jetstream

Sure use this one, my 15" report is similar I think, but not sure where it is.

Maybe, but will it not show the effect that aperture has on things vs "perfect" smaller aperture APOs?

Yes but now were comparing a near perfect refractor (Tak etc), optimised shall we say to a mass produced, highly obstructed SCT. Even so, if the optical quality of the SCT is the same as the frac, results might be surprising. What I'm interested in as well is how the graph shape might impact image processing.

Am I correct in reading this that the 4" refractor has more than 10% difference in the frequencies ranging from about 20 pixels to 100?

Hes gonna need it after he sees the next graph!

Vlaiv can you MTF plot a near perfect 4" refractor against a 12" newt obstructed 20% and with a Strehl of .90? This gets interesting IMHO. Assuming perfect focus, on axis (no coma)and perfect seeing.

Depends on your diagonal 😉😃

Vlaiv did- he assumed perfect optical quality for the scopes used for the examples. Optical aberrations will cause sags in the MTF graph. It would be interesting and very involved to discuss less than perfect optics.

@vlaiv keep any and all info on this coming please, it is much appreciated.

The way mine was set up gave massive astigmatism and signs of pinched optics. If I remember this mirror had the tape too.

I might just have understood some of this. I know of rectangle waves in industrial applications. Can you go through how the higher frequencies make the squarewave again please? Am I correct to say that the sharp edges of the rectangle wave relate to sharpness within the view or image? And that the softer, more prolonged transition of the sine wave softens the view?

Yes a full explanation of MTF vs resolution vs object feature size would be great. @vlaiv?

Its easy 1- make the secondary look round and centered under the focuser. Loosen stalk nut a bit for this, then tighten back up. 2-put the cheshire cross hairs on the primary dot. Use the 3 secondary adjustment screws for this 3- primary collimation- put the dot in the donut of the cheshire. Use 3 primary adjusting screws on the mirror cell for this. This 1 iteration gets close, maybe right on. Repeat if needed

Just collimate the primary with the 3 mirror cell screws. I have 3 cheshires, including the Catseye. Image from their site. Hope this makes more sense.

😀 My real world, unscientific tests reveal that when I lower the mag on the 15", with binoviewers, down to about 73x on the moon this combination gives much more detailed views than the TSA120. These days I just lower the mag on the 15"- if seeing can't support this I go back in the house.

Nice scope! I would tend to think the LZOS APO triplet would like the prism, but don't know for sure.

What scope are you getting exactly?

When collimating we go back and forth between secondary and primary collimation? Try putting the primary dot in the cheshire ring, then go back to the sec, then back to primary etc etc. IMHO.

Whats interesting about my image of the mirror and cell is the nylon supports screwed all the way down and replaced with small dabs of silicone. Why was this done? The mirror is touching the clips, with no room to raise the mirror up on the nylon supports.. Of course this is just a guess on my part.

My 200mm f3.8 mirror and cell

I'm in the same boat... So far the easiest thing for me to use is an assembled truss dob with wheelbarrow handles. Maybe a short Mak or something might work but for me its using the tripod- I stand using the dob and its so user friendly, at least for now.

I'm not sure about this- it it in reference to imaging? If it were me I'd use a good mirror diag , pop a good eyepiece in the scope and enjoy the good views.

Yes, but... had I known the prism could be fussy I might not have bought it! Sure glad I did though. Through all this discussion I must say that I have a cheap SW mirror diag that give vg views- its one downfall is scatter though. I had an expensive mirror diagonal that was a piece of junk years ago, so bad in fact I questioned my 90mm APO! In came the prism and voila- the 90mm gives wicked lunar/planetary views (for a small scope). Knowing what I know now and if I could only have one diag it would be a top mirror diag like the Baader BBHS.

When a lot of top refractor users talk about " this scope" has better color on Jupiter than "that scope" etc I think its down to the glasses used, f ratio etc and who knows what else. I do think a prism can also massage colours- but its a lot of money to gamble on. By all means try the prism! In my refractors the APO triplets like it- my one ED scope does not. This is not to say all ED doublets won't get along with it. Personally I'm not a gambler and I love known characteristics etc.

Actually no! lol!😀 As much as I like this prism I cannot recommend it for doublet ED scopes- its too much money to gamble on it working with a particular ED scope. The Baader BBHS is "guaranteed" to work with any refractor IMHO.