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Siebert 0.6X OCA for SCT
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By Suphot Punnachaiya
This is my personal experience with Siebert 0.6X OCA that I would like to share with you. My comment base on what I feel and I am not professional or expert astronomer, so this is not professional review of Siebert 0.6X OCA for SCT. But if you are the one who looking for low power wide field with Binoviewer, this comment may be your additional information.
I am a part time sky watcher in Bangkok, Thailand. My equipment is Celestar 8 and Burgess Binoviewer with 20mm Binolite eyepieces that I purchased from High Point Scientific. The Burgess Binoviewer give me a nice and comfortable view compare to single eyepiece mode. But when I need for wider field of view, my choice is purchase a pair of wide angle low power eyepieces or focal reducer. I go for 0.6X focal reducer from Siebert Optic that can work with my Burgess Binoviewer. Because it is cheaper and I think it is easier to use than switching a pair of eyepieces for wide field. Another concern is clear aperture of my Binoviewer may vignette wide angle eyepieces when their field stop is larger than 21mm which most of wide angle eyepieces are.
My 0.6X focal reducer is cell type, so it look like a 1.25 inch filter. It is very light weight. My focal reducer has some dirt on the thread, so I have to clean it by cotton bud. The lens assembly is clean and look good but it is not Multi-coat optic. The reflection on lens look purple like my prism diagonal which is single coat. The cell unit is not thread in the front for stacking standard 1.25 inch filter. If you need to stacking filter with this unit, you should stack this unit in front of other filter.
Celestar 8 is 2,000 mm focal length F/10, but when use with Binoviewer, look
like it magnify my Celestar 8 focal length about 30%. The effective focal
length is around 2,600 mm. When I use 20mm Binolite eyepieces, it give me
130X magnification with field of view 0.46 degree (assume Binolite has 60
degree viewing angle, so 60/130 is around 0.46 degree). Looking at the Moon,
I almost fit the whole Moon in the view, which confirm this assumption. You
can see in this simulated view.
The Siebert web site instruct to put this focal reducer in front of Binoviewer. It will give 0.6X focal length reduction to around 1,560mm with 0.77 degree field of view. But my view on the Moon look like it is not 0.6X reduction, it may look like 0.7X focal reduction in my opinion. I can see the whole Moon with some space around in the field of view. From my approximation it look like 1,820mm with 0.66 degree field of view. The following photo is simulated view when use the reducer.
I have not see any vignette in the field of view. The bright city sky in Bangkok look uniform and flat. The star in center and at the edge look good to my eye. No obvious aberrations that I can see. I can pan my Celestar 8 across the sky without pincushion in the view. Star in center and at the edge show no color aberration. Optical of this focal reducer perform very well for me. But the whole filed of view not seem to look brighter to my eye compare to normal view without the reducer.
When I view M42 in Orion and want to use my Lumicon UHC filter then I have a problem. My Lumicon UHC filter and 0.6X focal reducer is not thread in their front to accept standard 1.25 inch filter. So I can not stacking them together, all I have to do is put my UHC filter in front of my prism diagonal and leave 0.6X in front of Binoviewer. This way work but not convenience to me. But if your light pollution filter is thread in the front, it should not be a problem.
After viewing M42 for a while, the view is good, more sky to see compare to normal viewing without reducer. Then I think of switch the Focal reducer in front of my prism diagonal in place of UHC filter to see what will happen. At first I think my Celestar 8 will not reach focus with this configuration, but it did. The whole field of view in this configuration is wider. In this time I can see the Moon and a half in the field of view. That mean it look like 0.6X reducer with 0.77 degree field of view and the contrast is good. I can not accurately measure filed of view, so it is all my approximation.
Again, it did not show any aberrations, the field is flat and uniform. Star is sharp almost to the edge and no vignette. I really like this configuration because of the wider field of view. The field curvature is not obvious notice to my eye. You may need to refocus a little bit for sharper edge of field, but it not necessary for me. I can enjoy watching the sky with both eyes for a long time without to refocus back and forth for a good view. This mean I have 2 wide fields from this one Focal reducer, depending on the position of reducer on the telescope.
What I like in this 0.6X focal reducer is, it is small and light weight. The optical quality is good and easy to use with Binoviewer. No aberrations that I can see. They can focus when attach in front of my diagonal, give me more wider field of view in the price of one. If you have 2X Barlow lens, you may try put reducer in front of diagonal and put Barlow in front of Binoviewer. This configuration give you in-between magnification around 1.5 or 1.6X, a multi-magnifications for free.
What I think it should be improve in this unit is, it should be Multi-coat optic to improve light transmission. The aluminum cell that hold the lens is a bit difficult to hold and attach it to the Binoviewer. It is not black anodize as well. So if Siebert Optic mount this reducer in standard 1.25 inch filter style cell. It will be easier to hold and attach to Binoviewer. Black anodize will reduce light reflection and looking better on my preference.
If you are looking for wide filed Binoviewer with SCT, this 0.6X reducer may be your good option. It is not show any obvious aberrations to my eyes and I can not see vignette in the field of view. Low power wide field eyepieces may be better option, but more expensive and if you use 200$ Binoviewer like me, the small size of clear aperture in Binoviewer may cause vignette when use with Low power wide field eyepieces.