Design or Optics quality?

[david_taurus83]

1 hour ago, vlaiv said:

It does eat into violet part of spectrum a bit - so might be part of explanation

To be fair, I wouldn't even consider imaging in broadband or L without some sort of IR/UV cut. The IDAS works well enough from my back garden but if I was to go to a very dark site I would still probably use my clear IR/UV cut filter as well.

[ollypenrice]

1 hour ago, TareqPhoto said:

Halo with L filter? i thought it will show a mono result no matter if there is a halo or not, and also you mentioned Astronomik L2 and L3 which i know about long time ago, or even consider LP filter that is more suppression than L.

Monochromatic means 'one colour.' A luminance filter is only monochromatic in the sense that it can only deliver an image in one colour. By default that would be greyscale but you could colour it red or blue or whatever you like. But you have no way of separating the colours in a luminance image. However, a luminance filter is not a monochromatic filter because it passes all colours, so it is, in fact, a polychromatic filter. Therefore it needs a fully colour corrected lens.

A narrowband filter is nearly monochromatic in that it passes a very narrow selection of colours (Ha passes a narrow range of deep reds, OIII passes a narrow range of colours on the blue-green border, etc.)

A badly corrected lens will have blue bloat in luminance and blue. It will not have the same bloat in red or green so the bloat will appear blue in LRGB. It will be worse in blue than L because all the light in blue will be bloated whereas in L the green and red will not bloat. But the L will show bloat for sure.

Olly

Edited January 27, 2021 by ollypenrice

[TareqPhoto]

2 minutes ago, ollypenrice said:

Monochromatic has two meanings, here. An L filter will produce a monochromatic result, meaning that the colours are not distinguished separately. By default that will be a greyscale image. However, an L filter is a polychromatic filter because it passes all visible wavelengths (colours) so it needs a lens which brings all wavelengths to the same focal plane. A narrowband filter passes only a limited range of wavelengths meaning that it is nearly monochromatic in the true sense. An Ha filter is really a highly selective deep red filter. An OIII filter is a highly selective filter on the green-blue border. So Ha/OIII etc are nearly monochromatic filters whereas a luminance filter is polychromatic.

Olly

Ok, that will bring us into same question of beginning, does that really matter a lot with a doublet or triplet? So if it matters means a triplet is the answer without asking about which optics in the scope, now i really don't know what all that show and bubble about FPL-53 if a triplet no matter what is the answer? DSLR lenses has plenty of optics inside so isn't that for color correction then?

I feel i don't know about a triplet vs. fpl-53, some will cry for a triplet and others will cry for FPL-53 even if it is a singlet, so i can't understand much this dilemma here, good that reflectors are almost free of CA so not worried here.

I still think vlaiv idea about having 60mm doublet and do things like reducing it and stopping it down is the best solution here, i will not stop thinking about a scope at 280mm-320mm for Ha and Lum filters, not DSLR lenses.

[michael.h.f.wilkinson]

1 hour ago, TareqPhoto said:

Is this reducer any good with any scope [refractor] if using only 4/3" sensor camera?

https://www.altairastro.com/lightwave-06x-reducer-289-p.asp

That is the one I have, and gave this result on the ASI183MC. There are some gradients due to problems with the flats, but the eggy stars ar clear to see in the corners. Spacing might be an issue, of course, but I had the sensor at the rated distance of 55 mm. Note that there was no issue with field rotation as polar alignment was pretty good

[TareqPhoto]

1 minute ago, michael.h.f.wilkinson said:

That is the one I have, and gave this result on the ASI183MC. There are some gradients due to problems with the flats, but the eggy stars ar clear to see in the corners. Spacing might be an issue, of course, but I had the sensor at the rated distance of 55 mm. Note that there was no issue with field rotation as polar alignment was pretty good

I have this eggy stars or even worse with my ST80 + 0.8x reducer using 55mm spacing, and i posted results and people told me that it is not necessary to have 55mm always even if it is mentioned there, so it could be spacing needing more fine tuning, who knows, i saw results from someone using 0.6x reducer, i think it was better stars shapes, but i can 't confirm if that is without cropping or which camera.

[michael.h.f.wilkinson]

6 minutes ago, TareqPhoto said:

Ok, that will bring us into same question of beginning, does that really matter a lot with a doublet or triplet? So if it matters means a triplet is the answer without asking about which optics in the scope, now i really don't know what all that show and bubble about FPL-53 if a triplet no matter what is the answer? DSLR lenses has plenty of optics inside so isn't that for color correction then?

I feel i don't know about a triplet vs. fpl-53, some will cry for a triplet and others will cry for FPL-53 even if it is a singlet, so i can't understand much this dilemma here, good that reflectors are almost free of CA so not worried here.

I still think vlaiv idea about having 60mm doublet and do things like reducing it and stopping it down is the best solution here, i will not stop thinking about a scope at 280mm-320mm for Ha and Lum filters, not DSLR lenses.

Pure reflectors are completely free of CA, not almost. They may still suffer from monochromatic aberrations such as spherical aberration, coma and astigmatism. Catadioptric scopes like my Meade 6" F/5 Schmidt-Newton are practically free of CA, their corrector plates or meniscus lenses will correct for spherical aberration perfectly only at one wavelength, so a residual "sphero-chromaticity" may occur.

[vlaiv]

5 minutes ago, TareqPhoto said:

I feel i don't know about a triplet vs. fpl-53, some will cry for a triplet and others will cry for FPL-53 even if it is a singlet, so i can't understand much this dilemma here, good that reflectors are almost free of CA so not worried here.

It really depends on context.

If we are talking about planetary sharpness in refractors then I'll say that Takahashi is probably justified, but that kind of sharpness is useless in context of imaging and you'll be hard pressed to tell the difference between Strehl 0.8 and Strehl 0.98 scope in long exposure images because there seeing and mount dominate.

If we are talking about F/7 doublet scope - then yes, if you want to have visually CA free image - you need FPL-53 as for example FPL-51 simply won't be able to do it at that focal ratio. I would not mind having FPL-51 F/7 doublet for visual as I'm not that much sensitive to CA for visual - but that is different from imaging.

If we are talking about color correction - then yes, triplet over doublet. There are very good doublets - but with triplet good color correction is much easier to achieve. Again - there are exceptions and I'm certain that there are triplet scopes that don't have good enough color correction even if they are triplets.

9 minutes ago, TareqPhoto said:

I still think vlaiv idea about having 60mm doublet and do things like reducing it and stopping it down is the best solution here, i will not stop thinking about a scope at 280mm-320mm for Ha and Lum filters, not DSLR lenses.

That is probably most sensible option for that focal length - and you might even find that you don't need to stop down that scope - that it is sharp enough for you as is.

[michael.h.f.wilkinson]

3 minutes ago, TareqPhoto said:

I have this eggy stars or even worse with my ST80 + 0.8x reducer using 55mm spacing, and i posted results and people told me that it is not necessary to have 55mm always even if it is mentioned there, so it could be spacing needing more fine tuning, who knows, i saw results from someone using 0.6x reducer, i think it was better stars shapes, but i can 't confirm if that is without cropping or which camera.

55 mm is the starting point. I will have to get spacers to see what improvements I can make. It would be nice if I could work at 288 mm F/3.6, on the ASI183MC at least.

[TareqPhoto]

4 minutes ago, vlaiv said:

It really depends on context.

If we are talking about planetary sharpness in refractors then I'll say that Takahashi is probably justified, but that kind of sharpness is useless in context of imaging and you'll be hard pressed to tell the difference between Strehl 0.8 and Strehl 0.98 scope in long exposure images because there seeing and mount dominate.

If we are talking about F/7 doublet scope - then yes, if you want to have visually CA free image - you need FPL-53 as for example FPL-51 simply won't be able to do it at that focal ratio. I would not mind having FPL-51 F/7 doublet for visual as I'm not that much sensitive to CA for visual - but that is different from imaging.

If we are talking about color correction - then yes, triplet over doublet. There are very good doublets - but with triplet good color correction is much easier to achieve. Again - there are exceptions and I'm certain that there are triplet scopes that don't have good enough color correction even if they are triplets.

That is probably most sensible option for that focal length - and you might even find that you don't need to stop down that scope - that it is sharp enough for you as is.

Got it for the upper part.

I think i will be happy with any ED or semi APO or APO scope, if i used this ST80 for years mainly narrowbanding then upgrading to ED will be an improvement, and if that doublet is FPL-53 then that might help slightly, but you don't know much about that 61mm triplet i posted, so maybe i better look at that more around, at least it is a triplet after all, and previous one a doublet they mentioned it was good reviews by many, so the new triplet one should do better, they definitely won't make it worse anyway.

Also Askar option is very tempting, those new scopes are getting around, more people will have hands on them, i have time to decide until a budget will come, hopefully by that time i can be ready with a decision, it is just the more i wait, the more doubts and questions i will have.

[TareqPhoto]

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

55 mm is the starting point. I will have to get spacers to see what improvements I can make. It would be nice if I could work at 288 mm F/3.6, on the ASI183MC at least.

And it will be nicer to work at 288mm F/3.6 on ASI1600 / QHY163M / QHY294M, and even more nice if i can use that reducer with my 90mm F/6 triplet to give me 324mm F/3.6 on future plan of ASI2600MC [APS-C].

[michael.h.f.wilkinson]

 

6 minutes ago, TareqPhoto said:

And it will be nicer to work at 288mm F/3.6 on ASI1600 / QHY163M / QHY294M, and even more nice if i can use that reducer with my 90mm F/6 triplet to give me 324mm F/3.6 on future plan of ASI2600MC [APS-C].

I have reverted back to my 0.8x reducer, which gives better results, both on ASI183MC

and on the EOS 550D

Clear skies are so rare at the moment I don't want to waste time experimenting with spacing. If the weather turns for the better I might experiment a bit. I might put the ASI183MC on the Meade 6" F/5, and once that is clicking away, set up the little APM 80mm with the 0.6x reducer and a bunch of spacing rings with the EOS 550D.

[TareqPhoto]

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

That is the one I have, and gave this result on the ASI183MC. There are some gradients due to problems with the flats, but the eggy stars ar clear to see in the corners. Spacing might be an issue, of course, but I had the sensor at the rated distance of 55 mm. Note that there was no issue with field rotation as polar alignment was pretty good

Here is a result from someone used 0.6x reducer with his ES triplet, what do you think?

https://www.astrobin.com/full/tiartq/0/

[michael.h.f.wilkinson]

2 minutes ago, TareqPhoto said:

Here is a result from someone used 0.6x reducer with his ES triplet, what do you think?

https://www.astrobin.com/full/tiartq/0/

Difficult to say. That image has been processed quite heavily, I would guess from the appearance of the stars that some star size reduction filter has been applied. I do seem to see a residual elongation  that almost suggests field rotation, like my shot, which is more-or-less a stretched stack without any noise reduction or filtering

[vlaiv]

4 minutes ago, TareqPhoto said:

Here is a result from someone used 0.6x reducer with his ES triplet, what do you think?

https://www.astrobin.com/full/tiartq/0/

That was taken with Atik314L+ which is 11mm diagonal sensor - even smaller than ASI183

[TareqPhoto]

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

I have reverted back to my 0.8x reducer, which gives better results. Clear skies are so rare at the moment I don't want to waste time experimenting with spacing. If the weather turns for the better I might experiment a bit. I might put the ASI183MC on the Meade 6" F/5, and once that is clicking away, set up the little APM 80mm with the 0.6x reducer and a bunch of spacing rings with the EOS 550D.

Not good result, but just to see about the stars in the corners, sounds not bad, because the stars in the entire frame are weird anyway, so it corrected to have same shape, bad, not sure why, but some might like it, and maybe it can be better with different scopes

https://www.astrobin.com/full/o6c55t/0/?real=&mod=

 

We have many clear nights here, so i can experiment easily many times.