Maximum Field Of View?

[MimasDeathStar]

Hello all

Following a bit of  false start with a telescope that I bought a couple of weeks ago and had to return, I've now got another one. It's a Meade 70mm refractor with a 900mm focal length.

I've been having a lot of fun reading and learning about eyepieces on line but the one thing I cant figure out is what the maximum field of view my telescope can see is? I've been playing with the astronomy.tools thing but I suppose the information I'm getting is limited when it comes to field of view if I don't know how wide I can go.

Is this a really simple thing to work out that I just haven't figured out in my head? I've done some googling and I think its about 1 degree or so but I'm not sure. Can anyone help?

Many thanks everyone.

[Davey-T]

In Stellarium you can enter your scope and eyepiece spec's and it will give you an idea of the FOV.

Dave

http://stellarium.org/en_GB/

[John]

It depends on the eyepiece that you use.

I assume that your 70mm / 900mm scope uses 1.25 inch fitting eyepieces ?

A 32mm plossl will show pretty much the largest field of view that the 1.25 inch format can show and in your scope this equates to showing a true field of 1.86 degrees. This is around the same as 3.6 full moons.

To work out the true field that you can see you divide the apparent field of view of the eyepiece (52 degrees in the case of the 32mm plossl) by the magnification that the eyepiece is giving in the scope, 28x in the case of your scope. So 52 degrees divided by 28 = 1.86 which is the size of the true field (amount of sky) that you can see.

2 inch format eyepieces can show larger fields of view but you have to have a scope that is 2 inch accessory compatible.

 

[johninderby]

Worth using the Televue eyepiece calculator which will give you the fov and mag etc for various Televue eyepieces but you can find cheaper eyepieces that are about the same as the Televue ones.

http://www.televue.com/engine/TV3b_page.asp?id=212&plain=TRUE

Focal Length:	900.0mm
Aperture:	70.0mm
Focal Ratio:	12.9
Include 2-inch data:	False
Table corrected for Paracorr:	False
List only DIOPTRX eyepieces:	False
Max recommended mag. for aperture:	165x

 

Design	Focal Length (mm)	Apparent Field (°)	Eye Rel. (mm)	Barrel Size	Mag. (x)	Exit Pupil (mm)	True Field (°)	True Field @ 1000 yards (feet)	Mag. w/2x Barlow (2x)	Mag. w/2x Big Barlow or Power- mate (2x)	Mag. w/2.5x Power- mate (2.5x)	Mag. w/3x Barlow (3x)	Mag. w/4x Power- mate (4x)	Mag. w/5x Power- mate (5x)
Panoptic	41	68	27	2"	--	--	--	--	--	--	--	--	--	--
Plössl	55	50	38	2"	--	--	--	--	--	--	--	--	--	--
Nagler 5	31	82	19	2"	--	--	--	--	--	--	--	--	--	--
Panoptic	35	68	24	2"	--	--	--	--	--	--	--	--	--	--
Ethos	21	100	15	2"	--	--	--	--	--	--	--	--	--	--
Nagler 4	22	82	19	2"	--	--	--	--	--	--	--	--	--	--
Panoptic	27	68	19	2"	--	--	--	--	--	--	--	--	--	--
Ethos	17	100	15	2"	--	--	--	--	--	--	--	--	--	--
Panoptic	24	68	15	1¼"	37.5	1.9	1.7	90.0	75.0	--	93.8	112.5	--	187.5†
Plössl	32	50	22	1¼"	28.1	2.5	1.7	90.0	56.3	--	70.3	84.4	--	140.6
Plössl	40	43	28	1¼"	22.5	3.1	1.7	90.0	45.0	--	56.3	67.5	--	112.5
Nagler 4	17	82	17	2"	--	--	--	--	--	--	--	--	--	--
Ethos	13	100	15	2" & 1¼"	69.2	1.0	1.4	74.3	138.5	--	173.1†	207.7†	--	346.2†
Nagler 5	16	82	10	1¼"	56.3	1.2	1.4	73.7	112.5	--	140.6	168.8†	--	281.3†
Panoptic	19	68	13	1¼"	47.4	1.5	1.4	71.0	94.7	--	118.4	142.1	--	236.8†
Delos	17.3	72	20	1¼"	52.0	1.3	1.3	70.7	104.0	--	130.1	156.1	--	260.1†
Plössl	25	50	17	1¼"	36.0	1.9	1.3	70.7	72.0	--	90.0	108.0	--	180.0†
DeLite	18.2	62	20	1¼"	49.5	1.4	1.2	63.7	98.9	--	123.6	148.4	--	247.3†
Ethos	10	100	15	2" & 1¼"	90.0	0.78	1.1	59.0	180.0†	--	225.0†	270.0†	--	450.0†
Nagler 6	13	82	12	1¼"	69.2	1.0	1.1	58.7	138.5	--	173.1†	207.7†	--	346.2†
Delos	14	72	20	1¼"	64.3	1.1	1.1	57.7	128.6	--	160.7	192.9†	--	321.4†
Plössl	20	50	14	1¼"	45.0	1.6	1.1	57.0	90.0	--	112.5	135.0	--	225.0†
DeLite	15	62	20	1¼"	60.0	1.2	1.0	53.3	120.0	--	150.0	180.0†	--	300.0†
Delos	12	72	20	1¼"	75.0	0.93	0.96	50.0	150.0	--	187.5†	225.0†	--	375.0†
Ethos	8	100	15	2" & 1¼"	112.5	0.62	0.88	46.3	225.0†	--	281.3†	337.5†	--	562.5†
DeLite	13	62	20	1¼"	69.2	1.0	0.88	46.0	138.5	--	173.1†	207.7†	--	346.2†
Delos	10	72	20	1¼"	90.0	0.78	0.81	42.3	180.0†	--	225.0†	270.0†	--	450.0†
Plössl	15	50	10	1¼"	60.0	1.2	0.80	42.0	120.0	--	150.0	180.0†	--	300.0†
Nagler 6	9	82	12	1¼"	100.0	0.70	0.79	41.3	200.0†	--	250.0†	300.0†	--	500.0†
DeLite	11	62	20	1¼"	81.8	0.86	0.74	39.0	163.6	--	204.5†	245.5†	--	409.1†
Ethos	6	100	15	2" & 1¼"	150.0	0.47	0.66	34.7	300.0†	--	375.0†	450.0†	--	750.0†
Delos	8	72	20	1¼"	112.5	0.62	0.63	33.0	225.0†	--	281.3†	337.5†	--	562.5†
Nagler 6	7	82	12	1¼"	128.6	0.54	0.62	32.3	257.1†	--	321.4†	385.7†	--	642.9†
DeLite	9	62	20	1¼"	100.0	0.70	0.61	32.0	200.0†	--	250.0†	300.0†	--	500.0†
Plössl	11	50	8	1¼"	81.8	0.86	0.58	30.3	163.6	--	204.5†	245.5†	--	409.1†
Ethos SX	4.7	110	15	2" & 1¼"	191.5†	0.37*	0.57	29.8	383.0†	--	478.7†	574.5†	--	957.4†
Delos	6	72	20	1¼"	150.0	0.47	0.48	25.3	300.0†	--	375.0†	450.0†	--	750.0†
DeLite	7	62	20	1¼"	128.6	0.54	0.48	25.0	257.1†	--	321.4†	385.7†	--	642.9†
Ethos SX	3.7	110	15	2" & 1¼"	243.2†	0.29*	0.45	23.5	486.5†	--	608.1†	729.7†	--	1,216.2†
Nagler 6	5	82	12	1¼"	180.0†	0.39*	0.45	23.3	360.0†	--	450.0†	540.0†	--	900.0†
Plössl	8	50	6	1¼"	112.5	0.62	0.41	21.7	225.0†	--	281.3†	337.5†	--	562.5†
Delos	4.5	72	20	1¼"	200.0†	0.35*	0.36	18.7	400.0†	--	500.0†	600.0†	--	1,000.0†
DeLite	5	62	20	1¼"	180.0†	0.39*	0.34	17.7	360.0†	--	450.0†	540.0†	--	900.0†
Nagler Zoom	6 - 3	50	10	1¼"	150.0 - 300.0†	0.47 - 0.23*	0.32 - 0.17	17.0 - 8.7	300.0† - 600.0†	--	375.0† - 750.0†	450.0† - 900.0†	--	750.0† - 1,500.0†
Nagler 6	3.5	82	12	1¼"	257.1†	0.27*	0.31	16.0	514.3†	--	642.9†	771.4†	--	1,285.7†
Delos	3.5	72	20	1¼"	257.1†	0.27*	0.28	14.7	514.3†	--	642.9†	771.4†	--	1,285.7†
DeLite	4	62	20	1¼"	225.0†	0.31*	0.27	14.3	450.0†	--	562.5†	675.0†	--	1,125.0†
DeLite	3	62	20	1¼"	300.0†	0.23*	0.20	10.7	600.0†	--	750.0†	900.0†	--	1,500.0†
Design	Focal Length (mm)	Apparent Field (°)	Eye Rel. (mm)	Barrel Size	Mag. (x)	Exit Pupil (mm)	True Field (°)	True Field @ 1000 yards (feet)	Mag. w/2x Barlow (2x)	Mag. w/2x Big Barlow or Power- mate (2x)	Mag. w/2.5x Power- mate (2.5x)	Mag. w/3x Barlow (3x)	Mag. w/4x Power- mate (4x)	Mag. w/5x Power- mate (5x)

[MimasDeathStar]

Thanks all that's really useful 

8 minutes ago, John said:

A 32mm plossl will show pretty much the largest field of view that the 1.25 inch format can show and in your scope this equates to showing a true field of 1.86 degrees.

So are you saying that my telescope can see a maximum of 1.86 degrees? I think I understand the maths for working out the field of view for a particular eyepiece, what I'm trying to understand is at what point I can't increase the field of view any more because of the design of the scope or something.

Sorry if that wasn't clear, alternatively - sorry if you have explained that above and I haven't understood it.

I tried the FOV calculator on stellarium but I think I must have done it wrong. Using a really big eyepiece it says I can get a 2 degree FOV which seems much too big when I consider what I've read elsewhere.

[John]

A 40mm plossl shows the same amount of sky as the 32mm plossl. The field stop in the eyepiece barrel limits the field diameter that can be seen and the internal diameter of the eyepiece barrel limits the maximum size of the field stop. Thats why a 2 inch format eyepiece can have a larger diameter field stop and shows a larger piece of the sky.

To get a larger field of view than the 1.86 degrees that I've mentioned you will need either a 2 inch eyepiece or a scope with a shorter focal length. A 70mm 450mm focal length scope would show a 3.7 degree true field when used with a 1.25 inch 32mm plossl for example.

[MimasDeathStar]

Thanks everyone that's really useful - @John - its absolutely fine I don't actually mind too much what the maximum is, I was just wondering so I could make sure I didn't spend money on a widefield eyepiece at a longer focal length that was wider than my scope could see!

I think I'm going to be looking at the moon and planets mainly anyway so it no problem either way!

Thanks very much for all the quick responses.