Is the (main) problem with the stars in this image the focus?

[happy-kat]

If you split an image into the three separate colour channels, are the green and blue channels focused and good stars? Or are they all off.

Not sure if that will help much but it was a curious thought.

[LowGravitas]

3 hours ago, happy-kat said:

If you split an image into the three separate colour channels, are the green and blue channels focused and good stars? Or are they all off.

Not sure if that will help much but it was a curious thought.

Here's the blue channel from one of the subs:

https://imgur.com/a/AxvNJDq

And the green:

https://imgur.com/a/36WQW33

Stars look the same, I'd say.

[happy-kat]

Could you add them as jpg please, or instead are the separate channels all ok with good stars?

[LowGravitas]

1 minute ago, happy-kat said:

Could you add them as jpg please, or instead are the separate channels all ok with good stars?

Green and blue look bad. Red is the same.

[happy-kat]

I hadn't expected green to look bad, blue perhaps a little bit though these lenses tend to show CA on the red channel

 

[LowGravitas]

Yeah, it seems to be interacting poorly with all wavelengths.

[happy-kat]

To clarify is your camera full spectrum modified?

My understanding is if it is there needs to be a UV/IR filter otherwise stars bloat. Whether the bloating shows on all stars or just the bigger ones I'm not sure.

[LowGravitas]

Just now, happy-kat said:

To clarify is your camera full spectrum modified?

My understanding is if it is there needs to be a UV/IR filter otherwise stars bloat. Whether the bloating shows on all stars or just the bigger ones I'm not sure.

No, it's only had it's hot mirror removed. It still has its LP1 filter, which blocks UV/IR.

[Knight of Clear Skies]

Could be that the lens simply isn't reaching focus with the M42 adaptor, my modded 1100D had the sensor re-shimmed to ensure infinity focus with camera lenses. Another modded camera I've used will reach focus with some lenses but not others, it's fine with modern auto-focus lenses and telescopes as they will go past infinity but not all combinations of M42 lens and adaptor will work with it. Some adaptors are thicker than others and move the lens further from the body, a thinner one may work. Notice that the nebulosity isn't in focus.

Alternatively it could be some kind of internal reflection in the lens, possibly a problem with the coatings. There were applied to the lens 50 odd years ago and who knows what's been done to them since (an overzealous cleaner might have worn them away).

I'd recommend trying the camera with an auto-focus lens to confirm there isn't a problem there. You could also try the Takumar on a distant target in daylight, or on the Moon.

Hope that's some help, in its current form at least it gives quite a nice artistic effect.

[alacant]

On 25/10/2020 at 14:42, LowGravitas said:

Anyone know if that's right?

Hi

- Set the lens against the infinity stop and loosen the 3 grub screws holding the focus grip to the helicoid. Twist the grip so that after re-tightening the screws, you can now focus beyond infinity.

- The Bhatinov will give you focus on the red. The trick is to 'infinity' focus on a bright star until red fringing is seen. Now twist way from infinity so that the red just disappears. If it starts to go blue, you've gone too far. That's the best compromise I get with the 200/4 @ f 5.6. If you have time, f8 and smaller will allow you to focus 'normally' at infinity, but then it's painfully slow.

- fit an ir cut filter.

HTH.

Edited October 28, 2020 by alacant

[alacant]

On 25/10/2020 at 14:42, LowGravitas said:

 

...

Edited October 28, 2020 by alacant

[LowGravitas]

4 hours ago, Knight of Clear Skies said:

Alternatively it could be some kind of internal reflection in the lens, possibly a problem with the coatings. There were applied to the lens 50 odd years ago and who knows what's been done to them since (an overzealous cleaner might have worn them away).

I'd recommend trying the camera with an auto-focus lens to confirm there isn't a problem there. You could also try the Takumar on a distant target in daylight, or on the Moon.

The camera does seem to be fine with my cheap 50mm.

I got this photo with the Takumar before I tried it for astro. Not a great photo, but no obvious problems. Nonetheless, a problem with internal reflection sounds plausible. Will have to get another, if true. At least they're cheap.

[LowGravitas]

3 hours ago, alacant said:

Hi

- Set the lens against the infinity stop and loosen the 3 grub screws holding the focus grip to the helicoid. Twist the grip so that after re-tightening the screws, you can now focus beyond infinity.

- The Bhatinov will give you focus on the red. The trick is to 'infinity' focus on a bright star until red fringing is seen. Now twist way from infinity so that the red just disappears. If it starts to go blue, you've gone too far. That's the best compromise I get with the 200/4 @ f 5.6. If you have time, f8 and smaller will allow you to focus 'normally' at infinity, but then it's painfully slow.

- fit an ir cut filter.

HTH.

Thanks. I'll try this.

[happy-kat]

I would use the lens with a lens shield, helps with dew but also prevent random surface light hitting. Tube of rolled card from the brown side of a cereal box held with rubber bands would do if you don't have something none reflective black.

[Knight of Clear Skies]

1 hour ago, LowGravitas said:

Not a great photo, but no obvious problems.

Decent enough image but it's not focused on infinity, so doesn't help with the diagnostics I'm afraid. A quick snap of the Moon might be easiest way to test it.

[LowGravitas]

9 minutes ago, Knight of Clear Skies said:

Decent enough image but it's not focused on infinity, so doesn't help with the diagnostics I'm afraid. A quick snap of the Moon might be easiest way to test it.

Yeah. Will do so when the clouds next clear.

[John78]

Unfortunately in my experience (2x200mm F4, 1x135mm F2.5) these SMC lenses are just no good for astro - even though some people appear to get good results with them.

To reach proper focus you need to do the 3 screw trick, you need the thin version of M42 adapter, you need to remove the pin on the lens that pushes against the adapter.

Once you get to proper focus, without tilt in my experience; you get the most heinous star bloat - even with a Halpha filter in the image train its just provides totally unusable data using a Canon EOS body astro modded or my cooled mono CCD.

[John78]

[alacant]

1 hour ago, John78 said:

no good for astro

Really? Maybe you missed focus a bit? Faulty examples? Dunno... 

Granted, with their tiny apertures, they are very (read: painfully!) slow, but if you've a few hours (read: the whole night!) to spare, we find that they really are capable of fine results. This is an example of an area in Auriga taken with a €50 Super Takumar 200/4, and this is the same region with the even cheaper Takumar 135/3.5.

No need to remove the lens pins; choose the adapter without the flange.

Cheers and HTH.

Edited October 31, 2020 by alacant

[John78]

55 minutes ago, alacant said:

Really? Maybe you missed focus a bit? Faulty examples? Dunno... 

Granted, with their tiny apertures, they are very (read: painfully!) slow, but if you've a few hours (read: the whole night!) to spare, we find that they really are capable of fine results. This is an example of an area in Auriga taken with a €50 Super Takumar 200/4, and this is the same region with the even cheaper Takumar 135/3.5.

No need to remove the lens pins; choose the adapter without the flange.

Cheers and HTH.

Well, like I said my experience with my first 200mm was so poor I bought another and that was, if anything, worse.  I even built an ollypenrice type micro-focuser basically a 3D printed lever clamped around the focuser barrel which was some 150mm long with an M3 rod and a thumbwheel, then subsequently a fully motorised micro-focuser using HFR focusing.

Your images look like they are stopped down, these lenses aren't slow compared to telescopes - they're quite fast esp the 135mm F2.5 is in the very fast range, I will concede I found them to be somewhat better performing stopped down using a step down filter adapter (I find the aperture blades detract from the images) - but then its slow, which defeats the point, and in all honesty still nowhere near as good as the benchmark lens for this type of imaging: the Samyang 135mm F2, its flat across the field, fast and free from aberrations even wide open.

There are many more threads across the internet in general with people with performance problems with these lenses than success', and once I'd failed to see a noticeable performance improvement using a halpha filter I gave up and these lenses wont see the light of night again for imaging.

[Alien 13]

The original image is in fact perfectly focused as seen by the tiny faint stars in the corners, the big issue is that modified cameras are not good with camera lenses or any no full APO refractors... not sure I agree with a previous statement regarding camera lenses being painfully slow due to their smaller apertures (wouldn't call 70mm ish or larger with some lenses small though.) as the f ratio rule/myth works in these examples.

Alan

[Merlin66]

John78,

Did you try fitting a UV-IR filter to the lenses?

A good UV filter should suppress most of the "blue bloating"

 

[alacant]

12 hours ago, Alien 13 said:

camera lenses being painfully slow

Try doing a 5 minute exposure with a 24mm (millimeter!) telescope; e.g. the Takumar 135 @ f5.6. There's not a lot of light gathering capacity, believe me!

Cheers