Jump to content

Narrowband

Most aperture for 4° TFOV?


Recommended Posts

So here is a puzzle.

Using existing gear on the market - which scope / eyepiece / reducer / coma corrector - in simple words - kit combination would provide the most effective aperture for 4° TFOV (or more).

Things to consider:

1. Should not be significant vignetting - max 10% in outer field

2. Exit pupil should be kept below / up to 7mm  (or if it is larger than 7mm - it will be treated as aperture stopped down)

3. Realistic field stop should be taken into account when choosing max FL - we want 4° TFOV irrespective of any AFOV claims

4. Mirrors should be treated as 94% reflective unless specified otherwise by manufacturer (like 97%). Each air glass optical surface should be treated 99.5%. Central obstruction should be taken into account. Diagonal if used should be taken to be 99% efficient.

Comparing of effective aperture is used. Not sure how to account for lower contrast of achromatic scopes though, so for the time being - let's treat them as equal.

5. Stars should be "OK" at the edge. Some distortion is acceptable, but for example fast newtonians do need coma corrector (and CC transmission taken into account). This also restricts use of 4/5 element wide field eyepieces in fast scopes.

  • Like 2
Link to comment
Share on other sites

My 90mm f7 SV will do it, not a big aperture though. Using my Vixen 42mm with a 46mm fs gives 46/630x57.3=4.18 TFOV. The Vixen has a bit of astig at the edges so other options might be better. The TFOV of this scope is one of the reasons I bought it. TS sells similar I think.

Link to comment
Share on other sites

My Vixen ED102SS is F/6.5 and gave a 4.1 degree true field at 16.7x with the Aero ED 40mm. The exit pupil is 6.15mm with that combo.

Mostly I use the 31mm Nagler though so that reduces the true field to 3.83 degrees at 21.4x but with a 4.77mm exit pupil.

An ST120 could produce a 4 degree true field with a 40mm Aero ED but the exit pupil would be 8mm.

Edited by John
Link to comment
Share on other sites

Well, if it helps, here are some calculations:

46mm field stop: 650mm FL (658mm to be precise)
45mm field stop: 640mm FL (644mm)
44mm field stop: 630mm FL
43mm field stop: 615mm FL
42mm field stop: 600mm FL
41mm field stop: 590mm FL

Answers so far have been up to 100mm of aperture so let's spice up things a bit.

ST120 with ES82 30mm with field stop of 43mm fits in above table, and that sort of beats 100mm by 44% (aperture area) - although I have no idea what the effect of achromatic scope on contrast is

150 F/4 TS photon newtonian with ES coma corrector will work with 42 or field stop eyepiece. Problem is - not sure how large illuminated field is ES CC does correct for 30mm - but it does not show vignetting (I think it also extends FL by 6%). Accounting for central obstruction and reflections is bound to bring effective aperture down to 130mm or so.

Link to comment
Share on other sites

1 minute ago, Captain Magenta said:

I assume it's cloudy in Novi Sad? :)

Sometimes I just think of what M31 scope to get :D (and yes, mostly when cloudy).

I have bunch of different combinations that might work well if I compromise on field of view a bit. So far, best combo I think is this one:

https://www.teleskop-express.de/shop/product_info.php/info/p14000_TS-Optics-152-mm-f-5-ED-Rich-Field-Refractor-with-4--RAP-Focuser.html

+ some EP with up to 35mm and as wide AFOV as possible :D

Price is ridiculous and I realized that it won't fit veil properly - and it would be shame to have wide field scope that can't frame veil well - so I wanted to know what is the most aperture one could squeeze if there was hard 4° limit imposed.

 

  • Like 2
Link to comment
Share on other sites

One of possible solutions is AP 27TVPH photo visual x0.75 reducer.

It has has 56mm of clear aperture and it squeezes that into 41mm illuminated circle.

With 56mm we can go up to 800mm of FL for 4°, and this reducer has 118mm back focus - which is consistent with 2" diagonals.

130mm F/6 scope has FL of 780mm - so good candidate. I'm not sure if above reducer will work fine on such scope though, but that would give 130mm of clear aperture (give or take - mostly take due to number of elements) and 4°

 

Link to comment
Share on other sites

4 minutes ago, Captain Magenta said:

Are you talking AP only? If so, how would, say, a Canon 400mm f/2.8 fare? The early non-IS ones can be got quite reasonably, I think, although they are very heavy. 400/2.8 = 143mm aperture?

M

Actually - I'm talking visual only - hence all that mentioning of eyepieces and field stops :D

  • Like 1
Link to comment
Share on other sites

3 minutes ago, jetstream said:

Seeing as were going for the gold here how about jamming the monster ES 30mm 100 3" in the focuser?:D

52.2mm field stop and only about a thousand Euros.

es.jpg

Thought of that, and not sure what would be suitable scope for it?

With 52.2mm we can go almost to 750mm - 747mm to be precise.

Maybe 150mm F/5 with upgraded 3" focuser? :D

 

  • Like 1
Link to comment
Share on other sites

6 minutes ago, jetstream said:

Seeing as were going for the gold here how about jamming the monster ES 30mm 100 3" in the focuser?:D

52.2mm field stop and only about a thousand Euros.

I think we have winner - except it has a bit less FOV than 4°

https://www.teleskop-express.de/shop/product_info.php/info/p14000_TS-Optics-152-mm-f-5-ED-Rich-Field-Refractor-with-4--RAP-Focuser.html

+

https://www.teleskop-express.de/shop/product_info.php/info/p7392_Explore-Scientific-star-diagonal-3--with-99--reflection.html

+

https://www.teleskop-express.de/shop/product_info.php/info/p7379_Explore-Scientific-30mm-100--3--UWA-Eyepiece.html

And roughly 6000e later - we have best 4° view money can buy right?

:D

  • Like 3
  • Haha 1
Link to comment
Share on other sites

1 minute ago, vlaiv said:

Thought of that, and not sure what would be suitable scope for it?

With 52.2mm we can go almost to 750mm - 747mm to be precise.

Maybe 150mm F/5 with upgraded 3" focuser? :D

 

The very well respected Tammy Homma on CN has one in a frac if I remember. He is a vg resource. Yes they do work in fracs but I'd go slower on the speed and reduce the aperture a bit to avoid some issues. Either that or get wheelbarrows full of money out for the triplet f 5 fracs...:grin:

Link to comment
Share on other sites

My cash wheelbarrow is pretty small these days- its called a 24" dob lol!

Its handles are about 5 feet long :cheesy: Oddly enough, I prefer more image scale than the fracs give on the Veil espc. Upping the mag but keeping the exit pupil at 4mm-5mm with a tight OIII really works, for me anyway. Of course this means different scopes than the fracs.

It is pretty cool seeing the Veil in the 90mm at 4.3 deg TFOV, but contrast seems down- actually its not contrast its just lacking mag at the needed eye illumination, same goes for the TSA120.

Link to comment
Share on other sites

My best shot: 

If you ask really nicely OO might make you a 6" f/4 Newt. They made me an F/6 nonstandard version. It's a bit of a cheat but there is probably someone else doing the same thing out there.

These used to be quite common as wide field scopes. 

Team that with the ES coma corrector (mag *1.05) gets you to 630mm focal length. 

Add a 28mm Nirvana gives you a magnification of 22.5 and a field of 3.64 degrees, exit pupil 6.67mm, and the stars should be reasonably tight to the edge. 

  • Like 2
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.