Internal Reflections due to Hutech IDAS LPS Filter

[Gib007]

Unfortunately the Hutech IDAS LPS filter, as good as it's meant to be, seems popular for producing internal reflections of the telescope's optics. It is advertised as doing this due to rejecting light pollution wavelengths back out of the system before entering the optical train (hence why I place this filter at the very front, in front of the focal reducer). 

This is not an issue with my refractor, simply because reflecting light pollution wavelengths back out is not a problem if there's nothing in front to block the light from leaving the optical system altogether. Unfortunately, on my 8" RC, with its secondary mirror and spyder vanes, this rejected light causes a shadow of these optics in my images:

The above is the M51 Whirlpool Galaxy and is a raw image of 450 seconds binned 2x2. The Red filter was used as this accentuates the problem (due to most of the light pollution being on this region of the spectrum). 

I always heard the Hutech IDAS LPS was the best in its class but this is unhelpful. Mind you, this disappears in the most part when using flats to calibrate but the flats can be difficult to gather as I found out yesterday. I spent a good number of hours tweaking the optical train in the FeatherTouch focuser (tightening differently on each bolt, angling it and turning the telescope to shift the weight droop) in order to get this internal reflection to match from my flats on to my lights (to cancel it out as best as possible). 

I would really, really like this gone from my images and still get decent cancellation of light pollution, if at all possible. My question is simple. Can this be avoided to begin with? Has anyone managed? Perhaps a different light pollution suppression filter that doesn't produce this? Thanks in advance!

[johnrt]

This is upsetting. I know several users of this OTA and combination of filter & ccdt67 none of which suffer these reflections. I use the 6" version with the IDAS and same telecompressor. I'm not sure what to suggest, other than to try another filter as you say above.

[Stephen]

I know it's not a solution, but if you take the flats at the same time as imaging - the flat should perfectly match the position / orientation of the lights and cancel out?

Other than that, what about moving the filter to the other end of the train - right up to the sensor. Would this be possible?

[GuillermoBarrancos]

I know it's not a solution, but if you take the flats at the same time as imaging - the flat should perfectly match the position / orientation of the lights and cancel out?

Other than that, what about moving the filter to the other end of the train - right up to the sensor. Would this be possible?

I was wondering this too, as I thought you would only use these type of filters for the luminance when imaging with a mono camera?

[sloz1664]

I was wondering this too, as I thought you would only use these type of filters for the luminance when imaging with a mono camera?

If this was the case you would use an LP filter in place of your Luminance and do away with the additional Hutech IDAS LP filter attached to the front.

Steve

[johnrt]

I use mine in front of everything, LRGB and even Ha and OIII.

[Sp@ce_d]

Hmm...  I haven't noticed a difference on my fracs whether the P2 is in front or behind the Reducer. I've now got a P2 in the nosepiece of the FW (2"), behind the reducer in the Moonlite on the SCT. I'm still commissioning this setup but I haven't noticed this type of reflection. What I have noticed is quite bad internal reflections if there are any bright stars at the edge of the FOV. I'm considering taking it apart to flock all the internal surfaces. I've read of others on here that have done this with success, I don't know if this would help in your case?   

[Sp@ce_d]

I use mine in front of everything, LRGB and even Ha and OIII.

+1

[GuillermoBarrancos]

I use mine in front of everything, LRGB and even Ha and OIII.

Hmm ok. My bad then.

I guess it's the confusing description on some sites then regarding the use of LP filter with mono cameras.

[Sp@ce_d]

Hmm ok. My bad then.

I guess it's the confusing description on some sites then regarding the use of LP filter with mono cameras.

The D1 is the one that cuts to far into the NB so should be avoided leaving in.

[sharkmelley]

That's a very interesting example of a weird effect! 

However, this cannot be the rejected light causing that "shadow" on the image.  The "shadow" clearly shows the secondary and the spider vanes and there's no way the rejected light going out back out past the spider can get back in and cause this.  So it is caused by incoming light (possibly from a bright star?) passing through the filter and then possibly being reflected and refocused off multiple elements (including the filter) in a manner that puts a fairly focused "shadow" of the spider onto the sensor. 

I once had a very similar problem on an SCT with focal reducer and a reflective cover glass on the CCD.  Debugging this could take some time but here are some possible steps to take:
1) If you remove the filter entirely does the problem disappear?
2) If you move M51 off-centre does the "spider shadow" also move or does it stay in the same place.  This will tell you if a bright star is causing it.
3) Can the filter be mounted "backwards" so the anti-reflective coatings face inwards instead of outwards or vice versa.
4) Try placing the filter at a different points in the optical train - it is not always easy to predict in advance what the effects might be.

Good luck!

Mark

[ollypenrice]

Just an idea, but have you considered a baffle around the outside of the baffle tube? When Ralf Ottow looked at Yves' Orion Optics ODK14 he immediately spotted a design error and this was his solution. It greatly reduced our problems with internal reflections. I must admit that I didn't fully understand Ralf's explanation of the problem so I can't advise on how big the baffle should be, but trial and error might bring results. Just make some cardbaord washers of various outside diameters to slip over the outside of the baffle tube. In our case the baffle was about a centimetre wider than the diameter of the baffle tube.

From memory Steve (Kirkster) posted something like this showing on the flats from his RC. I'm not sure what was the outcome.

Like Mark, I'm suspicious of the filter theory.

Olly

[Gib007]

I e-mailed Ted at Hutech about it. Being as helpful as he always is, he replied immediately (he's excellent!):

This is the nature of coating - interference type. 

So unfortunately there is no complete solution. 

I'm not sure what practical solution is. 

I suggest to post at the discussion forum. 

Some experts should give you some idea. 

I know many reflecting scope users are

using LPS filter too. I wonder how they work

out to minimize reflecting. 

Only my suggestions are

1) set LPS after reducer

2) set LPS into filter wheel and use as L filter.

I don't think these solves the reflections though. 

Users of refractors shouldn't be seeing anything like this purely based on the fact that refractors don't have these obstructions on the front (secondary mirror, spyder vanes, etc). The nice thing about this Altair Astro 8" RC is the fact that it has internal baffling all the way inside the OTA. I'm not sure what you meant by baffling the outside of the baffle tube, Olly. It certainly sounds interesting. 

Mark, your suggestions sound interesting as well. Indeed removing the filter makes the effect disappear entirely. The same happens when I use my Geoptik Flat Field Generator on the OTA for creating flats. As soon as the filter is removed, the flats are clear. I have not tried moving the target off-centre and seeing the effect on the shadow. I did however notice that the shadow formed in a slightly different place for the R, G and B filters. Imagine the effort I went through to replicate this for the flats the next day at home!! Hahaha

Ironically, I place the filter the other way round for use in my refractor, as that's the only way it fits on the focal reducer. I don't see any problems whatsoever with my refractor, but then again, refractors don't have front obstructions so I'm not sure. I cannot put in the filter the other way round while in front of the focal reducer but I can test this on my filter drawer, further back. Well worth a shot! In any case, is there some kind of 2" male-to-male adapter thread? That would allow me to flip the filter in front of my focal reducer for the 8" RC. 

Thanks for the suggestions guys! I will report back if I find anything. 

[swag72]

If this was the case you would use an LP filter in place of your Luminance and do away with the additional Hutech IDAS LP filter attached to the front.

Steve

That's exactly what I do! Hutech replaces the Luminance filter.

[Sp@ce_d]

I guess an 8" version to go over the front of the tube would work out rather expensive.. ... sry.. couldn't resist

[ollypenrice]

I e-mailed Ted at Hutech about it. Being as helpful as he always is, he replied immediately (he's excellent!):

Users of refractors shouldn't be seeing anything like this purely based on the fact that refractors don't have these obstructions on the front (secondary mirror, spyder vanes, etc). The nice thing about this Altair Astro 8" RC is the fact that it has internal baffling all the way inside the OTA. I'm not sure what you meant by baffling the outside of the baffle tube, Olly. It certainly sounds interesting. 

Mark, your suggestions sound interesting as well. Indeed removing the filter makes the effect disappear entirely. The same happens when I use my Geoptik Flat Field Generator on the OTA for creating flats. As soon as the filter is removed, the flats are clear. I have not tried moving the target off-centre and seeing the effect on the shadow. I did however notice that the shadow formed in a slightly different place for the R, G and B filters. Imagine the effort I went through to replicate this for the flats the next day at home!! Hahaha

Ironically, I place the filter the other way round for use in my refractor, as that's the only way it fits on the focal reducer. I don't see any problems whatsoever with my refractor, but then again, refractors don't have front obstructions so I'm not sure. I cannot put in the filter the other way round while in front of the focal reducer but I can test this on my filter drawer, further back. Well worth a shot! In any case, is there some kind of 2" male-to-male adapter thread? That would allow me to flip the filter in front of my focal reducer for the 8" RC. 

Thanks for the suggestions guys! I will report back if I find anything. 

The baffle tube is a tube with baffles on the inside. What we did was place one baffle (a cardboard washer, in effect) around the outside of the baffle tube to stop light shunting between the mirrors. I would certainly give it a try. It needs to be blackended. Use barbecue paint which uses pigments, since paints using dyes are reflective in IR. It is dead easy to make one or two for test purposes.

Olly

[sharkmelley]

Would you post an image of one of your flats that shows the problem.  In any case, I think what is happening is that the optical system as a whole is producing an image of the spider that happens to be more or less in focus in the plane of the sensor.  Who knows exactly what combination of reflections and refocusings might be causing it.

On my C11 SCT with focal reducer, the combination of the secondary and the standard C11 focal reducer produced an in focus image of the corrector plate behind the sensor but this produced a large out of focus bright ring on the outer 2/3 of a DSLR sensor.  This caused lots of gradient problems in a heavy light polluted environment as this image from my earliest imaging days (in London) shows:  http://www.markshelley.co.uk/Astronomy/2008/horsehead2008.html Eventually I was able to verify the cause both by experiment and by lens equations from my schoolboy physics days.

Try placing the filter after the focal reducer - you might find this totally cures the problem.  This is something you can easily test on a cloudy night using your flat field generator. Certainly follow up Olly's suggestion - again you can test this on a cloudy night.

I hope you quickly find a solution.

Mark