jetstream

2 minutes ago, globular said:

I suspect another overlooked, and perhaps with the biggest impact too, thing when using SCT is focussing.  People seems to think that because the focuser will let you move the mirror a long way to bring things to focus, that it's fine if they do this - and so they don't think about the optimal position of their mirror and just focus as much as needed.

In my SCT an addition of 10mm to the focal length introduces 0.25 wave aberration.  And 10mm to the focal length is achieved when the focal plane is only 3.5mm away from optimal.

This is an excellent point and my buddy YKSE was big on this years ago. Your method of focusing preserves the optical quality of your SCT and the Edge models are reputedly of higher optical spec. I didnt mean to disrespect your scope with my previous comment.

I dont know why anyone who is possibly offended by Takahashi ownership would read and participate in a thread about the comparison of one?

Just now, John said:

That is a good point. Many features are more about contrast differences rather than angular resolution. I think this is where Modulation Transfer Function MTF comes into play but it is also where my knowledge peters out so I can't "shed any light" on MTF really 😄

Yes, I believe it to be about MTF as well possibly. I hope through conversation here, ideas about it and how seeing can potentially affect these certain features as described by spacial frequency sheds some light on it. I think there might be something to it as the MTF graph describes optical quality in a different but related way to the idea of "diffraction limited or above" etc.

2 minutes ago, Mr Spock said:

I know the type of telescope involved affects the outcome. I had a C9.25 which was extremely susceptible to seeing conditions. On its day it was a fine scope - finding a 'day' was another matter.
I had more excellent seeing (instrument wise) in the first few months of owning the 12" Dob than I had in ten years of the SCT!

Excellent observation Michael. Most people attribute this to cooling, thermal issues which is a factor. Another factor, possibly overlooked is the average optical quality of SCT's. I know some who have tested them and most are just below diffraction limited and a few at or just above. Combine this with the cooling factor and the seeing issues being discussed it does not surprise me at all that your dob shows better and more consistently I presume.

I'm not trying to disrespect SCTs or their owners by my comments, but many have reported the same thing as you.

4 minutes ago, John said:

My understanding of the term "diffraction limited" is that, seeing conditions allowing, the optical quality is sufficient for the instrument to perform to the theoretical limits of the aperture. 

I think but dont know that the term diffraction limited is a description of an optics ability with regard to angular resolution which may not describe everything we see on the moon and planets.

6 minutes ago, John said:

I suspect it's more likely that the better optical quality shows itself in being able to "see" to the theoretical limits a little more easily and under a wider range of seeing conditions ?

I believe this to be the case but Ive noticed the same sharp definition on certain lunar features with scopes of the same aperture and of differing optical quality. Strangely I have also noticed that on certain features the scope with the better optical quality will show them, whereas the other either does not or "blends together" so to speak.

It seems that certain features can be more prone to seeing in differing optical spec scopes?

5 minutes ago, Piero said:

 

Also worth mentioning that you have encoders in your 24".  

Yes for sure, and also on the 15". It amazes me that he old school SkyCommander can locate and more importantly re locate very small PN, galaxies using an ortho with tiny TFOV. Its too bad these are no longer available.

My most used 100 deg is the 20mm APM, edge astig and all.

My 10BCO does very well in its deep role eventhough I know you had a poor copy of one or it just didnt work for you. I use mine barlowed to check very faint observations.

7 minutes ago, Piero said:

if you really want to go deeper, increasing aperture is a better way to go, in my opinion at least. I

100% agree Piero. Put an average eyepiece in my 24" and it will go much deeper than the best eyepiece in my 15".  When going truly deep with these 2 dobs I use the Docter barlowed, the 10mm Delos but I always find myself confirming things with an ortho.

9 minutes ago, neil phillips said:

Exactly Nigel which tally's perfectly with my experiences

I truly appreciate real world observations of things and this is much appreciated Neil, thank you.

2 minutes ago, davidc135 said:

I should say that the term 'diffraction limited' is a bit loose; a smooth, 1/4 wave pv of SA or a Strehl of 0.80... and I can well imagine a scope with a Strehl of 0.95 taking a higher magnification than one with 0.80 although both are diffraction limited. In perfect seeing there would still be some difference. Performance decline would be gradual down to 0.80 and then increasingly rapid.

David

Excellent, thanks for helping me wrap my head around this.  A lot of my puzzlement involves the H130 and the mag it takes. I believe it to be diffraction limited or a little better. The 2.4 HR in this scope provides stupidly good views eventhough the structure is lacking at this level. The 2.4mm gives 270x.

The next question or thought would be assuming perfect seeing how much difference would be between our idea of diffraction limited- .80 strehl, .25 wave SA and one much better? For lunar/planetary I think the MTF graph could be of help eventhough this introduces the effects of aperture in there.

So if we assume that optical quality does affect performance with regards to seeing and that the sag in the MTF graph illustrates this optical quality - then the details observed in a scope at the spacial frequency where the sag occurs would be more affected than those more closer to the perfect MTF graph?

23 minutes ago, Mr Spock said:

I'm not sure of the technical aspects involved. All I know is both my scopes were hit equally last night by the appearance of the jetstream overhead and the consequent waves of poor seeing. Both scopes were rendered useless and I had to call it a night.

I agree, when seeing gets so bad that it degrades optical quality below diffraction limited, any scope is going to perform poorly and its time to pack up.

17 minutes ago, John said:

Perhaps not quite the same question but I have noticed that with eyepieces and scopes, good to excellent seeing conditions helps the better optical quality to stand out a little more.

It is all very related John, great observations.

Assuming we have very good seeing and for this consider it "fixed" the better optic will pull away as the lesser (but good) one is still degraded to a level that differences appear by the seeing, as related to what @davidc135 mentions. I think its related to Suiters wobbly stack idea.

Eventhough I can only speak for myself I think we might assume that excellent seeing is "perfect seeing" with respect to optical performance if you know what I mean.

I'm still very curious how much difference in the views there would be under perfect seeing between a diffraction limited scope and one much better optically. ie lets take seeing out of the picture. My feeling is they would be very close, but dont know.

19 minutes ago, michael.h.f.wilkinson said:

On average the wavefront is flat, is what I claimed. The idea is that there is no systematic deviation from flat, it is just a series of random wobbles, equally likely to go up or down at any point along the front. During really bad seeing there may not a single moment of truly flat wavefronts, but adding all the distortions up taking the average should result in something approaching flat. Put differently, it does not make sense to alter the optical design to correct for e.g. concave spherical wavefronts due to seeing, because the perturbations are random

Thank you Michael, I understand this now.

21 minutes ago, davidc135 said:

The different causes of poor performance such as seeing, Spherical aberration and roughness, thermals and obstruction each cause a loss in Strehl and are additive as the sq. root of the sum of the squares of each indivifual contribution.

Lets say seeing is "fixed" at "very good" . I use 2 different scopes of the same aperture and f ratio, one goes to "X" magnification and one goes to "X +" mag.  Even though both scopes are above diffraction limited the fact that one takes more mag could be an indication of differing optical specs seeing considered? If so then under prefect seeing the views should be the same and magnification very similar?

I have sat for years at the eyepiece shaking my head trying to understand how a diffraction limited scope takes a certain mag while another scope can take more for given seeing.

7 minutes ago, davidc135 said:

There's a bigger difference noticed between .35 and .25 than between .25 and zero waves SA, so that's the theory.

I do understand this, thanks David. In my own tests I have purposely decollimated my scopes to simulate poor optics and have noticed at a certain point the image degrades very quickly, I did this years ago. Anything .25 wave and above gives great views IMHO.

5 minutes ago, michael.h.f.wilkinson said:

Another way to look at it, is that the perfect wavefront is perfectly flat, Likewise, the average wavefront in poor seeing is flat, so optics that correct superbly for a flat wavefront are best on average. It is possible, but extremely unlikely that a wavefront distorted by seeing happens to correct for any imperfection in the optics, but a little maths will show the chance of this is vanishingly small, or at least far smaller than the chances of having fleeting moments of near flat wavefronts in situations of poor seeing.

Thanks Michael, I dont really understand how you put this- ie that the wavefront in poor seeing is flat- can you help me understand please?

Excellent, thanks guys. Ive wondered this for a while but never did know if there was anything to it.

Heres a question that is not meant to be controversial.

Over the years I have heard others, respected observers having had many scopes and having high end glass report that the better the optical quality of the scope the more resistant it is to poor seeing. The reports Ive seen refer to refractors basically .

I have zero interest in "bias" and dont want it brought up here. I do have a great interest in this idea of optical quality vs seeing and would appreciate anyones thoughts or experiences with this. Myself, I dont know and am going to test it out coming up. Ive never really tested it out because as seeing deteriorates I go back in the house.

Gerry

5 hours ago, Stu said:

Quite right Gerry! Similarly I reported my Vixen FL102S was soundly trounced by my Heritage 130P on Zeta Herc, yet apparently only bought my Tak because I was too swayed by others eulogising about them.

Thanks Stu. I think having the right tool for the job applies in all this. I have recently started to dissect the moon with some scopes and the results are a bit surprising. Under very difficult circumstances some scopes excel it seems. In the end a person will know what to reach for based on the observing conditions.

11 hours ago, davidc135 said:

A very interesting report, Gerry, showing the difference between a good and an excellent scope. If you ever test the two in DPAC and were able to attribute the difference to colour correction or smoothness of figure etc it would be worthwhile.

David

Yes, I would love to do this.  The 120ED, now with the focuser aligned is putting up vg views and will take around 500x . The TSA120 takes endless mag and the snap focus is brutally fast with my top eyepieces, the Zeiss zoom included. I'm noticing some things in these comparisons including the VX10/TSA. I can say this- I'm looking for features or circumstances that show up differences in these scopes and I have the seeing to do it in the spring.

When the terminator adds a great amount of contrast from placement the VX10 shows more detail-not sure about the contrast differences in the individual features shown yet. Under very difficult circumstances ie weak contrast from terminator placement near the edge, the TSA120 soundly bested the VX10. Dimming the brightness with very low scatter eyepieces at very high mag 300x+ the TSA 120 really did show the VX10 up. The TSA120 bested the 10" on Gassendi the other night as well.

Not sure if many are interested in these comparisons as some of the findings go against the grain. Right now I'm looking at a bigger refractor to go with the TSA and dont want to spend a pile on a suitable DPAC flat and I do think its flatness is important and thank you for the help on this.

I look forward to comparing my scopes in DPAC and hope to find a cost effective but suitable flat.

1 hour ago, Deadlake said:

With regards to red line testing, I think the current theory is that the blue line will be more optimised than testing in the green and the green line will also not be as high.

I understand this and also more than some might think regarding optics. It puzzles me when people think they can tell the difference between .90 strehl and .95 strehl when the mags they use are under 200x usually indicating seeing issues.  A good, smooth diffraction limited scope ie .80 strehl will give great views . The better the optics the higher the mag they can go.

If any scope including SV, Starfield, Tak, Tec A-P can get the colour lines all at .90 strehl and above the view will be very very good.

I never doubted for a minute the tester of the 180SV. I would guess this particular scope had issues and I would also guess the rest do not. Given the close results by @Mr Spock I would like to see a DPAC test of the 2 scopes- they are probably very similar and would be interesting to correlate the results to the visual through both.

1 minute ago, Deadlake said:

At the moment there really is not enough data on SV's line up to call this one way or the other.

Even what SV are going to do is not firmed up, originally they where going to move to testing in the green line and then that statement was removed from SV's site.

It is very possible the rest of the line will be well corrected for green and blue despite being corrected best in red. Its also possible that the 180mm tested had some issues stemming from a slip through QC.IMHO.

To me, the DPAC test and others, take bias out of the equation optics wise. No doubt many scopes from many makers will do well in these tests and some will not.  Time will tell as more and more scopes are run through the test, unless owners are scared they wont test well and therefore affect resale value. IMHO.

I am very interested in further SV tests and also scopes like the Starfield. AT scopes seem to be testing well and it will be interesting to see sample to sample variation in them, if and when more scopes become available.

5 minutes ago, Deadlake said:

I'm waiting on more reports but from the feedback I've seen very positive.

My next tracking mount will be a harmonic mount- some guys have been keeping an eye on them and gave me a heads up, thank you as well for the heads up. Not sure what brand yet but this will be the future in tracking mounts I believe, eagerly waiting our report on one!

I love the clamshell ring and my little 90mm is very easy and fast to balance with it, same goes for the Tak.

11 minutes ago, Mr Magoo said:

Congratulations on the DM6, its a brilliant mount.

Thank you! Great to hear and I cant wait to get and use it!

Gerry

@Space Hopper @TakMan I am in the process of ordering a DM6 from Tom, cant wait and should be here in a week and a half. Got it with encoders and the finder mount on the mount itself, what a great idea. Also coming with the DSC mount. Tom is also giving me pointers with regard to the WSP and where to stay etc. I hope to look through some top glass from TEC, Tak, A-P and help me in a future decision down the road.

I may try the Rowan down the road, we'll see where this refractor craze will bring me! lol!

Gerry

4 hours ago, JeremyS said:

Long way to snowmobil, Gerry. Good luck!

Im thinking of it!!

The  TAKAHASHI TSA120 and its super fantastic, tough old school sand cast parts would be about the only refractor that would make the trip in one piece!

Thats why its called a SUPER APOCHROMAT

Dont want to hurt anyone whos optically fragile here ..