Tamron 135mm First Light - Getting Weird Shaped Stars. Any Ideas?

[Xiga]

Hey guys

I recently purchased a vintage Tamron 135mm F2.8 lens for next-to-nothing, and on Friday night i finally got a chance to give it a test run. However, it's producing strange comet-like stars, and i can't work out why, so maybe someone here can help me figure out if it's something i've done wrong, or if the lens is just a lemon! 

The test subject was IC 2177 (the Seagull Nebula). I only shot it for around an hour in total, and it was pretty low on the horizon too, which i know was not ideal. Temps were around Zero C, i was using a dew strip over the lens. Imaging scale with the D5300 is about 6", and i was guiding using my finger-guider (perhaps overkill, but it just seems wrong not to guide these days). Guiding accuracy was a little over 1" iirc. I was using my 2" mounted IDAS-D1, which was screwed onto the end of the lens hood, and i then had some step-down rings to bring the lens aperture down to 37mm, which works out about F3.65. 

The issue is also present in short subs, so it's not a guiding issue. Here is a crop from a 20 sec calibrated sub which shows the issue (converted to Jpeg for upload):

And here's the same non-cropped sub (also jpeg):

Does anyone have any ideas?

Here's a link to a calibrated raw 20sec sub, just in case anyone wants to inspect it a bit more closely. 

IC_2177_20sec_ISO200_filter0_RGB_1_frame1-cal-NoSt.tiff

Thanks in advance guys! 

[Xiga]

FWIW, here is a quick 20 min process of the full stack, which was 17 subs of 200 secs, so a little under an hour in total.

No star reduction used, but i did i did make use of the Chromatic Abberation filter in PS:

 

[tooth_dr]

Ciaran, that is a bit disappointing.  It does look like CA.

[Uranium235]

The odd star shapes seem be down to the optics. You've described the lens as vintage which probably gives you a clue as to why (probably wont contain ED glass or have modern coatings). You could try stopping it down a bit further, trying some narrowband - or just cropping down to the FOV that looks the best. 

[davew]

If it's one of the old " Adaptall " lenses then I think it's the nature of the beast. CA reduction in either PS or lightroom is an option but it won't cure the rest of the flaring.

For its age I think they were a reasonable lens. I had one  

Dave

[Xiga]

3 hours ago, davew said:

If it's one of the old " Adaptall " lenses then I think it's the nature of the beast. CA reduction in either PS or lightroom is an option but it won't cure the rest of the flaring.

For its age I think they were a reasonable lens. I had one  

Dave

You'right Dave, it's an Adaptall 2 lens. It's in great shape i have to say. I had hoped the CA would be a bit better, but it is what it is. You get what you pay for in this game that's for sure! ?

[Xiga]

5 hours ago, tooth_dr said:

Ciaran, that is a bit disappointing.  It does look like CA.

 

5 hours ago, Uranium235 said:

The odd star shapes seem be down to the optics. You've described the lens as vintage which probably gives you a clue as to why (probably wont contain ED glass or have modern coatings). You could try stopping it down a bit further, trying some narrowband - or just cropping down to the FOV that looks the best. 

Adam & Rob, thanks for confirming. I thought it had to be CA, but not having ever imaged wit a lens before, i wasn't 100% sure. 

I was expecting some CA, but I had hoped that with just a cropped sensor, i might be able to get away without the worst of it. But looking at the subs it doesn't look like cropping down is an option, as it doesn't appear to be evenly distributed from the centre. It seems much worse on the right side vs the left side for instance. 

I will give it one last go, with another step-down ring to stop it down a good bit further and see if things become acceptable. If not, i'll consign it to the dustbin and just save up for that Samyang 135mm F2. 

[Uranium235]

1 hour ago, Xiga said:

 

I will give it one last go, with another step-down ring to stop it down a good bit further and see if things become acceptable. If not, i'll consign it to the dustbin and just save up for that Samyang 135mm F2. 

Hmmm I wouldnt bin it per se. If you eventually get the Samyang, you could use the Tamron as a spare portrait lens.  You could even aattach an EP to a camera lens and use it as a monocular...lol   (if you have the right bits)

[happy-kat]

Please don't dustbin it I'm sure there are more suitable ways of disposal should you decide to not keep it.

One thing these older lenses have is smooth focusing they feel so much nicer.

[Xiga]

14 hours ago, Uranium235 said:

Hmmm I wouldnt bin it per se. If you eventually get the Samyang, you could use the Tamron as a spare portrait lens.  You could even aattach an EP to a camera lens and use it as a monocular...lol   (if you have the right bits)

 

4 hours ago, happy-kat said:

Please don't dustbin it I'm sure there are more suitable ways of disposal should you decide to not keep it.

One thing these older lenses have is smooth focusing they feel so much nicer.

Thanks guys.

I mis-spoke tbh. I definitely won't throw it out. It's actually not that bad of a lens, so it would be a total waste to bin such an item. If I can't find a use for it myself, i'll give it to a friend or family member who will give it a good home.

Although Rob after reading your post, I'm now itching to see what sort of view it gives if I hold up a 10mm eyepiece to it. Will definitely try this out next time I get the chance, so thanks! ?

[Xiga]

Well as luck had it, it was clear last night for a bit, so i made a last minute decision (thanks @tooth_dr for the push) to head out and take advantage of what little i could. I was in a hurry to try and get some subs of the Horsehead Nebula, but first i wanted one last try with the Tamron to see how it performed when stopped down further, and i'm pleasantly surprised i have to say. I think it might be useable after all. 

The images from further up were all taken with a 37mm step-down ring, which yielded about F3.65. The smallest step-down ring i had was a 30mm (~F4.5) but i wasn't convinced this would be enough, so i set about quickly trying to fashion a quick 24mm one myself. After butchering a DVD case with a 10p piece and a stanley knife, i managed to end up with an opening of around 24mm (~F5.6), except it broke in 3, so i glued it to the 30mm ring. See below for the hideousness in it's full glory ?

So as i was in a hurry, i decided not to bother guiding or even attaching the dew bands. I just quickly polar-aligned (via Eqmod) then fired off 16 x 20s subs. 

Early inspection looked good, much better, so i stacked the 16 F5.6 (unguided) subs and compared it to one of the single (guided) 200s F3.65 ones from before. The F5.6 subs are obviously dimmer, with less detail, so i had to give it a bit more of a stretch to make the comparison a bit more even.

Here is a gif which shows the difference between the two, from the far left hand edge of the frame (although in reality, every single portion of the entire frame shows a similar level of improvement):

Obviously, at F5.6 and just 24mm of clear aperture it's not exactly sucking down photons, lol, but at hopefully it now means i should be able to have some fun with it at least. The necessity for longer subs won't be a problem with the HEQ5. And on the plus side, i'd like to use it for shooting Ha (and even the odd Oiii) to mix in with the RGB. And for this purpose i'm hoping that i can use the lens fully wide open at 48mm, as it will be my intention to always remove the stars from the Ha anyway, so hopefully any CA from those frames won't actually matter.  Well, that's the plan at least!

[happy-kat]

That was a productive test and I love the homemade step down ring.

[tooth_dr]

Looking forward to the results Ciaran. 

[Xiga]

5 hours ago, happy-kat said:

That was a productive test and I love the homemade step down ring.

 

1 hour ago, tooth_dr said:

Looking forward to the results Ciaran. 

Thanks guys