Telescope for visual richfield

[Matthieu]

I only do visual and have decided to move my small 70mm refractor that I use for widefield views to the city.

This leaves a gap in my scopes in the country (bortle 3-4) as my widest view will only be 1.38 degree with the C5.

I've narrowed things down to 2 achromats with great build quality so am interested if anyone's ever compared them.

SVBony SV48p 90mm f/5.5
SVBony SV503 80ED f/7


My main query is whether the ED glass in the 80ED will result in it pulling above it's weight for widefield low/medium mag views against the non-ED 90mm?

Note that I'm really not bothered by CA on the moon and planets as I'll get the C5 or dob out when the moon is out or if it's planetary season.

[EpickCrom]

Hi Matthieu, and welcome to IIS!

I am a visual only observer like you. I currently own a 10 inch dobsonian reflector which is excellent for visual but not exactly a rich field telescope. My 10x50mm binoculars do a decent job in giving me a rich field.

I do not own any refractor's, but plan on buying one in the future. From what I've heard, the ED refractor's eliminate CA. Sorry I can't offer more beyond this, but I'm sure others with experience with rich field refractor's will chip in shortly. Clear Skies!

[Stefan Buda]

Hi Matthiew,

If you tell me what eyepiece/s you intend to use, I can work out the better
option. The eyepiece is the other half of a visual scope, so it should not be left out the equation.

[MortonH]

I have a C6 and recently bought a used Skywatcher Star Travel 102mm f/4.9 for the same reason as you. It is substantially brighter than an 80mm scope and has greater resolution. On deep sky it's very nice and is better than expected on the moon, Jupiter and Saturn. The only minor caution is that at f/4.9 it needs decent eyepieces to perform well at the edges. I have 11mm and 14mm Maxvision/Explore Scientific 82° eyepieces and they are pretty good.

The choice between a 90mm achro and 80mm ED is more difficult. The 90mm would only be 26% brighter. Ignoring the price I'd probably go for the superior optics of the 80mm.

[Matthieu]

Thanks Joe!

@Stefan,

At this stage, for low power I’ll use a 2 inch GSO Superview 30mm 68 aFov but may upgrade in the future to the closest focal length in the ES 82° series or a second hand nagler. For medium and high power, I’ll use the 1.25 inch xcel lx at 60 aFov, I have the 25,12, 9, 7 and 5. But I’d be surprised if the seeing is ever good enough for the 7 and 5 on a widefield instrument.

So I guess, the 30mm, 25mm and 12mm would be used more often than not as I assume their exit pupils should be good for DSOs at either f/5.5 or f/7.

[MortonH]

Note that the Svbony 102ED is currently cheaper than the 80ED. It could possibly replace the C5 as well as fulfilling your wide field needs.

[Matthieu]

Thanks Morton,

That opens another can of worms 😅 as the sv503 is available at 102mm and ES has the AR 102 that also looks enticing.

If I understand correctly, provided I have the right eyepieces, the 90mm sv48p should outperform the sv503 80mm.

What would I need to look for in widefield eyepieces for faster refractors? I assume it’s not so much about coma so is it about field curvature so ultra flat field eyepieces would help?

[By.Jove]

Morton the classic "richest" richfield scope is a 6" f/5 (usually a newtonian). Not a 4" refractor and definitely not an 80/90mm.

The explanation as to why is somewhere in the old SciAm ATM books edited by Ingalls - it is related to the frequency distribution of stars by magnitude - which is not linear - vs the scope aperture, and field of view.

[MortonH]

Field curvature is actually dependent on focal length, not ratio. The Svbony 90mm and the Star Travel 102mm both have focal length of 500mm so FC will be the same. Where focal ratio comes in is the faster the scope the steeper the light cone arriving at the eyepiece. Complex designs like Naglers and the ES82's generally handle this better.

I've read that the ultra flat field eyepieces actually work best in flat field scopes so not sure how they'd fare in a short refractor. You might need to Google that.

[MortonH]

Agreed. I like the 4" f/5 refractor as it works on the same small mount as my C6.

[Matthieu]

Thanks for all your help Morton.

I realise now that I meant widefield rather than richfield visual observation. Apologies for confusing the two.

It does sound like the SV48p ($399 minus another 10% on eBay) would work as well for that purpose if not better than the SV503 80 ED ($637.49 minus a $40 coupon on Amazon) due to it's bigger aperture and slightly shorter focal length though it will be challenging on eyepieces at f/5.5.

[MortonH]

The Svbony 90mm seems nice but I've read quite a few reports on Cloudy Nights of issues with the focuser.

[Matthieu]

I reread all comments and ended up ordering the sv503 102ED. It’s a bit more than I wanted to spend but it sounds like a scope I can keep forever and that should work nicely with my current eyepieces.

Thanks for all the advice 😊

[MortonH]

Nice. Let us know how it performs.

[gerardgallagher]

Isn't there one available now in classifieds for a really good price?

[Stefan Buda]

Hi Matthew,

Sorry I'm late with the reply. I got hit by Covid.
Anyway I think you made a good decision. The 30mm eyepiece will give you a nice exit pupil. Don't know how old you are but an exit pupil of 4.28 should be good for all ages. Also the 2.8 degree field of view is not bad for a rich field scope.

[OzEclipse]

An RFT, Richest Field Telescope, is one which has an exit pupil that matches your pupil's maximum dilation. If the telescope exit pupil exceeds the maximum dilation of the observers eye, only a portion of light from the scope is transmitted to the eye, the rest is wasted and it's as though you're using a smaller scope.

Note:

1. It has nothing to do with whether it is a refractor or reflector and nothing directly to do with the f ratio other than some f ratios are impractical.

2. The maximum pupil dilation varies with age of the observer and at a given age, varies between individuals. so one persons RFT is not necessarily another's.

3. The scope's richest field view is always a very low power. Therefore you won't see much difference in the views through an achromat vs APO. APO performance shines at high not low magnifications.

HOW?
Look up the average pupil dilation for your age group (see graph) or measure it.

Multiply the exit pupil by the scope f ratio to get the eyepiece focal length required for Richest Field(RF).

examples:
A 60 year old has a pupil dilation of 5mm and a 10"f6 newt.
5mm x f6 = 30mm eyepiece required for RF.

A 20 year old with a 7mm pupil dilation using the same scope needs
7mm x f6 = 42mm eyepiece for RF.

If the same 20 year old was using an f10 scope, a 70mm eyepiece would be required - this is impractical.

However the circumstances for a 75 year old with a 4mm pupil dilation are very different - 4mm x f10 = 40mm eyepiece for a richest field.

cheers

Joe

[Stefan Buda]

There is one more thing that needs to be considered regarding the exit pupil: Many people have some degree of astigmatism in their eyes and in those cases a smaller than ideal exit pupil is preferable as the astigmatism becomes more noticeable for bigger exit pupil.

[Matthieu]

Hey Gerard, I do regularly check the classifieds and as far as I could tell, none of the scopes fit the bill for me.

Thanks for the info Joe and Stefan, I think the sv503 102 will work like a charm as it's going to give me 5.7mm Exit Pupil with my Omni 40mm and 4.27mm with my Superview 30mm so I'm sorted now (in my 40s) and for the next few decades. I'll go through the four seasons with the scope and may get an upgrade of the 2inch eyepiece if what I have leaves me wanting.

[dabbeldi]

I've been using a Skywatcher 120 mm f5 for the purpose of a RFT type scope. With a 30 mm ES82S I'm getting about a 4 degree field of view. The large magellanic cloud blew my socks off. I upgraded the mount as balance is becoming a serious issue.
I'm 46 and the 6 mm exit pupil seems to works for me.

I don't have experience with the two scopes you mention.

[Matthieu]

That’s good to know, I’m expecting the scope this week so should arrive just in time for Thursday and Friday nights which both look promising. The ES 82 is definitely a top consideration for a future upgrade as it would give me 3.44 fov in the f7. I’m hoping to join the gippsland star party in March so would be a chance to look through some premium eyepieces and see what I like.

[Matthieu]

Considering all the help I got choosing my widefield refractor, the SvBony sv503, I thought it was only fair that I would share my thoughts now that I’ve had 5-6 sessions with it. Hopefully it might help someone else make their own decision for or against.

I have been impressed by the build of the OTA. As far as I can tell, the whole OTA is either metal or glass which feels like quality.

The focuser has no slop with 1.25’’ diagonal and eyepieces (mine weigh about 400g combined). The rack and pinion focuser is butter smooth with both coarse and fine grained wheels. It is also rotatable 360 degrees which was a factor in my decision considering I’m using an Explore Scientific Twilight I which requires side-mounting. The focuser gets out of the way when observing which is really what a focuser should aspire to do.

The retractable dew shield follows suit by being tight enough to hold without being too hard to pull in or stow away. As a note, the cap seems to have been improved from reviews I’ve read as it isn’t a screw in model but simply slides in and out. Again, the fit feels right.

When it comes to optics, I clearly am no expert. I can see chromatic aberration but am not bothered by it. Using my X-Cel Lx 5mm for 143x, the moon looked bright and sharp. Considering the seeing was around 4 out of 5, I tried to Barlow a 7mm X-Cel Lx for 205x, but much preferred the view with the 5mm as the slight gain in visible features was unfortunately offset by the loss of light.

Moving on to Jupiter on the same night, I found the view in the 7mm and 9mm more pleasing at 102x and 80x respectively rather than in the 5mm. I could see 5 to 6 cloud bands though for some of them only the western side was visible.

As for DSOs, the Pleiades just fit in the 2.1° fov of the 25mm X-Cel Lx at 29x . I can’t wait to get a 2’’ diagonal to get a wider fov as it would show some of the darker surroundings and better frame the view. The Carina Nebula, however, is a fantastic fit for that fov and is bright enough across the whole field to show nebulosity without filter (bortle 3 or 4 skies). So is M42 in Orion.

As for open clusters, I’ve so far enjoyed the low power view and how it frames NGC 3532 and the Southern Pleiades (IC2602) though NGC 3532 could have done with a wider fov. Other open clusters such as NGC 3766 and 3114 worked really well at medium to high power.

Whilst I’m useless at star testing, the above views all looked good and gave me confidence that the optics are good for an f/7 achromat.

Finally, my mount pairing on an Explore Scientific Twilight I works very well for visual. It was easy to balance and once I realised I could just leave the azimuth axis unlocked became another piece of equipment that I could just forget about whilst I get lost into the stars. The pairing handled about 10 km/h wind at 143x though became too unstable at 200x.

All in all, I am very happy with this telescope so it’s going to be a mainstay for me. Now I only need to work out whether I keep my C5 which has sharp optics but can’t really compete with the refractor’s fov.