Excessive Star Glow

[brenski]

This is just a quick question on focusing.
Equipment: Super Takumar 300mm, Altair 183c (Cooled) & Altair Hypercam Lens Adapter
The Sharpcap captures are from upper-centre frame, 30-sec exposures. The bahtinov pattern appeared spot-on. However, all of the stars in the image have a bright centre and a wider radiant glow - it's more evident on the less-bright stars.
Is this a focus issue? Any help appreciated.

3x images 1.jpg (888.31 KiB) Viewed 7618 times

eosclah.jpg (38.02 KiB) Viewed 7618 times

[oopfan]

I'll hazard a guess:

1. Moisture.
2. Chromatic aberration. If possible try shooting through different colored filters to see if it mitigates the effect. I have a 400mm refractor that can't focus Blue to save its life. Ended up having to shoot with a Wratten #12 (minus Blue) for luminance.

If it's #2 try to get your money back.

Brian

[admin]

Hi,

Both of Brian's possibilities are sensible and worth investigating.

It also looks like the glow is slightly off-center too with respect to the star. If this direction varies across the frame then probably some sort of optical aberration.

cheers,

Robin

[TimHaymes]

Old lenses will have these aberrations. Its accentuated by high(er) resolution CCDs. You could stop down, and this should sharpen up the image.
I see the same problem on Canon-L lenses at full aperture on a 60Da, since they are not corrected for the IR or deep red (e,g Ha). So I take extra care with live-view focus. It should be better in RGB fiters, as suggested (or narrow band).

You may not be able to stop down if the lens connections are disconnected from the camera. This is a difficulty ! A manual iris lens would be nice or an ASCOM adapter that will control the lens settings (e.g. iris) by USB.

Just some thoughts - Tim

[brenski]

thanks Tim

The Tak 300 works manually with my Canon - and I have the Man/Auto switch set to "Man" so adjusting the Aperture settings does appear to work when attached to the Altair camera.

My Altair 183c has a Built-in UV/IR filter - so would that mean that no additional filter req'd?


From inspecting the stacked tif file, I've little more information:
The direction of "glow" appears relative to star position.

Star Position
Centre Frame = Glow radiates out equally
Top Left = Glow radiates toward 10 O'clock
Bottom Left = Glow radiates toward 8 O'clock
Top Right = Glow radiates toward 2 O'clock
Bottom Right = Glow radiates toward 4 O'clock

[Menno555]

I don't know if this is possible with this kind of lens, but it looks like a back focus issue?
The way you describe it, it looks like the lens is a bit too close to the camera sensor.

Menno

[brenski]

I don't know if this is possible with this kind of lens, but it looks like a back focus issue?
The way you describe it, it looks like the lens is a bit too close to the camera sensor.

Menno

i not sure about that. As i said in my top post:
The bahtinov pattern appeared spot-on.
it can't be a focus issue if bahtinov pattern is correct, no?

[Menno555]

The backfous distance is something else than focus.
It's the distance between scope/lens and the camera sensor where best overall focus can be reached (please someone correct me if I describe it wrong/weird ).
So, your lens can be too far or too close to the camera sensor but the Bathinov can still be perfect. But it's easy to check: search for a piece of sky where in your FOV are multiple bright(er) stars. When using the Bahtinov, use a star in the center to get a perfect Bahtinov pattern.
When that is done, check if the stars near the edges of your FOV are also in a good or almost good Bahtinov pattern.
If there is a backfocus issue, those patterns will differ a lot though.
Or you can use the Bahtinov and then make test captures and see if you see deformations like in the image below where you also can see how to move your lens.

Menno

[turfpit]

brenski

I have a couple of Super-Takumar lenses and have managed to get them working with a Canon 7d Mk2 and an Altair 183C (fan cooled).

See viewtopic.php?p=35432#p35432 post #42.

The optical train for the above is:
Super-Takumar 135mm f3.5 M42 lens + M42 to EOS adapter + Canon 7D Mk2.
The adapter is 1mm thick, Canon sensor to front flange is 44mm

I have the 183C working but only tested against a distant land based object (looking for a mounting bracket) which achieved focus to infinity.
The optical train is:
Super-Takumar 135mm f3.5 M42 lens + M42 to T adapter (10mm) + T spacer (20mm) + Altair 183C (fan cooled)

The Takumar has a back focus of 45.46mm. The Altair 183C fan cooled has a 12.5mm sensor spacing.

The Canon spacing is 1mm + 44mm = 45mm
The Altair spacing is 10mm + 20mm +12.5mm = 42.5 leaves me 2mm out which might matter for astronomical infinity. I have some Delrin spacer rings with which I can tune the 2mm shortfall https://www.firstlightoptics.com/adapte ... g-set.html.

As far as I can tell from the web, the Takumar 300mm has an M42 connector at the rear.

Looking at https://www.altairastro.com/dslr-adapters-54-c.asp tells me there are 2 DSLR adapters for the Hypercam - Canon EOS and Nikon.
The Canon (or Nikon) offer 2 adapters - fan cooled with 12.5mm sensor spacing or TEC cooled with 17.5mm sensor spacing.

3 questions to consider:

• Which DSLR adapter was purchased?

• What is the spacing of the model purchased?

• How is the Tacumar connected to the DSLR adapter?

According to the above figures your TEC camera has a 17.5mm sensor spacing.
That leaves 45.46mm - 17.5mm = 27.96mm.

That means ~28mm from the back of the lens to the front of the camera to be made up by the DSLR adapter plus any other adapters you need.

You mentioned you had the lens working with a Canon which implies you have an M42 to EOS adapter and a Canon EOS DSLR adapter.

I found this https://stargazerslounge.com/topic/4145 ... mar-300mm/ for a Takumar 300mm to Altair 183C (fan cooled).
Looking around the web a comment that stands out about these lenses is that some can suffer from chromatic aberration for which the fix seems to be stop down the lens by 1 stop. That would be 3.5 -> 5.6 for my lens.

You might want to check your additional star information against the diagram that Menno posted, your info seems to fit the picture "camera sensor too close"

XXX.JPG (85.28 KiB) Viewed 7508 times

Hopefully you have enough information to resolve this now.

Dave

[brenski]

thanks @menno & @Turpfit

My Tak> Altair chain is

Tak 300mm > Ef/M42 ring > EF/Altair adapter (see top post) > Altair 183C

thanks for the advice, will stop down from F/4 to F/5.6 and try again