ODK spider / 2y shadows

[DaveS]

I feel a bit of a numpty asking this, since I've had my ODK 12 for some time now, but has anyone else seen shadows from the spider vanes / secondary mirror in either their light frames or flats?

I get them in both , and no amount of stacking methods have so far been able to remove them.

I've been assuming that it's all down to my ham fisted processing, but I'm coming to wonder if there's something fundamentally wrong with the OTA. My best guess is the mirror spacing being off. I know the collimation is sloightly out as the "hole in the doughnut" is very slightly off centre in an out of focus star image, though the rings themselves look circular.

 

I've put this in the Telescope Discussion board, but the mods may think it belong in Imaging Discussion. I won't be upset if it's moved.

[andrew s]

I have an ODK and have never seen them.

 

I would guess that they should be way out of focus so a reflection from the corrector/ filter etc. must be focusing them. Just a guess though.

 

Regards Andrew 

 

 

Edited January 17, 2023 by andrew s

[DaveS]

I'm going to try raising the peak ADU. At the moment it's between 21k and 24k, I think I will aim for 25-31k.

[andrew s]

2 hours ago, DaveS said:

I'm going to try raising the peak ADU. At the moment it's between 21k and 24k, I think I will aim for 25-31k.

To be honest I don't see how this will help. Regards Andrew 

 

[Oddsocks]

Dave.

In this case a written description of the artefact is not really sufficient to determine the cause, if you upload an image showing the problem that might help pin down the cause should someone else have seen the same issue.

I have never seen shadows of the spider or secondary in images from my own ODK10, used with a FLI Atlas focuser and QSI683 camera, but I have come across images taken with a 10” RC that appeared to show out-of-focus “shadows” of the spider and secondary in lights and flats that also would not calibrate out.

The source for those artefacts were edge lensing from un-baffled; plain-black-anodised; spacer-couplers, close to the camera. You can produce the same effect visually if you look through any reasonably smooth piece of black anodised tubing held close to your eye, an image of the central bright opening at the far end of the tube is reflected back along the walls of the tube, slightly off-axis, to form a secondary image at the focal plane.

In the case of the RC the problem was solved by adding some flocking material to the inside of the spacer-couplers closest to the camera.
There was a thread running describing the problem and the resolution on the PixInsight forum about four or five years ago.

Any in-line reflector design with a large diameter secondary requires an equally large diameter and/or short primary baffle, which then allows off-axis “edge” focussing via any poorly baffled and blackened couplers or spacers between the forward end of the primary baffle and the camera to occur.

This secondary image is off-axis light that enters the focal path directly from the front of OTA and into the primary baffle opening where it reflects off the walls of any un-baffled plain-black couplers and into the camera.

The artefact created by these unwanted “edge” reflections look like fuzzy “shadows” of the spider and secondary in flats and lights, and have quite a strong gradient, darker and wider towards the outer edges of the spider arms and growing fainter and thinner towards the secondary shadow in the middle of the image.

The above case on the PI forum was the only one I recall reading about, and that had images attached showing the artefact, and may be describing a problem quite different to yours.

If you can show an example image maybe someone else will have seen the same and have the correct explanation and answer for you.

William.

Edited January 18, 2023 by Oddsocks
Spelling

[DaveS]

Thanks. I'm on my phone having only just surfaced, so it will be a while before I can post something.

[DaveS]

Here we go

Calibrated with Dark frames only and a linear stretch

And with Darks, Flats, and Dark Flats. Uncropped with a linear stretch.

[DaveS]

A problem may well come from using a T-Shirt stretched over the end of the OTA for flats. I'm trying flats from a flat box.

[Oddsocks]

Dave.

The dark spider shadows in your images are the same that were seen in the RC images on the PI forum that I mentioned above and the cause was identified on that system as described, off-axis light entering the primary baffle tube and reaching the camera sensor by way of edge reflections off some black-anodised but very shiny un-baffled camera spacing tubes, just in front of the camera port.

I can't remember all the details now, it was too long ago and the old PI forum was erased when they moved to new forum software back in 2020/21?, or was it earlier, memory not so wonderful these days, but I think the primary culprit was a distance spacer for the flattener on that RC system.

But other than that, your calibration is plainly not working, the image with flats added is over-compensated leading to the dark centre and bright edges (reverse vignetting).

Assuming these images are with the Moravian G3, you don't need Flat-Darks with these Kodak KAF 16200 CCD sensors, just a regular Master Bias, Master Dark and Master Flat.

Flat-Darks are generally for CMOS cameras with unstable outputs when taking zero-time Bias exposures, the Kodak KAF16200 is very stable and produces very consistent Bias frames which makes for much easier calibration.

HTH

William.

[DaveS]

Thanks. I will have to dismantle the imaging train and have a look at the spacing adaptors. As I recall they are the ones supplied by OOUK and have knife-edge baffles machined into them. I wonder if a spray with Black V3 might be more successful than trying to flock such a complex surface.

I will try again with my flats, calibrating with just a Master Bias frame.

[DaveS]

I've tried using flatbox Flats with an ADU in the 31k-35k range, calibrated with Master Bias only and there is an improvement. The diagonal shadows can still be seen but need an unhealthy stretch to be visible.

The situation now arises: If I have a go at the solution indicated above then I could end up with all the data collected previously rendered unusable, and there's a hell of a lot of data. I will likely have to keep two sets of calibration frames for Before and After treatment.

[DaveS]

This image of the Cetus A group is where I've got to so far. The shadows can still be seen but much reduced from what I had previously, and the negative vignetting has gone.

I'm tagging @peter shah on this as he knows the ODK inside out.

[Oddsocks]

That looks much better Dave, your Cetus A image is very similar to those I get with my ODK10 and QSI683 (KAF-8300 CCD sensor).

You are correct that any changes to the baffling to deal with those off-axis spider shadows would produce calibration frames incompatible with previously captured data.

(I know those Orion Optics spacer knife-edge baffles very well, still got the scars)

William.

 

[DaveS]

I bought some matt black (RAL 9005) paint and treated the Orion Optics spacer and 82mm-68mm adapter. It's not as black as I would like (I have some Black 3.0 on order) but an initial Bin 2 hydrogen flat looks promising

Calibrated with Dark Flat and Bias, given a light linear stretch.

[DaveS]

It also begs the question of how the hell could orion Optics supply a high end, expensive OTA with defective-by-design accessory components.?

[ONIKKINEN]

29 minutes ago, DaveS said:

It also begs the question of how the hell could orion Optics supply a high end, expensive OTA with defective-by-design accessory components.?

My VX8, from the opposite end of the price spectrum so maybe less of an issue, came with a non round tube (it was squared on the secondary end and triangular on the mirror end because of the attachments bolted to the side of the tube), a bent secondary spider, a secondary mirror that was physically impossible to collimate because a) collimation screws were too short, b) the spider was drilled to the incorrect position, c) the focuser was drilled to the wrong position, d) a bit of all. (secondary mirror could not be brought under the focuser = true collimation impossible)

Just a glance down the focuser tube and an attempt to bring the secondary correctly under it would have revealed the action to be impossible so this begs the question, was it assembled by a moron that does not understand how the product was obviously flawed, or was it assembled by someone who knew it was flawed and did not care to do anything about it, or third option: did nobody actually look down the focuser of the telescope to check that they are shipping a working scope? You tell me which one of the options is the least worst... Anyway, not a company i will throw any money ever again even though the scope worked out ok in the end.

[Oddsocks]

14 hours ago, DaveS said:

It also begs the question of how the hell could orion Optics supply a high end, expensive OTA with defective-by-design accessory components.?

Hello Dave.

Your sample flat looks ok now that you have dealt with those coupler reflections.

You might want to check out this short blog, I had this bookmarked from many years ago and came across it again yesterday, you'll be particularly interested in "Figure eight" I think:

https://diffractionlimited.com/flat-fields-stray-light-amateur-telescopes/

Regarding standards in amateur astronomy generally it's been my experience that "amateur" applies to both the target market and also many of the suppliers.

Go back a few decades and there were quite a few suppliers who had entered astronomy equipment production from an A.T.M. background but were not formally trained or qualified in optics, or engineering generally, and have no understanding, or the desire to learn, how to implement the Japanese business model "Kaizen" or principle of "Change for the Better" or "Continuous Improvement" of which rigorous Q.A. processes are a fundamental component.

It is sadly still common today in the amateur astronomy field that a small manufacturer will design and build a "widget" and then stick to that design come-what-may despite any shortcomings that may be revealed over the coming years, simply because they see it as too much trouble to change things and there's no market pressure for them to do so, there will always be new customers coming along with little experience in the field and will be influenced by the marketing hype.

Sadly too, I have seen far too many gushing amateur (and paid "influencer") YouTube reviews of astronomy equipment that has just arrived on their doormats saying how wonderful the product is and then when defects are revealed after a few months or years of use there is no follow-up informing would-be purchasers that their initial assessment might have been a little too favourable.

I realise I'm a little biased, coming from a medical engineering background where a small engineering oversight could lead to litigation costing millions, or possible imprisonment for negligence, it was impressed on me right from my trainee days that attention to quality, procedures, and reporting were paramount.

All of the imaging components on the medical systems I worked with for over forty+ years were properly blackened inside the focal path, not an unpainted plain black anodised surface or unfinished screw-head to be seen!

Good thing this astronomy lark is "only a hobby". 😂

William.

[DaveS]

Many thanks for the link to that blog, yes figure 8 does look horribly familiar!

Unfortunately OOUK only make telescope, they don't actually use them themselves, so egregious faults like this go unnoticed. Their "Customer Service" is notoriously poor, so perhaps customers just aren't bothering to report the issues.

[CedricTheBrave]

3 hours ago, DaveS said:

Many thanks for the link to that blog, yes figure 8 does look horribly familiar!

Unfortunately OOUK only make telescope, they don't actually use them themselves, so egregious faults like this go unnoticed. Their "Customer Service" is notoriously poor, so perhaps customers just aren't bothering to report the issues.

reporting issues makes no difference 

[DaveS]

A quick 2 hours on the Intergalactic Wanderer. Just given Dark Frame calibration and a humongous stretch.

The diagonaal shadows are gone, just need to get some Flat Frames , and give the filters a clean..

Many thanks to @Oddsocks For identifying the cause of the problem, otherwise I'd be no nearer the solution.

[harry page]

Hi

I Know this topic is a year old but I also had this problem with my ODK10 and cant blame orion optics this time it turned out to be the adaptor on my sx filter wheel

was causing the problem - bit of black paint and all is well.

Never had this problem with all my other scopes - must be down to the optic configeration off the scope

I would have gone mad looking for the problem and cure - so Thanks you SGL and all you great people

Thanks

Harry

[Oddsocks]

Harry.

With the hundreds of PixInsight beginners that your video series have helped, myself included, I think we’ll always be more indebted to you!

Glad to know that you found this thread useful.

William.