What is this strange thing in my picture?

[MylesGibson]

I was attempting to image the Horsehead nebula for the first time last night, and unfortunately, the wind was quite strong so didn't get many useable subs. However, I decided to do a very quick process of the light frames just to see what I managed to capture and noticed a weird glowing spot that I didn't notice in the individual subs. Does anyone have any idea what possibly caused it so next time I can try to avoid it?

It's rather obvious where it is, but just in case, just to the upper left of the horsehead.

I thought it might be something wrong with the equipment, so I quickly went over to M42 and took a few snaps of that, but there was no anomolie there, and the picture was absolutely fine, so I have no idea what may have caused this. 

I'll attach the light files, just in case anyone is able to notice anything I missed, but just be aware, this was only a little play around with some new equipment, and no calibration frames as it was a very windy night and I wasn't intending it to be a serious imaging session!

The scope used was an Altair Starwave 102ED-R and the imaging camera was an unmodified Canon 1000D

L_IC 434_300s_ISO800.zip

[andrew s]

Looks like a reflection from the bright (over exposed ?) central star.

Regards Andrew 

Edited November 19, 2020 by andrew s

[Paul M]

I'd agree, it's an optical artifact caused by Alnitak being reflected within the imaging train. It's a lot of star to be sharing the same frame as the faint Horse Head!

It's not uncommon to see various similar artifacts in wide angle Horse Head images.

 

[Seelive]

Were you using a filter of some type? I certainly wouldn't expect any reasonable straight optic train to create that sort of 'reflection', dispite how bright nearby stars are, and it's certainly not something I've ever experienced with a similar field of view image of the Horse Head. Prehaps you were very unfortunate to just happen to have Alnitak aligned on an optical defect of some form.

[MylesGibson]

2 hours ago, andrew s said:

Looks like a reflection from the bright (over exposed ?) central star.

I did think that, but I have imaged around other bright starts without this artefact being present. 

 

1 hour ago, Paul M said:

I'd agree, it's an optical artifact caused by Alnitak being reflected within the imaging train. It's a lot of star to be sharing the same frame as the faint Horse Head!

It's not uncommon to see various similar artifacts in wide angle Horse Head images.

 

Hmm, do you have any tips on how to get rid of it, or make sure it doesn't appear? With an unmodified camera, I can't do short exposures, otherwise I won't pick up the horsehead!

1 hour ago, Seelive said:

Were you using a filter of some type? I certainly wouldn't expect any reasonable straight optic train to create that sort of 'reflection', dispite how bright nearby stars are, and it's certainly not something I've ever experienced with a similar field of view image of the Horse Head. Prehaps you were very unfortunate to just happen to have Alnitak aligned on an optical defect of some form.

No filter is being used. Just the scope and camera. I have imaged near bright stars before and not seen any reflections. 

[michael8554]

Here's one of your subs heavily stretched, so the reflection is on your subs:

Michael

 

[andrew s]

Try rotating the camera and take the same shot and see if it stays in same place on the camera or on the sky. That will help in it down possible causes.

Regards Andrew 

[MylesGibson]

@michael8554 damn, didn't think of stretching a single sub! 

@andrew s good idea, I'll give that a go next time.

Not too sure what to do about it if it is a reflection though?

[vlaiv]

I'm quite surprised with poor color correction for FPL-53 doublet.

[MylesGibson]

3 hours ago, vlaiv said:

I'm quite surprised with poor color correction for FPL-53 doublet.

On the stars you mean? Could possibly be my camera at fault there. Think I'm going to need a new one soon. It's old, getting close to 30k shutter count now. And unmodified. Plus LP from my back garden. Could all be contributing perhaps?

Edited November 20, 2020 by MylesGibson

[happy-kat]

30k shutter isn't particularly high.

[MylesGibson]

Just now, happy-kat said:

30k shutter isn't particularly high.

Was having a bit of a guess. I can see colour in the image though.

[vlaiv]

34 minutes ago, MylesGibson said:

On the stars you mean? Could possibly be my camera at fault there. Think I'm going to need a new one soon. It's old, getting close to 30k shutter count now. And unmodified. Plus LP from my back garden. Could all be contributing perhaps?

Yes, the stars, or rather blue part of the spectrum. I highly doubt it is due to camera. It can be due to processing somewhat, but even then it needs to be in the data in order for processing to emphasize it.

That is just too much blue bloat around stars. On the other hand, it is fast at F/7 for doublet scope even if it has FPL-53 glass. On the other hand TS in Germany placed this scope in their Photoline series - which clearly means it is meant for imaging.

In any case, you might want to look at Astronomik L3 filter to try to see if it will reduce star bloat. It is UV/IR cut filter but cuts off a bit more on far ends of spectrum where telescope has the worst color correction.

To give you idea of why I say that - this is image taken with very simple f/5 achromat and guide camera:

That is attempt at Crescent nebular, and while image itself is very poor - it does show you level of bloat that you'll find in very cheap fast achromat of the same size. Telescope costing about x4, being slower and having exotic glass should not have the same issues with stars.

While my image above is not really good, here is another one that I've found via google - credits go to SGL member here:

you can see other work here:

 

[MylesGibson]

@vlaiv Ahh I see what you mean now. Yeah, I used to suffer from star bloating a lot in my old scope. This one is much better but I did expect it to be a bit less than it is to be honest! I do use focal reducer/flattener with it too, although I'm yet to start using filters

[vlaiv]

33 minutes ago, MylesGibson said:

@vlaiv Ahh I see what you mean now. Yeah, I used to suffer from star bloating a lot in my old scope. This one is much better but I did expect it to be a bit less than it is to be honest! I do use focal reducer/flattener with it too, although I'm yet to start using filters

Indeed, I expected to see less of it too. This one has CA on the level I would expect from 102ED model (one that uses FPL-51 glass) - not 102ED-R.

What FF/FR are you using? Maybe try couple of subs without it to see what happens. I believe it is also responsible for reflection, but it could as well cause issues with CA. I'm no optical expert so I can't claim that, but I've seen that specific model of FF/FR is recommended for F/7 ED doublet by TS. Not sure why particular FF/FR would be required and if regular one would work as well - but it might be something to do with above CA levels?

[MylesGibson]

@vlaiv it's the one inwas recommended to come with it. You think the FR/FF could be responsible for the reflection too? It's possible I suppose!

[vlaiv]

3 hours ago, MylesGibson said:

@vlaiv it's the one inwas recommended to come with it. You think the FR/FF could be responsible for the reflection too? It's possible I suppose!

FF/FR is most definitively cause of reflection.

You can calculate total path of light to and from reflection by inspecting what diameter reflection is and taking into account F/ratio of telescope. If you do that, it is very likely that you'll find it is back side of reducer that is causing reflection.

If it is matched FF/FR then I don' think it will be responsible for poor color correction.

[Delboy_Hog]

Following this one with interest - I've encountered a similar thing in my last attempt at the Alnitak area, though my blueish blob appeared in a slightly different place.  5 min subs, ED80 and Canon 1100d.  I'd assumed it was a reflection, but wasn't sure what to do about it.  Is your image above cropped at all?  If not, both our bluish blobs appear to be roughly equally distant from the centre (or edges) of the frame from Alnitak, so I'm guessing that's the culprit, but which piece of glass is doing the bouncing was beyond me.  Given its location I'd struggle to process it out convincingly...

Like you MylesGibson, I was pushing the exposure length to give me a sporting chance of catching the nebulae...and similarly , ran short on total number of subs, and had no calibration files.  I guess a greater number of shorter exposures might help, and I gather that you can use star masks and the suchlike to 'contain' Alnitak, though those are processing skills well above my current skill level!

How long an exposure is too long, for a bright star like Alnitak?

[MylesGibson]

2 hours ago, Delboy_Hog said:

Following this one with interest - I've encountered a similar thing in my last attempt at the Alnitak area, though my blueish blob appeared in a slightly different place.  5 min subs, ED80 and Canon 1100d.  I'd assumed it was a reflection, but wasn't sure what to do about it.  Is your image above cropped at all?  If not, both our bluish blobs appear to be roughly equally distant from the centre (or edges) of the frame from Alnitak, so I'm guessing that's the culprit, but which piece of glass is doing the bouncing was beyond me.  Given its location I'd struggle to process it out convincingly...

Like you MylesGibson, I was pushing the exposure length to give me a sporting chance of catching the nebulae...and similarly , ran short on total number of subs, and had no calibration files.  I guess a greater number of shorter exposures might help, and I gather that you can use star masks and the suchlike to 'contain' Alnitak, though those are processing skills well above my current skill level!

How long an exposure is too long, for a bright star like Alnitak?

My image is cropped slightly if I remember correctly, I'll have to double check when I am at my home laptop. Is your camera also unmodified? And did you also use a reducer/flattener?

I think the longer exposures are needed to bring out the nebulosity. Perhaps next time I'll try a few shorter ones to blend in as well? And when I get a good amount of time on it, I'll take calibration pictures too!

[wxsatuser]

On 20/11/2020 at 14:36, MylesGibson said:

On the stars you mean? Could possibly be my camera at fault there. Think I'm going to need a new one soon. It's old, getting close to 30k shutter count now. And unmodified. Plus LP from my back garden. Could all be contributing perhaps?

Over exposed, your over 50% on the Canon histogram.
Plus the stars on the right hand side are egg shaped, left looks ok.

[pete_l]

On 20/11/2020 at 11:39, MylesGibson said:

Not too sure what to do about it if it is a reflection though?

Since it is only on that one image, the pragmatic approach is simply to photoshop it out and not worry about it!

[Delboy_Hog]

21 hours ago, MylesGibson said:

My image is cropped slightly if I remember correctly, I'll have to double check when I am at my home laptop. Is your camera also unmodified? And did you also use a reducer/flattener?

I think the longer exposures are needed to bring out the nebulosity. Perhaps next time I'll try a few shorter ones to blend in as well? And when I get a good amount of time on it, I'll take calibration pictures too!

Yes, unmodified camera, and yes, with the field flattener / focal reducer that's specifically recommended for the Skywatcher ED80...

I think the approach you suggest is right - taking different exposure lengths to cater for the bright star and the faint nebulosity, and combine them in processing?  I guess though that the reflection would still be there in the longer subs, so perhaps Pete's suggested approach of attacking it in photoshop might be the way forwards?

I've also wondered whether having Alnitak absolutely central in the subs would help prevent light bouncing around at strange angles...some testing required, I guess!  

[MylesGibson]

On 28/11/2020 at 04:10, wxsatuser said:

Over exposed, your over 50% on the Canon histogram.
Plus the stars on the right hand side are egg shaped, left looks ok.

Not really the answer to the question I was asking to be honest!

On 28/11/2020 at 09:00, pete_l said:

Since it is only on that one image, the pragmatic approach is simply to photoshop it out and not worry about it!

My photoshop skills are nowhere near good enough lol

On 28/11/2020 at 12:06, Delboy_Hog said:

Yes, unmodified camera, and yes, with the field flattener / focal reducer that's specifically recommended for the Skywatcher ED80...

I think the approach you suggest is right - taking different exposure lengths to cater for the bright star and the faint nebulosity, and combine them in processing?  I guess though that the reflection would still be there in the longer subs, so perhaps Pete's suggested approach of attacking it in photoshop might be the way forwards?

I've also wondered whether having Alnitak absolutely central in the subs would help prevent light bouncing around at strange angles...some testing required, I guess!  

Yeah, looks like a bit of experimentation is needed! Will have to wait until we finally get some clear skies again!