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.
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!
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.
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.
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.
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?
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.
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?
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.
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.
Agreed. I like the 4" f/5 refractor as it works on the same small mount as my C6.
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.
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.
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.
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 😊
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.
Location: '34 South' Young Hilltops LGA, Australia
Posts: 1,436
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.
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.
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.
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.
