Looking for a Refractor

[The Wave Catcher]

Thanks Steve. I have not tried that yet. Will have to give it a go. I suppose the effect basically is to increase the focal length.

Yes, but focal ratio, not focal length. Another way to look at is that the outer portions of the lens have to do the most bending of light and therefore have the most chromatic error. By using only the center, sweet portion you get a much cleaner image. With my f/4.5 achromat (I know, horrors!), the improvement is dramatic at high powers, effectively an f/7.65. At low powers I don’t need the mask. I’ve taken some good planetary photos using a simple SV305 camera while using the mask.

[SkyHiker]

Good luck with the choice. I only have a 6" Achromat. It's fine for DSO's visually but not perfect for planets. I've been thinking of getting a Mak for planetary work. But I haven't pulled the trigger yet.

If you haven’t done so already, you may want to try an aperture mask. I use a 60 mm mask with my el cheapo 102 mm Bresser achromat refractor and it turns it from a wide field comet hunter into an “APO like” planetary scope for visual use. At high powers, the resolution with the mask seem greater than without because if all the colors don’t focus at the same point, things aren’t sharp either.

Thanks Steve. I have not tried that yet. Will have to give it a go. I suppose the effect basically is to increase the focal length.

Not at all. The focal length is a fixed attribute of the objective. Decreasing the aperture increases the F ratio just like a longer focal length does, which is more forgiving of imperfect optics. For planetary work the diameter is important because the resolution depends on it - more so for perfect optics than with imperfect achromatic optics. This is why you get the best planetary images with 14" scopes and higher. I presume that this is what you meant to say, I am just trying to be clear so as not to confuse novices who may believe that this is a trick to increase the focal length. Use a Barlow for that.

[helicon]

If you haven’t done so already, you may want to try an aperture mask. I use a 60 mm mask with my el cheapo 102 mm Bresser achromat refractor and it turns it from a wide field comet hunter into an “APO like” planetary scope for visual use. At high powers, the resolution with the mask seem greater than without because if all the colors don’t focus at the same point, things aren’t sharp either.

Thanks Steve. I have not tried that yet. Will have to give it a go. I suppose the effect basically is to increase the focal length.

Not at all. The focal length is a fixed attribute of the objective. Decreasing the aperture increases the F ratio just like a longer focal length does, which is more forgiving of imperfect optics. For planetary work the diameter is important because the resolution depends on it - more so for perfect optics than with imperfect achromatic optics. This is why you get the best planetary images with 14" scopes and higher. I presume that this is what you meant to say, I am just trying to be clear so as not to confuse novices who may believe that this is a trick to increase the focal length. Use a Barlow for that.

Gotcha.

[Lady Fraktor]

Any set of lenses placed together produces colour blur.
When you use a aperture mask you are comparatively extending the focal length but what you are actually seeing is a reduction of the colour blur through the smaller aperture.
The light cone is still at its native length, you are just looking through a smaller area of it.

That is a bit of a poor example and there are formula to figure out how much blur a objective produces compared to another. (refractive index)
It affects resolution and contrast though and focal length affects the amount produced as well.
It has been a long day but I can write something up later if anyone is interested.

[Bigzmey]

If you haven’t done so already, you may want to try an aperture mask. I use a 60 mm mask with my el cheapo 102 mm Bresser achromat refractor and it turns it from a wide field comet hunter into an “APO like” planetary scope for visual use. At high powers, the resolution with the mask seem greater than without because if all the colors don’t focus at the same point, things aren’t sharp either.

Thanks Steve. I have not tried that yet. Will have to give it a go. I suppose the effect basically is to increase the focal length.

Not at all. The focal length is a fixed attribute of the objective. Decreasing the aperture increases the F ratio just like a longer focal length does, which is more forgiving of imperfect optics. For planetary work the diameter is important because the resolution depends on it - more so for perfect optics than with imperfect achromatic optics. This is why you get the best planetary images with 14" scopes and higher. I presume that this is what you meant to say, I am just trying to be clear so as not to confuse novices who may believe that this is a trick to increase the focal length. Use a Barlow for that.

Barlow increases focal length, but it does not address CA and other aberrations which fast achro optics has. Stopping down aperture reduces CA and cleans the image. It does reduce resolution, but to take an advantage of large aperture on planets you need a night of perfect seeing which are far in between.

[planecrazzzy]

You guys are making me Dizzy
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But dizzy is fine

[Lady Fraktor]

You guys are making me Dizzy
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But dizzy is fine

We are just getting started!

Read about apodizing masks, they work the same as using a aperture mask and will probably explain what is happening easier.
I use a apodizing mask with my 150mm f/8 for planetary and doubles.

[The Wave Catcher]

Read about apodizing masks, they work the same as using a aperture mask and will probably explain what is happening easier.
I use a apodizing mask with my 150mm f/8 for planetary and doubles.

I learned a new word today, “apodizing”. It makes perfect sense!

[SkyHiker]

Dang I stopped my 20x80s down with perforated cardboard. Had I known the word "apodizing" I could have sold them for $300 a pop I bet. Just imagine, apodized skymasters!

[Lady Fraktor]

I really wonder about you men at times
https://csastro.org/a-do-it-yourself-apodizing-mask/

[planecrazzzy]

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No Doubt , She's good...
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Gotta Fly...
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[planecrazzzy]

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It's in the delivery "Window"... but didn't make it before the weekend.
Probably Monday.
https://optcorp.com/products/explore-sc ... gKtNvD_BwE
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Gotta Fly...
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[planecrazzzy]

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My NEXT refactor will be an 80mm,
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That puts me in low to medium range...

Both should grab and go easy...

That would be 4 scopes ( 5 really , but we won't count the 90mm spotting scope )
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Both should be good for Visual and AP
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This was a pretty interesting video...tech at times...stuff I'll know more about later.

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This was another one about the ES 102mm.

[Lady Fraktor]

I am not sure what you mean by low to medium range?

[planecrazzzy]

Focal and FOV...?
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Gotta Fly...

[Lady Fraktor]

You do not know either?

Focal ratios of f/10 and longer are slow telescopes, f/7-f/9 are mid range, f/5- f/6.5 are fast, anything below f/5 is very fast.
Slow telescopes have a narrower view compared to faster telescopes.
FOV is determined by okular field stop or camera sensor size.

[planecrazzzy]

Still haven't re ordered the 102mm yet.
It's close to the holidays... waiting for sales...
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Found one that has 20% off ( $239 ? ) but an oddball company...

B&H will price match... So they said.
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They gave me some code that wouldn't enter.... to find out how much discount...
So I called , took a while to find it... their best try was $100 bucks off.
i'll wait more.

kind of disappointed they didn't match the price

[JayTee]

It does reduce resolution, but to take an advantage of large aperture on planets you need a night of perfect seeing which are far in between.

Not really. Good seeing yes, perfect seeing, not really. This is the whole point of "lucky imaging", it does not require perfect seeing.

[Bigzmey]

It does reduce resolution, but to take an advantage of large aperture on planets you need a night of perfect seeing which are far in between.

Not really. Good seeing yes, perfect seeing, not really. This is the whole point of "lucky imaging", it does not require perfect seeing.

I guess good vs perfect is relative, depending on your local conditions. At my desert location I can hardly ever push any scope productively above 150x and at that power larger aperture (9.25" vs 4" in my case) just adds more glare. Also, I am talking about visual, imaging could be different.

[JayTee]

Also, I am talking about visual, imaging could be different.

You and I are in complete agreement. Planetary / Lunar AP does not require the same level of good/excellent seeing that visual observing does.