.3 adjusted to 105mm so hopefully I'm in the right place +- a few millimeters.

 

All that's left now is to take some pictures and see how things look. Since I've finished making home made cell phone adapters for the above EPs the weather is crap with 100° F temps, high humidity, haziness, etc . . lol




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