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Old 16-05-2019, 08:21 PM
Averton (P and C)
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Focal reducer question

Not strictly a DIY project but hopefully the DIY people will have the answer. We purchased a cheap 0.5x focal reducer which when we used it had the most awful image imaginable. All stars had a flare coming out one side that made them look like small street lights. When we disassembled the reducer it had 2 elements cemented together. One is a normal looking convex lens and the second is concave on one side glued to the lens and flat on the other side. The elements were mounted in the holder so that the convex lens faced the camera or eyepiece and the flat side faced the telescope/subject. This appeared incorrect to our thinking so we reversed it and have only had one chance to test it thus far but it was a huge improvement. No more flaring.
Questions - which way round is the correct way? Is it possible that the manufacturer assembled it incorrectly or didn't know the correct way around? Pictures on line of this reducer seem to show the elements mounted as supplied to us it does not appear to be a one off mistake.
Thanks for your help.
Peter & Clare
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Old 16-05-2019, 08:50 PM
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Ukastronomer (Jeremy)
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You will need to supply more pertinent information than that I am afraid

you don't say the make of the FR, show an image of the FR and what you have shot, what scope it is used on, what cameras yo0u are using
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Old 17-05-2019, 12:25 AM
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mental4astro (Alexander)
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P & C,

I have a couple of those cheap 0.5X reducers. One is a GSO, and the other a Svbony (really cheap). The GSO one works really well. The Svbony is a shocker, same scope and camera, with terrible flaring as you described. The two reducers are of slightly different design and appearance.

Your experience and what you did in reversing the position of the elements have given me an idea to do the same with this Svbony item - nothing to lose by doing so.

Yes, it is often the case an issue with QC with many Chinese made products. It only takes on element to be carelessly reversed, or an incorrect spacer ring, or any other small component to be wrong, and the whole unit you are using is rendered useless. The is mostly the case with the really cheap stuff. With high end products there is tighter quality control, and duds are very rare.

There is another possibility - did you use that focal reducer in a Newtonian/dob? These reducers will not work well in Newts, and often will result in flaring and even ghosting. These cheap focal reducers are designed to work best in refractors, SCT's and Maks. And even with say refractors, these modest reducers are only a "general purpose" item, and some small flaring will be noticeable particularly as the chip size of your camera gets larger or when used with eyepieces. For a perfectly formed, flat and pinpoint star field across the entire chip, a scope-specific reducer/flattner needs to be used. What works with a standard f/10 SCT will not work with a refractor, even if it is also f/10, and a reducer/flattner for an EP100 scope will not work with an ED80.

Alas focal reducers, just like eyepieces, do not follow the notion of "this is a scope, this is an eyepiece, it will work". Optics are more complicated than that.

As for a focal reducer for Newtonians, I've heard of one or two, but I've never seen one, and have not read reports from anyone that's used one, only "spin" from the businesses selling them, and spin is not something I base my purchasing decisions on.

Like Jeremy said, if we knew what scope you were using it in, and how (that is visual or with a camera), and maybe a photo, all of this will help us give you the best info possible for your question.

Alex.

Last edited by mental4astro; 17-05-2019 at 12:45 AM.
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Old 17-05-2019, 08:38 AM
Averton (P and C)
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Hi Alex & Jeremy, Thanks for your replies.
We had not wished to mention the manufacturer in case it was just our inexperience. However, Alex, you have nailed it in one - it is the SVBony.
We are using it in a Newtonian 130mm F5 scope with a Toupcam GP701200C td p { margin-bottom: 0cm; }p { margin-bottom: 0.25cm; line-height: 115%; }(marketed in Australia as Prostar). We think you have answered our question. It appears unlikely that its going to be much use. We would however say that reversing the elements has made an amazing difference so we will try some subs and see if we can get a reasonable result. Its worth a try. It might be worth a try with yours as well Alex.

Peter & Clare
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Old 17-05-2019, 10:24 AM
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mental4astro (Alexander)
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P & C,

Note also that when using a reducer on a Newt, coma will also become more apparent around the edges, and there will be other aberrations visible too which may include field curvature and spherical aberration (due to the reducer, not the scope if it's using a parabolic mirror).

If this set up is for astrophotography, this is not the right tool for the job, be it a Newt or any other scope. In a Newt you will need a coma corrector. In a refractor a reducer/field flattner for the specific apochromatic scope you are using (there are not scope specific reducers/flattners for achromatic refractors). SCT's, those specifically made for the optical design used by your SCT, standard or Edge or ACF - all are different. There may be a reducer for Maks - I seem to recall something along those lines, but as Maks are typically so long in focal ratio, it is somewhat futile to use a reducer with them.

If it is for video astronomy for an on-screen fix, then you may be happy to tolerate the coma with a Newt, or a little bit of field curvature in a refractor when using one of these inexpensive reducers. Or you may still prefer to look into using the appropriate corrector unit.

Note also that there is a significant difference between "reducer" such as this inexpensive 0.5X item, and a "reducer/flattner/corrector". An inexpensive reducer is just a simple lens arrangement. This is why these are so cheap. A reducer/flattner/corrector is more complex as it also produces a flat focal plane that is necessary when projecting the scope's image onto the flat chip our cameras have. This is even more important as chip size gets larger and larger. As a result these items are much more expensive and very scope specific as these need to be designed for the very specific curved focal plane each scope produces, which includes scope design, focal length & focal ratio, all of which mean very exacting design requirements. A coma corrector does the same field flattening job but for Newtonians.

And here we all are when we start out in Astro that this is all easy! It can actually very quite technical and complex... if you want it to be.


Thanks for noting your experience with the modification you did to your reducer and when you used it in your Newt. I will do this mod too and see what results I get with a Newt, a fast achromatic refractor and an ED80. I only indulge in video astronomy by way of imaging, but if I can find ways to better control aberrations, then I'm all for it

Alex.
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Old 17-05-2019, 09:41 PM
Averton (P and C)
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Hi Alex, Thanks for the clarification regarding these devices. At this stage our expectations are very low. Really we are just experimenting and learning as we go. For the money the reducer was worth a try.
P&C
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Old 31-05-2019, 08:15 PM
Averton (P and C)
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Accepting all of the limitations that have been pointed out for the focal reducer with our equipment, we still took the opportunity to try the reducer with the elements in the reverse direction. It took a while to get the weather to co-operate but we finally had an opportunity last Sunday am.
We fully understand that the attached pictures are of very poor quality with many issues but they do seem to indicated that the problem with the reducer has been mitigated to some degree. The picture of Southern Pleiades is a single exposure taken on 6/5 with the 0.5 reducer in its original configuration which shows the "headlight" effect on the stars. The picture of the Butterfly Cluster was taken on 26/5 with the elements reversed in the reducer. It is a stack of 15 subs at 4 sec exposures and even though we still have significant issues to fix, the "headlights" and flaring appear to have gone.

We understand that the stars are not round, our biggest issue with this is movement caused by our DIY tracking platform which is limiting exposure time (working on this). Even at 4 secs there was really no usable exposure. Also attached is an image of the moon taken on the same occasion as the Butterfly Cluster, using the reducer which also appear to have worked reasonably well. Once again this is with the elements reversed. Without the reducer we cannot fit the moon in a single image.
We can now achieve focus using the reducer and it is at least suitable for our standard of imaging.
Peter & Clare
Attached Thumbnails
Click for full-size image (Southern Pleiades0048.jpg)
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Click for full-size image (Moon redu_g5_ap59_Drizzle30_Registax_GIMP.jpg)
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Click for full-size image (Butterfly Cluster_DSS_Darktable_GIMP cropped_Registax.jpg)
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