jetstream

I'm very interested too.

Do you lose detail this way? at what f ratio are you sampling and what are you imaging?

More reports from the 200mm f3.8 would be happening if the focuser worked- I might mention that the dob base ordered was not shipped with the scope. Thankfully Wolf at TS came through and had one to me a a very short time.

With regards to roughness- one of my main requirements in a set of newtonian mirrors is smoothness as is the optician I deal with, Terry Ostahowski. Very interesting that roughness shows as an asymmetric PSF. Also interesting that there are things that can degrade the image and not show on the MTF.

This is interesting https://www.telescope-optics.net/mtf3.htm "None of these effects that can significantly lower image quality and detection level is indicated by the MTF. exception being, of course, low contrast level in the case of spherical aberration (spherical aberration also causes contrast reversal at the roughly doubled error magnitude)." ^^^ " Defocus causes contrast reversal, changing the appearance of both images. Coma causes obvious asymmetry. Astigmatism causes an interesting effect that appears as contrast reversal in the star pattern, but only as intensity shift in the frequency continuum pattern (the difference in appearance for the lower and upper left portion of the image is caused by the diffraction pattern, rotated 22.5 counterclockwise, being nearly aligned with its "best" axis with the former, and with its "worst" axis with the latter). Primary spherical has lower contrast transfer in low and lower-mid frequencies, compensated by higher transfer in the high-mid frequencies, but the only change in image appearance is due to the insufficient contrast at some frequencies. Balanced secondary spherical, even if generally similar to the primary spherical, and at a roughly similar magnitude level, causes noticeably different effects. And the effect of seeing is in some ways similar to coma having, due to the extent of PSF energy spread, lower an early contrast drop and, due to its asymmetry, detail deformations. However, unlike coma it causes quite pronounced contrast reversals." as is this: "ROUGHNESS: ~1/14 and ~1/7 wave RMS of roughness producing asymmetric PSF pattern, thus having contrast transfer vary with its orientation relative to MTF bars. Due to the random nature of the aberration, the P-V wavefront error can vary significantly for any given RMS/Strehl level. IHere, the P-V error ranges from -0.169 (blue speck on the wavefront map) to 0.374, and -345 to 0.745, respectively. RMS error also may not be as reliably related to the Strehl as with most other aberrations."

Thanks @vlaiv will check this out tonight, right now getting ready for the -45c to -50c (wind chill) that the polar vortex is bringing. Gotta do some wind blocking, insulating on the propane lines, water lines etc. I'd rather be learning about optics...

I have some semi specific questions regarding the MTF sag in some telescopes caused by various aberrations. This is more of a programming/algorithm question than the actual MTF deal. Question: 1 what optical aberrations cause issues that common planetary frequency restoration algorithms use for processing ie the non symmetric ones discussed? 2 in my current situation the plan is to use exceptional optics to hopefully mostly avoid some of the mentioned issues in (1) above. However I'm looking at an 8" Edge SCT in the near future which will have a fairly big sag caused by CO but also who knows what. I hope I asked a question that makes sense!

And I hope you write some, so us amateurs can use them, excellent info Vlaiv

Each party is expressing their positions and justifying them by various means. If their were no different positions there would be no educational debate and I'm not sure why anyone would be put off by this and especially when they don't have to read it.

Its very easy- if not interested,check out other things. I hope this informative thread continues.

How much is that optics testing camera set up Andrew?

I used my best eyepieces for the scope test so that the results reflect the scope performance and not the eyepiece. I will rig up the ultra sharp, zero scatter Zeiss 25.1-6.7 zoom too...it loves the VIP and a zoom will show the "boundary" of the limits of the scope well. Testing eyepieces could be a whole "nuther" matter. I like the idea of that camera rig to test the optics, I wonder what it costs.

I should be able to test after this coming week. The extreme cold will twist up the lens cells on the 90mm SV, 120ED but not the TSA120. The first 2 will show astig sometimes under these conditions. It varies by cooldown- if I put them away in seacan after va successful cooldown no astig will emerge, its a crapshoot. "Down market" 😀 I don't believe in this term! I have inexpensive Circle T's that do very very well as well as my 10 BCO. I have not reached the limit of barlowing this EP with the VIP- and I'll show you my hillbilly zoom I made with it lol!. Not saying your saying this, but I don't believe that things have to be expensive to be good.

Yes I started with the little peashooter refractors- the real scopes will be out soon In reality this 90mm really surprised me eventhough it takes the 2.4mm HR on the moon with ease.

Not sure why you think this- challenging positions are a matter of discussions IMHO. Btw Vlaiv and I had a good one with respect to aperture and light throughput or "entendue". Opposing positions on it which taught me a lot from his position.

I'm confused why you think this-optical characteristics and their limitations are a fact of life. Before assuming that refractors are being bashed because of limitations in aperture, look through a good, largish-doesn't have to be expensive- reflector. I'm learning a lot from this thread, in many ways and hope it continues on.

If you mean those 2 belts in the marble then yes. The blue was easier than the yellow and I could see a very sharp top, bottom and middle contrast difference line. The yellow at 393x started going away but the blue could take more. I can see from this the colour makes a difference, at least to my eyes. I rushed to set up and observe this as today its snowing and -35c is right after this for a week, so no more marble tests for a bit. Anyway this is why I didn't try to get an image through the eyepiece. I do have a TV Fonemate coming, kind of. We'll see if it shows up. I'm going to set the 15" dob with the marble on the ice once I figure out the distance.

The marble and set up. I used a Vixen 2.4mm and the VIP barlow. Not sure if the fs is at the shoulder but if it is the mag is around 393x. Scope is a 630mm 90mm f7, APO triplet that star tests well. Distance is 54 yards on the Zeiss rangefinder. I was shocked at how well this scope did, I could see the 2 separations of colors- yellow and blue very well. The Nagler 3-6 was first tried but it was too easy. The scope is always cooled and I test straight through.

I just came in from a marble test and might start another thread unless its ok to post pictures etc here. Very very interesting... the little 90mm SV outperformed my expectations. Phone got cold and went dead, images after it revives.

Excellent Vlaiv, in my limited experience around 20% CO doesn'r really hinder things too much and espc when used with good optics. Makes me wonder why I have a weakness for good refractors lol! Another interesting thing is to compare the fracs with real world test results, I think a lot go 1/8pv-1/10 and with high strehls. Is it possible to include the effect of miscollimation in those graphs? Your work is truly appreciated Vlaiv.

Always welcome Mike! Lots of scopes here to play with, I'm going out with a marble pretty quick lol!

Agreed! and I invite you 3700 miles to compare your 100mm to my 15" under those conditions!😃

and how the length of the optical path in the prism affects the refractor 😃 I test my fracs "straight through" just for its and giggles

Thank you Vlaiv! Yes my GSD wants to go out now, back soon.

You know John 😃 and many of your reports indicate which ones show more?