pez_espada

1 hour ago, Solar B said:

These look great Pez 😃 ... You very clearly love and enjoy longer FL  refractors  👍

I do have one question ... do your neighbours ever get a bit nervous 😂

Brian 

🤣 I haven't received any formal complain or so sofar, but sure they must be nervous every time I deploy the big gun 😁   Seriously I do most of my observations from the rooftop, but sometimes I am bit lazy and try to catch up some doubles and the planets behind the buildings from my tiny balcony. Then you see all neighbours  shutting their windows up at once  🤷‍♂️🤣

Enjoy your new scope, a double star cracker!

... ut one you go long you get hooked, here my Japan made Carton F13 before restoration.. 😁   same tripod and mount

3 hours ago, Solar B said:

Just some fun that’s all no review 😄

Brian 

Wow that's exactly my same scope/tripod combination. Only my alt az is a Giro Ercole. Great taste Solar B!

On 06/02/2020 at 22:39, Don Pensack said:

The Paracorr offers a top with a bit less wobble and slightly better correction of eyepiece field stops as large as 46mm and better correction at f/3-f/3.5.  Other than that, the HRCC would be fine IF you have sufficient in-travel of the focuser to accommodate it.

You should note that the visibility and appearance of coma is related to apparent field.  Use 50° eyepieces in your scope, and you likely won't notice coma.  Use 100° eyepieces in your scope and you will.  Though the linear size of coma changes at the edge of the field with true field (it gets smaller as the focal length of the eyepiece shrinks), the apparent size (what you see) of coma only changes with apparent field. 

Also, note that a short focal length scope will have inherent curvature in the focal plane that will mildly defocus eyepieces that have large field stop diameters.  Another way of looking at it is that coma is made worse in appearance by field curvature, so keeping apparent fields narrow will reduce the ability to see field curvature.

So, suggesting an apparent field wide enough to yield fairly large true fields, as well as avoiding defocus from field curvature, and having just about zero astigmatism, I think I would recommend TeleVue Delites.  You might find you don't need a coma corrector with them, at 62°, but a coma corrector would yield perfect stars from center to edge in them.  Also, they're all parfocal, so would use the exact same setting of the coma corrector for each +/- 0.5mm or so.

Thanks Don, too much to learn and digest from your posts. I just bought myself the HRCC and am waiting for a clear night to test It.

1 hour ago, vlaiv said:

I think that really important part of the puzzle here is precision of the focus. Next time if you feel like experimenting - try not having perfect focus and seeing how it affects threshold visibility of stars.

Will do, and I will keep on going trying to improve my collimation skills, etc. I am pretty new to Newtonians.  Perhaps an autocollimator will do? any experiences? I was looking at one of these https://www.firstlightoptics.com/catseye-collimation-tools/catseye-infinity-xlkp-autocollimator.html

Are those better than the typical Cheshire/SightTube than I am currently using?

On 27/01/2020 at 13:13, vlaiv said:

Indeed - here is quick calculation:

102mm vs 130mm

102mm at about 98.5% transmission will be equal to 51^2 * pi * 0.985 = 8048.71mm2

130mm with 32% Central obstruction and 91% reflectivity on two mirrors will be equal to (65^2 - (65 * 0.32)^2) * pi * 0.91^2 = 9866mm2

Still larger and therefore more light gathering, but not 3cm larger in diameter, more like only 1cm larger as 9866mm2 is equal to about 112 - 113mm of clear aperture (sqrt(9866 / pi) * 2).

Add to this that F/5 scope has much tighter critical focus zone - harder to obtain proper focus, especially if scope has single speed 1.25" focuser that is not very precise, and the fact that larger aperture will suffer more from seeing effect and also that strehl numbers of two scopes can differ considerably. If 5.1" newton is mass produced it will probably have system sthrel of about 0.8 and F/11 refractor will probably have over 0.9.

Put everything together and you get result that you have.

I see what you mean but still I am surprised that all these factors put together can defeat an advantage of 27% more aperture in the reflector. This is 62% more light grasp, and the smaller 4" scope still not only matches the larger but defeat it. I am surprise that nobody else  seem to be surprised by this observation.

Well It seems that the large CO (35%) of the 130mm is enough to impact the image as to obviously show less stars in a similar FOV, magnification and exit pupil than a 102 mm refractor despite of the 3cm of more aperture of the former. Lesson learnt.

5 minutes ago, vlaiv said:

That is short version of that saying. Longer and proper version of that saying goes like this: Aperture is King everything other being equal

 

That would mean that a 6" F8 refractor and 6"F8 Newtonian are expected to perform around the same, I guess. To pickup just an example of the top of my head..

4 minutes ago, newbie alert said:

It's focal length.. your f11 scope has a focal length of 1122mm... your 130 f5 has a fl of 650.. aperture is for brightness..fl for depth..

I'm sure your targets have more reach with the f11

I see this in the practice indeed but how's that from the theoretical point of view? What about the "aperture is King"  tenet in astronomy then?

"Aperture is King"  for "seeing finer detail", but also for "seeing deeper". This is what is repeated ad nauseam in every astro forum since I know of.

19 minutes ago, vlaiv said:

Might be so for small apertures, but as aperture gets larger - differences get smaller.

For example - we have seen that 130mm is equivalent to 112mm of clear aperture, so that is reduction of about 13.8% by diameter.

If you do the same with for example 8" F/5 newtonian, you will get different result. It has 26% CO, so math will be sqrt((101^2 - (101*0.26)^2) * pi * 0.91^2 / pi) *2 =  ~183.5mm of clear aperture. That is difference of 9% by diameter,

For an 8" @ F5 to have a fully illuminated field one is looking more to 30-32% of CO, i.e a secondary of 60-63mm.

So all that said I can confirm that a reflector to be able to compete with a refractor, the former has to be at least 27-30% larger in apertura. 

The myth becomes truth. 

As the title says.  My  102mm F11 achromat is showing fainter stars  at 46.7X than my new  130mm F5 Newtonian does at 40.6X.   I' m aware the effect magnification has on seing fantier point light sources, but according my math 6X more should not overcome 3cm of larger apertura of the reflector. On the contrary, my math tell me that the reflector should see around  0.1 magnitude deeper than the frac does...

I have check mi collimation to the best of my current abilities. 

Clear skies

C

32 minutes ago, Charles Kirk said:

https://www.assa.org.au/resources/variable-stars/observing-variable-stars/

Experienced visual observers can see 0.1 mag differences under ideal conditions, and 0.2 mag more typically.

 

That's right, estimates down to  .1 mag are expected. And yes, field of view should be fully illuminated or at least symmetrically illuminated.

13 minutes ago, Ships and Stars said:

I've the ES HR coma corrector, works a treat on my eyepieces, especially the 21 Ethos, in a f4 dob. Bought mine second-hand for around £160-180 (can't recall exactly), couldn't find a single used Paracorr 2 online after months of searching - which speaks volumes about how good those are too! The ES has a tunable top similar to the paracorr, so no faffing around with extra extensions and spacers. For a 130PDS I don't know if it makes sense though? 

You guys  think it makes not sense because  maybe not worthy to buy a coma corrector costing more than twice than the Chinese scope needed to be corrected?   I have seen some folks with eyepiece collections costing many times more than the scopes they are used on.  But again, I am not sure that this would be like buying the latest generation low noise cold camera to shot mag 14 carbon stars with your ShortTube 80. On the other hand,  I have heard this 130PDS is used by some guys here for AP,  and I guess some of them plug a CC  into this scope for this application.. why wouldn't one plug a CC for visual then?  I'm not interested now in obtaining an larger/more worthy Newtonian for the kind of observation I pursue at the moment (visual observation of brighter variable stars ).  Probably some day I will get something like am 8" but I don't think that would happen any time soon. 

 Maybe I should consider a fast refractor, but I would be struggling with CA instead of coma (not enough money for an APO). However I m not sure what would be worst (or less bad) for visual variables, coma of the fast reflector or false color  (and field curvature) of a fast refractor.

54 minutes ago, Don Pensack said:

Don't worry about central obstruction.  It is unimportant for anything except lunar and planetary observing at high powers.

As for what you typically see at the edge of the field in ultrawide eyepieces at f/5:

Coma (from the mirror--needs a coma corrector to solve)

Astigmatism (all ES eyepieces have this except the 92s, but it can be in the optics as well)

Field curvature (causes slight defocus, making other aberrations look worse)  This can be in the eyepiece or the scope if the focal length is short enough (say, under a meter).

Lateral chromatic aberration (sometimes referred to as chromatic smear, looking a bit like a prism has spread the colors)

Edge of field brightening (EOFB) wherein the center of the field looks like it has better contrast than the edge

Edge of field vignetting, wherein the edge appears to be darker than the center of the field

Of these, astigmatism is the worst, and if combined with coma, you get stars at the edge that look like seagulls.  

But all f/5 scopes have visible coma.  Only the central 2.75mm of field (remember, 2" is 50.8mm) is essentially coma-free, so:

--can you accept 2" eyepieces?  Because all currently-available coma correctors are 2".

--If you cannot afford the TeleVue Paracorr II, I also recommend the Explore Scientific HRCC, which is a lot less.

--do you have quite a bit of in-travel of the focuser left in the scope?  The Paracorr needs about 14mm of in-travel at the focuser compared to where the eyepieces focus without it, and the HRCC needs 32mm of in-travel at the focuser to get to a focus position.  The good news is that a coma corrector will allow all your eyepieces to focus at the same point of focuser travel, so you won't need any additional in-travel except to accommodate your near-sighted friends, and that's only a few millimeters at most.

You could be where I was many years ago.  I saw horrible coma, and got a coma corrector to eliminate it, which it did, only to show me my low power eyepiece had strong field curvature (it got replaced).  Once coma is removed, all the other aberrations become apparent.  Alas, eyepieces without astigmatism at f/5 are few and far between.  Only 2 from ES, none from Baader, 49 from TeleVue, none from Olivon/Barsta, a few from Nikon, none from Vixen, none from Pentax, none from APM, etc.  But, zero astigmatism may not be a necessity if coma is gone and astigmatism is very small.  But if you want refractor-like star images to the edge, prepare to pay.. As we say: Wide-field, well-corrected at the edge, low price--pick any two!

Great info here Don, thanks. I have the SW 130PDS which has a 2" focuser.  Which ES and TV eypieces you would recommend then at F5? As you say, probably I wouldn't need perfect astigmatism correction right at the edge! I am happy with the level of correction my Fujinons 10x50 provides (or my F11 refractor for that matter). Also, in the practice what does offer the Paracorr over the ES HRCC?

Thanks,

C

2 minutes ago, pez_espada said:

Gerry thank you for pointing me to this!

Looking at Bartels' web got me thinking that he uses himself  some not so large and very fast Newtonians (F3.3!) which would have (proportionally) huge  secondary mirrors. Ergo the contrast in these instruments should suffer for visual. However Mr. Bartels use them happily  (I guess with Paracorr and Ethos) for visual rich-field astronomy. So there's something I'm missing or maybe the role of CO on contrast is not that great as common knowledge implies?

5 minutes ago, jetstream said:

This calculator from Mr Bartels will show you exactly whats happening re CO IMHO.  Yes the larger the aperture the smaller the CO you can use,fully illuminated field considered. No experience with achromats but I do like triplet APO's.

http://www2.arnes.si/~gljsentvid10/diagonal.htm

Gerry thank you for pointing me to this!

Just now, John said:

F/5 is very much more demanding on eyepiece correction than F/11 or F/13.

Some ES eyepieces are well corrected in faster scopes though. Which ES eyepieces do you use ?

 

John, I use at the moment an ES68 16 and an ES68 24. But I was thinking in getting some from the 82 series for my wide-field endeavors after reading about them for years and hoping they are any good..

2 hours ago, jetstream said:

You might want to look for a scope with less CO, 35% is getting right up there. A Paracorr will increase perceived contrast (fo me anyway) but the large CO might not be your friend here. 30% CO is my max, 25% and less preferred and my best scopes are 21% or less.

It seems, or at least I think I remember reading something along these lines,  that in the reflector world the size of secondary mirrors scale inversely proportional to the focal length, which is  independently of the focal ratio , meaning that, for instance a 5" F6 would need a proportionally larger secondary than a 10"F6 scope (?).  I have seen from a popular brand a 6"F6 with a 46mm secondary (~30% of the primary),  and from the same manufacturer an 8" F6 with a 50mm secondary (25% of the primary). Both, they claim , optimized for visual..

If that's true then for a wide-field Newtonian with good contrast (i.e. proportionally smaller secondary) one necessarily hits a trade-off region, as the larger the scope so it can have  a proportionally smaller CO, the larger the focal distance that negates wide fields...  would that be a sound reasoning?

Maybe my best bet is to go the fast achromat refractor route?

10 minutes ago, Peter Drew said:

A degree of your problem could be due to the eyepieces.  TV eyepieces are probably better corrected than the ES equivalents, specially if the ES's are wider field than the TV's.  The Paracorr is also optimised for TV eyepieces, particularly the wide field ones.    🙂

I though that Paracorrs even when designed with TVs Ethos and Naglers in mind, would work fine for whichever wide AFOV eyepieces reasonably well corrected for off-axis astigmatism and other aberrations at the eyepiece's border. And I heard that ES are reasonably good eyepieces (good value) for not having the green lettering on them.. The ES eyepieces do work flawlessly over the whole field of view in my F11 and F13 refractors! 

This is the question. I have probably been spoiled by the very well corrected fields of both my long refractors and my 10x50 Fujinons that I seemingly cannot take the coma showing at the edges of my new 130PDS Newtonian at F5. I bought this as an entry level reflector to learn collimation and especially to team up with my binoculars as a wide field telescope for estimating magnitudes of bright variables. As you may know, very often you need to compare your variable star with stars of known magnitudes (called comparison stars) in the same field of view. So one wishes to have an instrument able to provide wide fields (such as small refractor or reflector), but obviously with a well corrected field for both curvature (especially in fast refractors) and/or coma (especially in fast reflectors).  My 5.1" Newt is showing its ugly coma in my ES eyepieces (but less in my TV Plössls).  I also think that, because of coma I am not detecting faint magnitude stars due to a low perceived contrast.

Should I invest in a Paracorr for such an instrument? i.e. the Paracorr would cost more than twice the small reflector!  would the corrector "clean" the image enough and provide more contrast as to show me stars close to the theoretically limit magnitude for its aperture? or the CO in the design (secondary is allegedly 46mm) is too much? (I heard this small Newtonian is optimized for AP and not too much for visual). I am aware that if I purchase a Paracorr/coma corrector I would be able to plug it in other larger fast reflectors I may have in the future but at the moment this little guy is the only Newtonian I have living among two long focus refractors....

Many thanks and your opinions and insights are very welcome

C

My Fujinon 10x50 together with my Carton 100 F13 are my  two most valuable and loved  pieces of astrogear.