Bahtinov mask different orientation / different focus!

[Robculm]

Quick question / wondering if anyone else has had this problem...

If I set focus with the mask rotation giving a 'horizontal' pattern, but then rotate the mask so the pattern is vertical, I'm then slightly off (i.e. no longer passing exactly through the center).

At the moment I'm basically just fixing focus somewhere in between, so it's closest to central for both orientations. We're not talking a huge difference, but it's definitely noticeable & it's always nice to know you're exactly focussed before spending hours imaging...!

Wonder if maybe it's a problem with the mask? I'm using the 200mm versions of this one: https://www.ebay.co.uk/itm/373432397287

Cheers,

Rob

[LaurenceT]

I also bought one from there and my experience is similar.

[markse68]

Could indicate your collimation is a bit off and the focal plane is slightly tilted I think? You can use a tri-Bahtinov mask for checking collimation:

https://www.cloudynights.com/topic/536410-a-tri-bahtinov-mask-for-sct-collimation-and-focusing/

Mark

[Robculm]

Thanks Mark, that's a very good suggestion. 👍

Will check the collimation & explore this while the moon is back but the nights are clear! (after I've recovered from the 3 sleep deprived nights of trying to image M16 this week!).

I find that collimation is somewhat underrated! There are lots of youtube vids which show how fast you can collimate a Newtonian, but rarely do they highlight the importance of getting the secondary mirror aligned! If you're not careful you can easily end up with great alignment of the primary to a badly aligned secondary! 😖

Thanks again,

Rob 

Edited July 17, 2021 by Robculm

[Robculm]

Finally got a few hours of clear skies to experiment on this a little & not convinced it's the collimation. I've spent some time checking the collimation. Stars are all of equivalent focus across the field of view & this 'effect' of a difference between 0 deg & 90deg of the bahtinov mask is evident when positioning the star I'm focussing on in any position in that field of view...

So maybe it is indeed the mask that's a bit crap!

Does anyone have any recommendation for good (but not stupidly expensive) bahtinov masks available in the UK?

There are certainly some interesting ones in the link (like 'gold astro'), but can't see those available in the UK...

Cheers,

Rob

Edited August 7, 2021 by Robculm

[bobro]

A small amount of mirror astigmatism perhaps? Tight mirror clamps can cause mirror astigmatism.

[Robculm]

Coming back to this topic & looking for some advice please.

So I bought a different mask, RVO's 'advanced' mask which gives me a kind of 6 axis pattern, but I still see the problem, different orientations / axis give different focus!

Here's what I see with the standard Bahtiniv (easier to see from the standard mask):

I've spent quite some time going through the collimation. Bought & used a 'long' Cheshire, rather than my usual laser collimator. Secondary nice & circular filling the eyepiece & all circles / dots aligned etc.

Managed to get a few test sessions in past few days (nice clear skies of course now that the moon is up!). Still seeing this problem & I'm stuggling to get really accurate focus in my images.

So I tried 'defocussing' on a star to look at the ring patters:

 

The circles look reasonably concentric to me. They aren't as clear in the photo as they were visually! Sorry.

I am a little confused about the two features (left side & top right corner). One of them seems to be the focus tube (top right I think), but not sure about the other!?

There's definitely something not quite right though. As I come out from the focus point, the star loses shape on one side:

 

And at focus point looks something like this:

 

Any thoughts / suggestions please (aside from buying a refractor!)?!

Cheers,

Rob

 

[markse68]

What do your flats look like stretched? It maybe that your secondary isn’t perfectly centred height wise? 

Mark

[Robculm]

Hi Mark, I don't have one since the latest collimation attempt, but here's how they usually look from my previous flats:

There's definitely a 'flat' on the bottom. Is that normal, or it should look more symmetrical?

Edited August 26, 2021 by Robculm

[markse68]

it should be a round hotspot in the middle of frame. The teardrop shape shows cone is hitting sensor at an angle i think. What is top and bottom here? (top of ota) This is a method that i’ve been playing with to try and perfect my own collimation/axis alignment. I think it’s a useful measure but yet to hear feedback when i’ve mentioned it before. So i may be wrong  Sounds sound to me though…

Mark

Edited August 26, 2021 by markse68

[alacant]

35 minutes ago, Robculm said:

'flat' on the bottom.

Hi

That's the shadow of the camera's mirror. 

The star changes shape because the focus tube is entering the light cone resulting in 'D' shapes instead of circles. Fit longer springs and screws to the primary to bring the focus position away from the tube.

If that doesn't cure it...

If you are certain that you're collimated then the next thing to try would be substitute the secondary mirror. An astro club is good (essential?) For this. 

After that, rotate the primary relative to the tube to see if that changes the mask pattern.

Cheers

Edited August 26, 2021 by alacant

[Clarkey]

1 hour ago, Robculm said:

I am a little confused about the two features (left side & top right corner). One of them seems to be the focus tube (top right I think), but not sure about the other!?

I would agree that one is the draw tube - the other is not so clear.

The picture of the defocused star seems to suggest the secondary may not be quite centred as there is an angle between the two vertical supports? Might be worth checking.

[ollypenrice]

The mask cannot, so far as I can see, give a variable result in a perfectly collimated beam arriving at a perfectly tilt-free chip. The problem must lie in the optics or the plane of the chip. Have you tried rotating the camera rather than the mask?

Olly

[Robculm]

Thanks for the replies / suggestions.

Rotating the camera is an easy one! Will give that a try this weekend. Will also 'poke around' and try to figure out where the other 'feature' that's not the draw tube is coming from.

Also plan to check again the collimation in terms of secondary positioning (centering), checking it's central in the vanes & I thought to check how central the primary is & check the centre spot really is in the centre!

Alacant, regarding your comment, I have definitely noticed the draw tube exteds quite some way in to the tube, becuse at prime focus it's almost fully 'in'. When you say extend the screws / springs to move the mirror forward, you mean in order to push this out, so prime focus is a little further out? I had comtemplated taking a hacksaw to the draw tube, but thought that might come back to bite me if I ever change camera / set set up which requires it to be extended further out!!!

Ta,

Rob

[markse68]

On 26/08/2021 at 15:40, Robculm said:

Also plan to check again the collimation in terms of secondary positioning (centering), checking it's central in the vanes & I thought to check how central the primary is & check the centre spot really is in the centre!

Ignore my earlier suggestion about the stretched flat hotspot centring Rob. I had recollimated after adjusting secondary height to get good centring but then wasted a rare evenings session as I had coma across the entire frame!! 😡 I think this would only work if you were sure you had your secondary offset 100% correct (which i guess mine isn’t!) 

In the end I field adjusted my secondary 1/2 a screw turn each time, then recollimated, then took a test sub and pixel peeped the star shapes. Took 1.5 turns the opposite way to how i’d previously adjusted it to get the stars vaguely round again.

I have since checked secondary centring to focuser tube and it looks “perfect” but the vignetting of the sensor isn’t centred anymore. Better that the collimation is good though as flats will sort out the uneven vignetting.

Live and learn…

Mark

Edited August 30, 2021 by markse68

[Robculm]

Ha, nightmare. I'm always a little nervous about making any significant changes to the secondary for fear of making things worse.

Actually I did brave a major re-alignment though & I do appear to have somewhat reduced the difference in focus between the two orientations. Secondary centration in the vanes was out a little and I think the centration in the focus tube slightly off, also spent a lot of fiddling time to get the circularity as good as possible. While I was at it I brought the primary mirror forward as far as possible (with the stock fittings) which drops the draw tube out by a couple of mm's. There is a lower profile focus tube adaptor available https://www.firstlightoptics.com/adapters/astro-essentials-2-inch-compression-ring-adapter-for-sky-watcher-newtonians-and-72ed-refractor-m54.html but it only gains ~1.5mm (and out of stock), so as per Alacant's suggestion I will probably look at custom springs / screws for the primary to push it forward more significantly.

Should add here that I also discussed the topic with Alex at FLO. As always, their level of knowledge & advice really is outstanding 👍.

Anyway, I'm keeping the set up as is for now until I get chance to try it out for a decent imaging session. My plan was to try & catch NGC6960 in this next moon free phase, while the nights are getting longer & it's still just about reasonably positioned...

However...

 

Edited August 30, 2021 by Robculm