Wide field views with ETX 105

I realise I'm never going to get massive wide field views with this
scope, but I'm looking for comments / recommendations for wide field
views with etx105.

I'm considering a meade SWA eyepiece, but what about focal reducers &
nagler or erfle eyepieces. I'm confused by all the possible
combinations. What is the minimum magnfication I can use before I see
the central mirror spot?

TIA

john

"John Collins" wrote in message
...
I realise I'm never going to get massive wide field views with this
scope, but I'm looking for comments / recommendations for wide field
views with etx105.

I'm considering a meade SWA eyepiece, but what about focal reducers &
nagler or erfle eyepieces. I'm confused by all the possible
combinations. What is the minimum magnfication I can use before I see
the central mirror spot?

Don't confuse 'wide field' with 'wide angle' (of view). The magnification
is what controls the former, in inverse manner; the latter is a
characteristic
of the lens format.


--
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Dorset UK Astro Society's Web pages, Info, Meeting Dates, Sites & Maps
Change 'news' to 'sewn' in my Reply address to avoid my spam filter.

Hi John,

I used to own an ETX-105 and never managed to crack the rich-field nut
quite as I'd have wished. The limiting factors were the constraint of
1.25" eyepieces combined with the long focal length of 1470mm.

In practice, the maxiumum 1.25" eyepiece focal length that I could get
my hands on was the usual 40mm, a Meade in my case, which has a 44°
afov, as the plössl design gets 'pinched' as the focal lengths get
much over 30mm. I also bought a 32mm (52° afov) during an eyepiece
buying frenzy, not expecting to really need it.

1470mm / 40mm = 36x
44° afov / 36x = 1°12' true field of view

....but then with the 32mm you get a 1°08' true field of view. Guess
what, I just used the 32mm in practice. :-) It's a wonderful
eyepiece, IMHO, and makes the 40mm redundant. But I wanted richer
fields, so I looked at buying an eyepiece with a wider apparant field
of view than the usual 52° afov plössl, sooooo, the next step was to
try and go 'ultrawide' with something expensive like a Panoptic or
Vixen or whatever. Well, the largest Panoptic in 1.25" is the 24mm
with its 68° afov, which gives 1°06'. Hmm... And then the Meade SWA
24.8 with 67° afov gives... erm... 1°08', which is the same as the
32mm superplössl.

Buggah.

That's when I gave up my fruitless quest, saved up for an expensiven
short-focal length refractor with a 2" focuser, (a Borg 100ED f/6.4,
thanks for asking), and then flogged my ETX at a considerable loss,
which did little to offset the pile of dosh I handed over for the
Borg. I'm now in rich-field heaven, 'cos my very own 2" Moonfish
eyepiece of 30mm and 80° afov gives me me about 3°45' true field of
view, which is well over seven moon widths in old money.

Should be adequate.

However, if you're happy with your ETX in all other respects, then I'd
like to give you a bit of advice that might save you a few quid: use
that £200 you're prepared to pay for a Meade SWA, and instead buy
yourself an ST80 refractor as a second telescope. Put it on a
Manfrotto camera tripod with a pan-head, and you have a sweet
low-power telescope of 80mm f/400mm. Shove your 32mm 52° afov
superplössl into that, and you've got yourself about 4° of true field
of view at about 12x, which is possibly too wide.

Ric

wrote:
Hi John,

I used to own an ETX-105 and never managed to crack the rich-field nut
quite as I'd have wished. The limiting factors were the constraint of
1.25" eyepieces combined with the long focal length of 1470mm.

In practice, the maxiumum 1.25" eyepiece focal length that I could get
my hands on was the usual 40mm, a Meade in my case, which has a 44°
afov, as the plössl design gets 'pinched' as the focal lengths get
much over 30mm. I also bought a 32mm (52° afov) during an eyepiece
buying frenzy, not expecting to really need it.

1470mm / 40mm = 36x
44° afov / 36x = 1°12' true field of view

...but then with the 32mm you get a 1°08' true field of view. Guess
what, I just used the 32mm in practice. :-) It's a wonderful
eyepiece, IMHO, and makes the 40mm redundant. But I wanted richer
fields, so I looked at buying an eyepiece with a wider apparant field
of view than the usual 52° afov plössl, sooooo, the next step was to
try and go 'ultrawide' with something expensive like a Panoptic or
Vixen or whatever. Well, the largest Panoptic in 1.25" is the 24mm
with its 68° afov, which gives 1°06'. Hmm... And then the Meade SWA
24.8 with 67° afov gives... erm... 1°08', which is the same as the
32mm superplössl.

Buggah.

That's when I gave up my fruitless quest, saved up for an expensiven
short-focal length refractor with a 2" focuser, (a Borg 100ED f/6.4,
thanks for asking), and then flogged my ETX at a considerable loss,
which did little to offset the pile of dosh I handed over for the
Borg. I'm now in rich-field heaven, 'cos my very own 2" Moonfish
eyepiece of 30mm and 80° afov gives me me about 3°45' true field of
view, which is well over seven moon widths in old money.

Should be adequate.

However, if you're happy with your ETX in all other respects, then I'd
like to give you a bit of advice that might save you a few quid: use
that £200 you're prepared to pay for a Meade SWA, and instead buy
yourself an ST80 refractor as a second telescope. Put it on a
Manfrotto camera tripod with a pan-head, and you have a sweet
low-power telescope of 80mm f/400mm. Shove your 32mm 52° afov
superplössl into that, and you've got yourself about 4° of true field
of view at about 12x, which is possibly too wide.

Ric

Very nicely put. I new that I was never going to get huge fields, but I
think I just want a little more than the 50 arcminutes of the 26mm. I'm
not sure I want 4 degree fields, but I take your point. Is it possible
to get a focal reducer to get the scope's fl down to about 1000mm, then
if my maths is right I'd end up with something around 1.5 degrees at 30x
with the 32mm plossl

john

Very nicely put. I new that I was never going to get huge fields, but I
think I just want a little more than the 50 arcminutes of the 26mm. I'm
not sure I want 4 degree fields, but I take your point. Is it possible
to get a focal reducer to get the scope's fl down to about 1000mm, then
if my maths is right I'd end up with something around 1.5 degrees at 30x
with the 32mm plossl

john

Hi John,

I'd buy the 32mm as a starter (I did!) because that eyepiece will be
useful whatever you decide to do. I've never noticed a 1.25" focal
reducer, but then again I've never looked for one. It might work if
you've got one in mind, so you might have fun experimenting...

Ric