problems with a focal reducer

[mark123]

When I brought my 8” LX90 it came with an F3.3 focal reducer, I finally got to try it last night. After setting up an peering thought the camera viewfinder there was what I can only describe as a large airy disk in the centre, adjusting the focus didn’t change the size of this disk but I did notice the behind the disk the real targets were behaving normally.

Where is this disk of light coming from? I think it could be stray light from the near full moon. The big question is how do I get rid of it? I tried a makeshift lens hood but it didn’t seam to do a lot.

Help!

[mark7331]

I would have thought that you're seeing vignetting caused by the focal reducer. I don't think there's any way to get rid of it. A more gentle reducer would cause less vignetting but even then you'll still get some.

If you remove the focal reducer the vignetting should almost disappear.

Mark

[mark123]

Thank for the speed reply Mark, I have seen vignetting when using a camera with a wrong lens fitted, but this is exactly like a bright, very out of focus star in the centre of the viewfinder. If only I had thought and taken a photo last night.

[mark7331]

Sorry Mark, I might of missed something here. You'll get vignetting from the focal reducer but you saw a bright out of focus star in the camera view finder. What was the scope trained on?

If you were looking at Jupiter it can appear to be a star, and requires very short exposures. I tooks some quick shots last night of about 0.005 seconds with my CCD. Any brighter and Jupiter is way over exposed. So maybe you're seeing two effects - vignetting and over exposure?

If you get out tonight take some pics - with and without the focal reducer, then vary the exposure length and post them if you can't work out what's going on.

I shall be setup (the weather looks good) and imaging M51 - so I'll have some time on my hands if you need some help.

Mark

EDIT: Did you check for dew on the corrector lens?

[Merlin66]

The x0.33 reducers on a SCT are a PITA!

They will not give a good coverage on anything larger than a webcam chip. That's what they were designed for. Absolutely useless for visual!

The distance from the reducer to the chip must be very close to 88mm.

IMHO go for a x0.63 reducer and cut your losses.

Ken

[riklaunim]

For visual you can use no reducer and just eyepieces that give less magnification. Both SCT focal reducers have their weak points and are sometimes very annoying.

[mark123]

I have taken a photo to show the effect.

The camera is a Canon 350D.

The central disk does not repond to the focuser but last night I could see the stars focusing behind (tonight we have 100% cloud cover again!).

Any ideas?

[mark7331]

Mark, is that with the focal reducer fitted? If so what does it look like without? Can you post a pic?

When the stars appeared behind the "doughnut" were they in the blue or the black area. I think you have a focal reducer that's either very low quality or just too much for the scope. You're going from slow(F10) to almost super-fast (F3.3) and for my money I would think the reducer isn't up to it.

If the view looks good without the FR then therein lies the problem....

Mark

EDIT: I think your pic shows the secondary mirror which is why focusing doesn't change it. I may be wrong.

[OzDave]

To me that image looks like you just can't achieve focus and what the camera is seeing is actually an out of focus version of what you'd see with your eye if you just look into the back of the scope with nothing attached.

I had the same problem with a 0.5x reducer on my SkyMax127.

You may be able to solve the problem by varying the distance from the reducer to the camera, but I'm not sure whether you need to increase or decrease the distance. At a rough guess, I'd say you maybe need the reduce the distance.

With my SkyMax, I found that I could not achieve focus with my DSLR with 50+ mm gap between reducer and CCD, but when I tried my Webcam, it worked ok, which I assume is because the gap was much smaller in that case, like about 13mm. So if you're trying to do this with a DSLR, which it sounds like you might be, I would say you're out of luck. Either get a 0.63x reducer or switch to a camera with a shorter distance to the focal plane.

David

[Merlin66]

What is the distance between the reducer and the front of the camera?? It should be around 30mm.

[mark123]

Here is a snap of the setup I was using. The focal reducer is a Meade 4000, is it possible that I need an extension between focal reducer and camera?

[Psychobilly]

Your going to get a lot of vignetting with the f3.3 reducer on the APS-C sixed sensor and the outer edges of the field wil be "iffy"....

I would have used an SCT-T adaptor and spacer with an EOS T-Ring to attach the camera rather than the SCT Visual Back (1.25") and a T-1.25" adaptor... but really the f3.3 is only useful for small chip cameras...

here's a test shot through mine...

Peter...

[riklaunim]

Note that the sensor in the DSLR is deep inside the body so the distance from it to reducer is bigger than it looks like. You could try low profile SCT to T-thread adapter without that narrow spacer which probably adds some vignetting (as well as the f/3.3 reducer will not illuminate the sensor compleately).

[Merlin66]

Doesn't look too bad from the photo.

The camera body to outside of the T adaptor is 55mm, so the spacer behind the x0.33 reducer should be about 33mm - which it looks to be.....

Try focusing on a distant object and see if you can find the focus.

From your earlier image you appear to be well out of focus.

Ken

[mark123]

Thanks, I will have a play this evening and let you know if I get any better results. This time I will remember to take some photos !!

[mark123]

Hey, the problem has been solved.

I needed to ad an extension tube between reducer and camera. I tried first with a 20mm extension and that improved the image slightly, so I kept extending, and extending, and extended a bit more just for fun. At 180mm I was finally able to focus on a rooftop about 150M away. So I have attached the two photos, one of the roof top and one of the setup. I am feeling quite pleased with myself. 1 problem down, 9999 to go. I think I will move onto PHD Guiding next, or Registax.

I have made a cool tool for tring to get PHD setup, I will have to take some photos and find the correct place to put it on here. Its rather Heath Robinson, but seems to work and cost £0.00 (my father was Scottish).

Thanks again everyone who helped me, I don't think I would have solved it myself.

[Psychobilly]

Ok you have an image but thats just wierd...:icon_scratch:

[Merlin66]

I was just checking the formula for the x0.33 reducer - the optimum distance is only 50mm or so. The lens has a focal length of 88mm

I think what you have done is positioned the lens so close to the prime focus of the telescope that it acts like a projection lens not a reducer. The roof tiles look the right way up...they should be upside down!

The attached spreadsheet (substitute 88 for the 240) will give some answers.

reducers_V2a.zip

[ncjunk]

Is that normal to have such a long extension between reducer and camera?

I ask out of curiosity as i have a reducer that i aim to use but wont have that many spare extensions.

Peter, whats the distance between yours and the camera and can you put them the wrong way around?

Aahh Merlin answered!

[mark123]

Merlin, I rotated the image when I reduced its size to upload.

[Psychobilly]

I used the SCT -T adaptor and the shortest part of the Meade Variable T adaptor... the variable T adaptor and f3.3 reduce can be used to give a range of reduction ratios with small chipped cams...

Peter...

[davy999]

I have a 6"SCT and a focal reducer, I screw it onto the end of the standard nosepiece that came with my atik titan CCD and it works fine, there is some vignetting but this is only to be expected, I didnt need or use any extension tubes.

[mark123]

Merlin, If I am reading your spreadsheet correctly at the moment the setup is working at about F6.0 but if I reduced the distance between reducer and sensor to 65mm the the F number would increase/reduce(?) to F3.1 (what it is designed to be)

[Merlin66]

Don't know where you get the F6 - the table doesn't reach that unless the distance is only 35mm or so, your currently 150mm plus! - for the 65mm spacing - Yes, that's correct. But also note the reducer itself has to be 250mm inside the prime focus...which means that the "normal" focus should be set that far behind the rear cell before adding the reducer. Or that the focuser must be significantly rotated to give this distance.

Ken

[mark123]

I think that is something to try another evening Merlin. I will let you know the results.