Eyepiece & filter selection 1.25" or 2" for Visual vs Imaging

[ozboyz1]

Hi all experienced observers/imagers,

I'm very new to this hobby and have been spending the past 2 months doing research ... and got valuable advices from you guys in the forum on different topics, telescope, mount etc ....

One of the most confusing topic for me is selection of eyepieces and filters for visual observation and imaging with DSLR.

Please allow me to break it down to my best knowledge and have input from your experience to help other beginners like myself.

All estimation below is based on 8" Newtonian with focal length = 1000. For other telescope size, please use appropriate FC and aperture diameter.

If I made correct statement, please kindly confirm; otherwise, much appreciate the correction from you.

Eyepieces:

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I found a formula from other forum that gives quite accurate estimation of Field Stop (since the information is not widely available). I tested with data from Tele Vue, and it yielded 95% accuracy !!!

Field Stop = AFOV * EP Focal Length /(180/3.1415926538 * (1+(AFOV/1450)))

True Field of View (TFOV) = Field Stop * 57.3 / Telescope focal length

For example: 

TV Nagler eyepiece 26mm with field stop 35, on the telescope with focal length 1000:

TFOV = 35 * 57.3 / 1000 = 2.01 degree

The TFOV determines whether I should get 1.25" or 2" eyepiece/filter. I don't an exact figure, but from TeleVue data, I'm guessing TFOV above 1.45 - 1.5 degree, we need to go for 2", below that we can use 1.25". 

Am I right thinking if I choose 2" for TFOV below 1.45 deg. I'm wasting lights and money ????

For planetary observation:

- Use 1.25" Eyepiece with magnification 180x - 240x (approximately only. Magnification = Objective focal length / EP focal length)

For planetary imaging (DSLR):

- What magnification I should use for eyepiece projection ??????????

For Globular cluster and small nebular:

- Use 1.25" EP with magnification 90x - 130x

For planetary imaging (DSLR):

- What magnification I should use for eyepiece projection ??????????

For large nebular observation:

- I got a mixed opinion on this : 30x - 60x ???? So both 1.25" and 2" are needed to cover this range of magnification (based on 8" scope and FC = 1000)

For large nebular imaging:

- Use barlow ???? What is the highest magnification I can go before losing sharpness ???

Filters :

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For planetary observation:

- Use colour filters (i.e. #21, #23A etc ). and since the eyepiece for planetary imaging is usually 1.25", use 1.25" filter to match ???

For planetary imaging:

- Can I use the above filters ???? I read it somewhere in the forum, planetary imaging and observation filters are 2 different sets ?????? If so, can you please give me some example for each type?

For nebular observation and imaging:

- Light Pollution filter like UHC ???? I live in outer suburb, where street lights and neighbour lights are my problem. No high-rise city lights.

- Since both 1.25" and 2" eyepiece are used to cover the magnification range (as mentioned above), do I need 1.25" and 2" UHC filter? If I buy 2" one, is there any 2" -> 1.25" adapter ???

Specific question about imaging with DSLR:

- Can I use any observing eyepiece for imaging?

- What adapter I need for eyepiece projection?

[Pete Presland]

i am certainly not going to get invovled in the maths of 1.25" v 2" eyepieces. 1.25" are the most popular for good reasons they do the job. have a wide range of focal lengths and you will be more than covered. 2" eyepieces are pretty expensive compared to 1.25" but obviously that is your choice.

i am not sure what scope you are going to be using, but i have a range of eyepieces from 40mm to 8mm and a couple of barlows. i have never felt the need to purchase any others.

as for magnifcations for objects? there is no sustitute for deciding what is best for any given night, other than what the conditions are like when you are at the eyepiece. some nights you can push the mag higher than others due to the seeing conditions.

as for the imaging for planetary imaging i would not be using an eyepiece and DSLR i would use a dedicated planetary webcam. filter wise unless you are doing RGB imaging a UV/IR filter would be fine.

for DSO imaging it is a huge subject with many questions! what scope are using? what mount? are you guiding your mount or just tracking? is your mount motorised even? but barlow is definitely not needed. i would recommend a light pollution filter though for your DSLR camera

summing up all of your questions, i would say doing all the theory is fine but a lot of it goes "out the window" when you are confronted rubbish seeing conditions or a mount that will not guide tonight for some unexplained reason. or when you push the magnifcation up to 350x against all the advice and get the best views of Jupiter you have ever seen in you life!

[Merlin66]

+1 from me!

Also consider your exit pupil and need (if any) for extended eye relief.