Can a soft dew shield interfere with flats? Rigid ones seem hard to find...

[pipnina]

I've basically diagnosed my imaging issues to be down to stray light. So I figure one of the things I need to do is get s long-ish Dew shield for the front to minimise how much stray light there can be.

I had a soft one for my 130-PDS, which sort of worked well, but I found it hard to take flats with, as my flat generating panel would move the shield and keeping it square to the front aperture was pretty hard, and too much movement from flat to light frame could cause my flats to improperly correct.

 

I found celestron's rigid dew shield, but while it was a somewhat reasonable price of £70 for the 6" version, the 8" that I would need was £170!!!

I haven't found any other rigid alternatives off-the-shelf yet. I do have some black velour for flocking though, so if there is an option for constructing a rigid shield with plastic tubing for example, that could work!

 

Any suggestions are welcome please!

[Dan_Paris]

I have a soft dew shield on my Newtonian, that  I remove for  taking flats (with a flat panel). Never seen anything suspicious on calibrated subs. Note that I fasten the Velcro at some angle, such that the dew shield is slightly  conical, in order to be sure that it does not cause vignetting by itself.

 

 

Edited September 7, 2022 by Dan_Paris

[pipnina]

42 minutes ago, Dan_Paris said:

I have a soft dew shield on my Newtonian, that  I remove for  taking flats (with a flat panel). Never seen anything suspicious on calibrated subs. Note that I fasten the Velcro at some angle, such that the dew shield is slightly  conical, in order to be sure that it does not cause vignetting by itself.

Sadly I am not so lucky! I have noticed that flats introduce a gradient towards the top right of my images, more strongly in narrowband, because light from the flat panel is able to get into the coma corrector and bounce around in there at odd angles. When un-flat calibrated the background has an odd mottling effect which changes with colour. I'm hoping to put the flat panel further away from the aperture, baffle the CC better, and also block more stray sky light while I'm at it with this.

Maybe I can do this with a Flexi, maybe not. It will make flats a bit harder but I had considered buying a photographic background screen and illuminating that while it's hung from the shed, then pointing the telescope at it from a distance (since this would increase the distance even more)

[Dan_Paris]

28 minutes ago, pipnina said:

baffle the CC better,

I see that you use a GPU corrector, with mine I had some issues with internal reflections as well. I found it useful to thread to the corrector (telescope side) a short M48 extension tube linen with flocking material. Of course you need to be careful that it does not protrude, or at least not too much, into the light path. 

 

 

 

 

[pipnina]

10 hours ago, Dan_Paris said:

I see that you use a GPU corrector, with mine I had some issues with internal reflections as well. I found it useful to thread to the corrector (telescope side) a short M48 extension tube linen with flocking material. Of course you need to be careful that it does not protrude, or at least not too much, into the light path.

 

That sounds like a very good idea! Could you send me a pic of your extension-flocked addition so I could mimic it in my setup please?

Sadly to reach focus my GPU CC almost starts to obstruct the primary as it is, but I'd rather have the extra diffraction than my current stray light filled images!

Cheers for the suggestion!

[carastro]

Why do you need to keep the dew shield on for flats?  Surely you have a flats panel over the aperture for flats, or a fabric or paper diffuser if you are doing flats in the day time. 

Carole 

[Dan_Paris]

11 hours ago, pipnina said:

Could you send me a pic of your extension-flocked addition so I could mimic it in my setup please?

Sorry I upgraded to a 2,5 inch corrector and sold the GPU so I cannot send you a picture.

For the extension tubes, it is important that their external diameter is 2 inches, otherwise it would not fit in the focuser (and would need to be screwed from inside the OTA, not convenient). Those that I bought on Amazon are I think the sames as those ones:

https://www.firstlightoptics.com/adapters/astro-essentials-m48-extension-tube-set-5mm-10mm-15mm-20mm.html

Among the set choose the extension that is flush (or  protrude just by few millimeters) inside the OTA when it is screwed to the GPU corrector (telescope side), at the correct focus position. The longer it is the more efficient it will be, but if it protrudes to much it would cause undesirable diffraction effects.

Then line the inside of the extension with self-adhesive flocking material (in my case, Protostar):

https://www.firstlightoptics.com/telescope-flocking-material.html

You would have of course to buy more flocking material that needed for the extension tube, but it is in any case a very good idea to flock the inside of the OTA, at least the area facing the focuser, to increase contrast and control stray light.

Edited September 8, 2022 by Dan_Paris

[pipnina]

4 hours ago, carastro said:

Why do you need to keep the dew shield on for flats?  Surely you have a flats panel over the aperture for flats, or a fabric or paper diffuser if you are doing flats in the day time. 

Carole 

My focuser is close to the front aperture, so when I put the flat panel on, light from the edges of the front aperture flies directly into my coma corrector and is causing my flats to over-correct and introduce gradients and background mottling. By pushing how far my flat panel sits further away from the aperture, the angles get a lot steeper and less light can get into my cc. I hope.

 

[carastro]

Ah, it's a Newtonian.  I found that that leaked light at the Primary when I had one, so was forced to do my flats with a panel at night.  I never got any light leak from that into my focuser though.  Are you perhaps using a flat panel that is uncovered?  I had mine contained inside a box, so very little chance of light leak from that.  Otherwise  can you not wrap up your focuser with something black.  I have done that sort of thing with my Newtonian at one time.

This was might light box on a later telescope, but showing what I mean by having a box around it.  It's just a cardboard box

Carole 

[pipnina]

3 hours ago, carastro said:

Ah, it's a Newtonian.  I found that that leaked light at the Primary when I had one, so was forced to do my flats with a panel at night.  I never got any light leak from that into my focuser though.  Are you perhaps using a flat panel that is uncovered?  I had mine contained inside a box, so very little chance of light leak from that.  Otherwise  can you not wrap up your focuser with something black.  I have done that sort of thing with my Newtonian at one time.

This was might light box on a later telescope, but showing what I mean by having a box around it.  It's just a cardboard box

Carole 

Yes I have light leak around the primary as well. Thankfully my garden is well shielded from street lamps, and a black shirt of mine works very well to cover the back. I also take my flats during twilight before an imaging session as you suggest.

 

My flat panel isn't cardboard up like yours but it does sit nice and flat to the aperture, and given the short exposures needed for flats I think it unlikely that enough light enters around the edge.

 

I think instead that the flat panel itself is shining light into the focuser, which itself is not particularly anti-reflective. (Smooth internal bore!). Not helped by the fact that the GPU CC is 10cm long and extends past the end of I think every focuser barrel, leaving the first lens element exposed.

The inside of your obsy looks very nice by the way, surprisingly bright looking!

[Ratlet]

My gut instinct would be to try and make something cheaply to test the theory first.  I'd be thinking cardboard tubes that come inside carpet (you might need to do a bit of a bodge to get the right diameter) or parcel tubes/ducting if you want to drop some cash on it.  Probably set you back £20.