Eyepice size suggestions for 8" SCT

[Solanum]

I just bought a Nexstar 8SE from another forum member and I'm thinking about eyepieces. The ones I have filled the range I liked for my dob, but this scope has a 2032 mm focal length, 203 mm aperture and is F10, so they have very different magnification.

I used to find 50x was handy for locating stuff, 80-100x best for general viewing and 150-200x good for detailed viewing in good seeing. My better eyepieces are now in the 150-350x range....

Would adding a 40 mm (approx 50x) and 30 or 25 mm (approx 70x-80x) be sensible for this scope, or have I made a mistake somewhere? I guess ideally, I'd want 50x, 75x and 100x, though I have the latter.

I don't see a lot of 1.25" eyepieces at the 30-40 mm size. I guess my budget would be looking at something of the level of a second hand Vixen NLV (SLV is it now?) type of thing. Any suggestions?

My 6mm didn't get a lot of use before, I think it will get very little use now, should I just sell it?

[Renato1]

When I was at a dark site hunting down galaxies and other faint stuff, a wide angle 20mm eyepiece with 1.25" barrel was hands down my most used eyepiece with my 8"SCT. My 15mm, 26mm, 32mm and narrow field 40mm 1.25" eyepieces just didn't get much use in it, because the 20mm teased out faint objects better than the others did - though they were fine on open clusters and bright nebulas.

An 18mm or 19mm wide angle eyepiece might be better for faint stuff from a light polluted backyard.

A 1.25" 32mm will give you the biggest field in this telescope for that barrel size. Avoid the 1.25" 40mm because it gives the same field of view as the 32mm.

If you want really wide fields of view, you need a 2" diagonal with two inch eyepieces - like a 30mm ultra-wide angle or 40mm wide angle. Cheap 2" 30mm ultra-wide angle eyepieces were around a few years back from Andrews and Bintel for $80, and I thought them pretty impressive in an 8" SCT (but sub average in other telescopes). Basic 2" 40mm wide angle eyepieces made by GSO can be gotten from Andrews for $49.

For high power viewing of planets, I found a 10mm was good on any night, and 8mm was good on many nights, a 6.4mm was good only on some nights, and a 5mm was only good on a few nights of the year ( but that was the case for my backyard, where you view may be better or worse).

I suggest also thinking of eyepieces in terms of exit pupil rather than just magnification, as that gives you an idea of image brightness in the eyepiece. As you have an f/10 telescope, a 40mm eyepiece has a 4mm exit pupil, 30mm has 3mm, 20mm has 2mm etc.

Regards,
Renato

[Solanum]

Thanks, that is just the sort of experience I wanted to hear about. Especially pointing out that 32 mm is the max field for 1.25", I hadn't thought about that for some reason.

I had calculated out the exit pupil sizes actually and was a bit concerned that to get the same magnification as with my old scope the exit pupil size would be quite a bit smaller with this one. Not much that can be done about that though I guess!

Everard

[Don Pensack]

I owned a scope like this for 11 years and came to the conclusion the following magnifications were useful:
50X, 100X, 150X, 200X, and 250X. Very rarely higher.
The eyepiece focal lengths are:
40mm, 20mm, 13-14mm, 10mm, 8mm
It will depend on what you want for largest true field as to whether you convert to a 2" visual back, but because of the slip clutch altitude axis and the lack of clearance with the base, i don't recommend it on this scope.
That would confine your maximum true field to 0.76 degrees, and that's just not wide enough.
So I advise adding the Celestron f/6.3 focal reducer to the scope. That would mean your all-1.25" eyepiece selection would become:
24mm 68 degree eyepiece (1.21 degrees!)
13-14mm widefield or ultrawide field.
9mm wide to ultrawide field
6-7mm wide to ultrawide field
5mm wide to ultrawide field.
The reducer/corrector flattens the field and improves star images across the entire field.
It allows a widest-field to be accomplished with smaller and less expensive 1.25" eyepieces.
It doesn't overweight the back of the scope and require a counterweight.
It's removable for use at f/10 if desired--2 scopes in one!
Now a judicious application of a good barlow would reduce the number of eyepieces needed. The 13 to 14mm could become a 6.5-7mm if barlowed, and a 9 to 10mm could become a 4.5-5mm if barlowed.
That might give you a bigger budget for the 3 eyepieces you then need.
I have my favorites, but because I am in the business, I won't recommend specific brands or models.
I think you'll find when you need a wide field, you'll go down to the 24x68 and when you want to do general observing you'll jump to the 13. having an eyepiece for 75X probably isn't necessary.
Don Pensack
Los Angeles

[Solanum]

Thanks for this, I was actually going to ask about a focal reducer in a new thread, what are the downsides? Presumably there is less back-focus? Does this also affect the eye-relief? Does focussing become more difficult (the travel on the focus knob is a lot less than I am used to, which makes it a bit more fiddly)?

Everard

[Don Pensack]

It's effect on back focus is that there is a specified distance at which the reducer has the 37% reduction. Farther back, the focal length of the scope shortens. For instance, with a 2" star diagonal, the f/ratio could be f/5.5 or even f/5. It doesn't change the overall amount of focus available in the scope, though.

Eye relief is not affected, as far as I know.

I used one for 11 years and never noticed that focusing became more sensitive. I did increase the focus knob diameter to 64mm, however, to slow focusing down and make fine focusing easier. That larger focus knob also made focusing at f/10 easier.

A focal reducer does NOT mean the scope has more illumination at the edge of a particular true field, though. If the scope starts displaying significant vignetting with a 1.2 degree field at f/10, it will at f/6.3 as well. So a focal reducer doesn't allow any larger a maximum possible field in the scope than the scope can have without it. it does, however, allow that maximum field with smaller, lighter, and less expensive eyepieces. You could easily spend as much for one 2" long focal length maximum field eyepiece as you would for a whole set of decent 1.25" eyepieces.

Don

[Solanum]

Just worked out the effect of the f/6.3 on my current eyepieces and it brings them right back to almost exactly what they were on my old scope. So I am sold. For the future I'll definitely try and shift my EPs to ones more suitable for the new scope, but for the meantime the focal reducer will be a bl**dy cheap way getting the the useful magnification range back with what I have. Especially as I've found somewhere in the US that will ship it out (new) for almost 1/3 of the cheapest price I can find in Australia. What a crazy system they have to hike stuff up that much here (the Australian site even claims it is 40% of the full price of AU$600 - vs <US$150 overseas).

[Renato1]

I've never had a problem getting any eyepiece to focus with my f/6.3 reducer. That said, I rarely use it.

While it worked fine on open clusters, I always felt that when trying to observe the really faint stuff at a dark sky site, using eyepieces that gave equivalent focal lengths, I could see the really faint galaxies better without it than with an eyepiece and it.

I did think that it would be handy for my more expensive eyepieces to get more use out of them - at the time I had a 14mm ultrawide angle, which acted like 22mm UWA with the focal reducer. Instead I found I just preferred to use my simple 20mm wide angle.

The focal reducer is certainly good for astrophotography.

Regards,
Renato

[Amaranthus]

I have a Nexstar 8SE, and find my two favourite eyepieces with it by far are a Nagler 16mm and 9mm, which, when the 0.63FR is fitted (which I typically do for non-planetary observing), yields FOV of roughly 1 degree (80x) and half a degree (142x). They cover most objects you're likely to want to observe with this telescope (if you want to go wider and stick with 1.25", you can go a Panoptic 24mm, but really, it's better to get a rich field scope for such viewing >1deg). Lovely eyepieces, and with the FF fitted, pin-sharp to the edge.