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Walking on the Moon

orion 250 dob

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as i have now ordered my scope :clouds2: (orion 250 dob f6.3 hi-lux 1/8 PV wave) here is an e-mail from martin mobberley which helped me decide on the f6.3 (a very busy man but replied within 24 hours)

Hi David,

Personally I'd always go for the 250mm f/6.3. The only advantage of the

faster 'scope is a wider field with the same eyepiece. But you can get

1 degree even with a 1.25 inch barrel on the f/6.3 scope.

Two things you may not have considered:

The[glow=red,2,300] sweet spot [/glow] at the focus of the f/6.3 Newt will be 5.5mm diameter.

With the f/4.8 Newt it will be 2.3mm. The f/6.3 'scope will probably

hold it's collimation well, whereas if the f/4.8 is knocked it could easily

lose collimation and, therefore, fine planetary detail.This is less important

for low power Deep Sky stuff.

The other point is comfortable viewing height. For an average adult male

the f/6.3 Newt will have quite a nice eyepiece height for objects high up,

with just a bit of bending lower down. With the f.4.8 you will always be

bending, or, maybe trying to find a stool of the right height.

The only downside of a longer tube is that they are more prone to wind

vibration and less transportable perhaps.

Eyepieces are much more difficult to recommend. I like quality stuff and

nearly always buy TeleVue eyepieces. Their barlows, 2 and 3x are excellent.

Their wide field Naglers are mega-expensive but give you a space walk

quality.....It all depends what you want to observe. High powers with Dobs

are tricky as the field keeps moving! TeleVue Panoptics are a bit cheaper.

All the TeleVue stuff seems to be parfocal...in other words, if you swap

eyepieces or barlows, you don't have to re-focus.

If you go for the f/6.3 and want a 2" eyepiece, something giving around

50x mag would be very nice for wide fields......around 30mm focal length,

as you mention...... but it depends so much on personal choice.

Venturescope sell TeleVue stuff.

My SPX 250 f/6.3 is optimised for planets so has a very tiny flat mirror....

I presume Orions Dobs have a much bigger flat........Because Orion

scopes are so lightweight they can be a bit shakey when focussing...I

fitted a JMI Motofocusser on mine....but for planetary imaging this is a

lot more critical of course.

Hope this is of some use!



thanks again martin :clouds2:

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The[glow=red,2,300] sweet spot [/glow] at the focus of the f/6.3 Newt will be 5.5mm diameter.

With the f/4.8 Newt it will be 2.3mm.

I'm glad somebody bought that up; it is a very good point that is rarely discussed.


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