Stars present as bars aligned with DSLR

[Ratlet]

I'm using a Tair 3 and stock canon t3i for imaging mounted on a GEM 28.  One thing I've noticed is that whenever I image my stars have a tendancy to show a horizontal smear to them.  I'm certain it's not guiding as it appears to be aligned with the sensor of the camera.  I've attached a snip from an autostretch from siril.  The tair has the ability to rotate the lens on its mount and this was done with the sensor and lens vertical relative to the mount but I've seen similar in my images on M31 here the lens was horizontal relative to the mount.

The lens was stopped down to about F5.6 in this case, but was broadly similar when fully open.

Is this just because of the correction of the glass?  It seems odd that the effect is worse aligned with the sensor.  I've attached a stack and autostretched image as well.  This is only about 21 minutes of data.  I'm not really attempting to produce images yet, just working my way through nina, guiding, etc to figure everything out so any usable data is a bonus.  Crescent nebula was selected because it's a dense starfield and a bit of nebulosity (quite chuffed there is anything as the dslr is stock)

 

[Ratlet]

On second thoughts it's fairly even across the whole image and there is a less severe vertical component.  To me this does suggest guiding as the issue.

I'm wondering if it could be because I'm using a hotshoe mount for the guide camera and it could be a small amount of flex.

[happy-kat]

I think @alacant has the same lens

[alacant]

3 hours ago, Ratlet said:

I'm using a hotshoe mount for the guide camera and it could be a small amount of flex.

Hi

Mmm. Hotshoe mounted guide camera so, as you can see,  a large amount of flex.

Try something like this.

HTH

[Ratlet]

I didn't want to bother alacant 🤣.  The guru of the tair and the 130pds.

I checked the hot shoe bracket after the post and it had worked itself loose through the evening.  I'm going to try and jury rig it for this evening but I'll sort out a more permanent solution in the morning.

[happy-kat]

Sorry, good luck in the retest later

[Ratlet]

23 minutes ago, happy-kat said:

Sorry, good luck in the retest later

I'm grateful for you summoning them.  Alacant is usually correct. 

[Ratlet]

Did a test on M33.  Got sewed out after and hour but focused as best as I could.

When focusing I used live view without guiding on focused on deneb without a bahtinov mask.

As I moved the lens through focus the stars went from having a horizontal bloat to a vertical bloat.  Odd.  I will need to try stopping down the lens some more next time

[alacant]

1 hour ago, Ratlet said:

having a horizontal bloat to a vertical bloat

Hi

You were unlucky as your Tair has astigmatism. As related previously, this is probably just a case of the front element being distorted by too tight a retaining ring. Loosen, then in a vertical posición, shake to re-seat the glass. Next, re-tighten until the glass just stops rattling when agitated. With this adjustment, and  your 72 to 58 step down ring, it should all but disappear at focus. If still intrusive, try 72 to 55; still affording a respectable f5.5.

Otherwise and as Tairs are quite common, they rarely reach their reserve price at (on-line) auction, so another alternative would be to just get another:)

Cheers and HTH

[Ratlet]

2 minutes ago, alacant said:

Hi

You were unlucky as your Tair has astigmatism. As related previously, this is probably just a case of the front element being distorted by too tight a retaining ring. Loosen, then in a vertical posición, shake to re-seat the glass. Next, re-tighten until the glass just stops rattling when agitated. With this adjustment, and  your 72 to 58 step down ring, it should all but disappear at focus. If still intrusive, try 72 to 55; still affording a respectable f5.5.

Otherwise and as Tairs are quite common, they rarely reach their reserve price at (on-line) auction, so another alternative would be to just get another:)

Cheers and HTH

For £40 I am learning a lot about optics so it is not a complete bust.  Fortunately we have lots of clouds so plenty of time to play with it.  I shall update when I have attempted to fix.

[Ratlet]

I am almost disappointed.  I thought it would be difficult and involve some serious work.  The ring with all the writing can be easily unscrewed by hand.  The retaining ring is below and having attempted to move it with a screwdriver, it is stuck harder than the amount of pressure I am happy to apply without the correct tools.

I have ordered a lens spanner which should be here tomorrow.

[alacant]

24 minutes ago, Ratlet said:

a lens spanner

Ah, OK. Yes, indeed. I should have mentioned.

Sorry. You need one of these. No idea in English.  Is this a lens spanner?

[Ratlet]

4 minutes ago, alacant said:

Ah, OK. Yes, indeed. I should have mentioned.

Sorry. You need one of these. No idea in English.  Is this a lens spanner?

Yes, I think lens wrench is more common.  I think there is probably a technical difference, but I have no idea.

I will say that I have no regrets buying the lens.  For the cost it is brilliant and gives me something to practice with and learn and even with the astigmatism and my shoddy focus it is making decent results.  Approximately 40 minutes of M33 last night at F5.

[Ratlet]

Alas the astigmatism remains.

I managed to get the ring to move and after a few tap and tests the astigmatism remains.  It does appear to have rotated somewhat though, probably due to the front element rotating.

What I have noticed is that there does appear to be an difference in the level of astigmatism.  When defocused with red halo you get a very marked oblong, however defocused with a blue halo is very nearly circular.  The colour fringes is also greatly reduced.  When focused you get the worst of both worlds and get both atigmatisms.

I think testing on stars, slightly out of focus with blue halo could be a winner.

On the plus side, I tested my takumar 200mm and it is much better.  Even fully open.

Attached crop of F5.6 from the tair 3 slightly defocused with blue halo

[alacant]

2 hours ago, Ratlet said:

out of focus with blue halo could be a winner.

Yes. That's it. Most old camera lenses benefit from this 'compromise' focusing. Your observations also explain why you can't use a focusing mask.

Focus until the red halo just disappears. With the emphasis on just. No need for blue halos.

If you want to fix it completely, you'll need a new front element and/or try adjusting the other two elements. Details of dismantling here; make certain that the seat for the front element AND its retaining ring are perfectly clean and even. Place the latter on a non-yeilding flat surface e.g. glass and make sure it sits perfectly flush.

But I think you'll be OK as you are. You could always stop to f6 if you still find it's annoying.

Cheers

Edited September 4, 2022 by alacant

[Ratlet]

Thank you for the help.

I think for now I will put it on the shelf and give the takumar a try for the next run.  I've marred a couple bits on the Tair 3 already and I don't fancy dismantling it as from what I've read it is not an easy task and I tend to be a bit rough when it comes to mechanics.