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Post by jamesling1000 on Jun 1, 2012 21:58:22 GMT -5
Hi All...... I am just wondering anyone using focal reducer for their long focal length refractor? The common one that people used for their CG scope are either celestron and meade, which reduces the focal length from F10 down to F6.3......, which I have the meade type....for my C8 and C14..... As I am curious to find out whether the meade FC will also work for both my C6R F8 and 8" Istar F8.8, which if it works, I am able to change it to F6 or lower.... And I am very sure it should work simply they're quite close in the FL for both CG and refractor scopes that I have.... Of course is not that direct to mount the meade focal reducer onto the refractor. Celestron and meade has a threaded visual back, while refractor is fitted with a focuser, and the end is a sliding tube with locking screws.... I went ahead , without asking around, from the expert point of views or advice, and get my machinist to machine 2 attachments. One attachment is a threaded end to thread onto this meade Fc and other end insert into the refractor focuser.... The other attachment is also a threaded end to other end of the meade FC and other end is has a 2" opening for inserting the eyepiece. The above method can only be used directly with the eyepiece, coz when inserting a diagonal, the focusing will not be able to achieve, based on my preliminary testing without the attachment...... I just received my 2 machined attachments yesterday, and after rushing back to from my oversea workplace, was not able to setup and testing it, coz the weather is really bad... I will try it out over the next few days, as I am on vacation leave preparing for this coming Venus Transit.... My real intention of mentioning using of focal reducer in this forum, as I think many of our ISTARSCOPECLUB members owned refractors. For those of the members with long FL refractor(s), you may already been using FC, which is also great to hear from you, your experience on using them.... My understanding of FC is also a field flattener, so isn't it is going to improve the image......over the full FOV?  ? And my another intention, is to see whether the result is also giving us a better, sharper and better contrast image, and perhaps lower CA? And is especially of great interest to most of us who owns achros.... Perhaps only experts like Ales and Mike are able to tell us more on all the advantages of using focal reducer for refractor, since most of us know how it benefits the older CG scope, against the latest Celestron edge HD scopes.... Regards James Ling Attachments:
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Post by Mike on Jun 3, 2012 19:31:44 GMT -5
James, there is no reason the 6.3 focal reducer/corrector won't work on your refractor. There may be a couple of issues though. Spacing between the focal reducer and the image plane is very important for the performance of these reducers. Also, there could be vignetting. Although reducers are achromatized using crown and flint glass (usually BK7 crown and F2 flint) it's doubtful there will be any reduction in chromatic aberration with a short focal length achromatic scope. I've only seen SCT reducers used on APO triplets where CA was not an issue. Since you already have the necessary adapters to try it, I'll be anxious to hear your results. One more thing... having a guy who will machine these custom adapters for you is a valuable commodity  Mike |
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Post by jamesling1000 on Jun 4, 2012 20:21:36 GMT -5
Hi Mike.... Will bring along my meade FC to the university for tonight star gazing activities, which will continue until tomorrow noon time, for this century Venus Transit..... As for machinist that I have sought help for my scope accessories fabrication, I will say I rather have no choice, as many of us who live in Cities and apartments, do not have the facilities to setup a mini workshop in our home for machining.... But i am lucky that I have 2, one in my own country, and the other, in the other oversea country I am working.......And best of all, they are also interested in Astronomy....... Hi All. I am very sure many members here also have been fabricating accessories for their scopes, as manufacturers does not cater their design to suit the end users..... And one good example is PRO mount from Celestron, as well as the G11 mount are not built with the ability to set it at latitude zero, and my country is at latitude 1 deg N? So that is how local machining comes in...... Hope members can put their designs into this forum, although it may not related to ISTAR products, but at least generates more interest and participation....., and after all, a scope like ISTAR still need to be used....., hahaha..... REgards James Ling |
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Post by tonybegg on Jun 23, 2012 0:07:00 GMT -5
James - this is interesting because I have been wondering about the opposite - increasing the focal length of a scope like your 8 inch f/8.8. I know that although this would apparently increase the focal ratio it does nothing for CA because both the image blur and Airy disk increase in exact same proportion. But might it not increase the depth of focus? Turns out it can only do this if the Barlow or PowerMate goes ahead of the focusser (otherwise it is too late). In fact the ability to insert optical accessories ahead of the focusser might be a good idea for other reasons (such as a TriSpace like, or possibly Raycorr CA compensator). I remember observing Mars with my 127 mm f/7.5 APO refractor with a 4mm TV Radian EP and being a bit frustrated with how the seeing interacted with the focussing (plus a stray cat I call Tycho kept rubbing against the tripod legs) and wondered whether I used a longer F/L EP and a Powermate it would help - but apparently not unless the focal increaser is ahead of the focusser. That's what I read in a book anyway so it must be true :-). clears skies Tony.
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Post by jamesling1000 on Jun 23, 2012 10:59:41 GMT -5
Hi Tony...
Although my 8" ISTAR is F8.8, with my TV 55mm 2" ep, is giving 33X power ....
But if can further lowering down, if I add the focal reducer... that will be great for DSO....
Hi All...
Again tonight, with the weather extremely clear , after a hot and sunny day....,
I test the meade focal reducer on my ISTAR.... and it looks great when I point it at the crescent moon......but is only for straight through visual.....not using the diagonal...
I used my baader zoom ep, so that I can adjust the power to compare with and without the focal reducer....
I believe the slight reduction of CA is due to the light that travel through the focal reducer, dims down slightly, but is not eliminating it....
After testing the focal reducer, I move to Mars, and now is already very small even at 450X, but still very glaring...and the polar cap is easily seen, unlike few months ago when is bigger, the polar cap is less distinguished.
After this short session, I dismantled the scope from the mount, as I have to go out early next morning.......
REgards
James Ling
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Post by jamesling1000 on Jun 24, 2012 8:27:12 GMT -5
Hi All.... The following photos which I have taken last night, will briefly explain what I have said earlier about the meade focal reducer and CA level reduction. The following photo is shot using a canon DSLR, which shows roughly the level of CA you expect to see from the 8" ISTAR achromat...... REgards James Ling Attachments:
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Post by jamesling1000 on Jun 24, 2012 8:33:40 GMT -5
Hi All.... You can improve photo taking result, especially the CA level by lowering the ISO level....(from 1st photo ISO 400 to improved one using ISO 200), same exposure time..., but this is not actual visual result, but photo result.... Note:- these photos are taken without any filter.... Regards James Ling Attachments:
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Post by jamesling1000 on Jun 24, 2012 8:47:44 GMT -5
Hi All... As I have earlier explained that if using the meade focal reducer, you can only use the eye piece directly inserted at the end of the FR, and both the diagonal or the camera is not able to come into focus..... So I am trying to demonstrate the CA level by inserting my baader zoom ep, and adjust to a similar magnification level, then use my DSLR to shoot this photo, over the eye piece lense surface.... to show a similar level of slightly reduced CA...., in this last photo... I will say if use on DSO, I can get a lower magnification , thus bigger FOV... For planetary result, I may have to do another round of comparison next week when I am back from my oversea job assignment again.... REgards James Ling Attachments:
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Post by tonybegg on Jun 24, 2012 14:33:47 GMT -5
Fascinating James. Just a light violet around the limb. The (usual) figure of merit (FOM) is just 1.1 for this telescope (I am thinking focal ratio 8.8 divided by aperture in inches 8). Fraunhofer's Dorpat refractor was 9.6 inch f/17.7 giving an FOM of 1.84. The Mt Stromlo replacement Tim Wetherall made with a 220mm f/15 achromatic Istar lens has an FOM of 1.73, Istar's shortest 6 inch Perseus (f/10) is 1.69, its longest (F/15) is 2.54. The Lick 36 inch (f/19) is only 0.53. You can see I have been worrying about CA with achromats :-). Best Wishes Tony
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Post by jamesling1000 on Jun 25, 2012 5:47:32 GMT -5
Hi Tony..... Yes, is strange why for 8" at F8.8, with FOM 1.1 can perform better than my 6" F8 achro, FOM 1.33, in terms of CA and performance.... Even my astro friend, who has the 6" F8 TMB triplet APO, also make such a comment, when we did the 1st light of this 8" ISTAR when I received it................................. vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv It's an achromat after all, so of course we expect to see CA. However the visible purple fringe is apparently less than what I would theoretically expect of a 8" f8.8 achromat. I observed that the CA is a deeper violet, which suggest to me it may be deliberately designed with some kind of a shift in chromatic focus, as compared to other achromats. This CA in the 8" seems less extended in area and dimmer than my previous C6R 6" f/8 achromat from my earlier observations. In fact, after I stopped down the C6R to effectively perform as a 4" f/12, the resultant CA is reduced by half, but it still seem not as good as this 8" achromat, if my recollection is accurate. It would be interesting to compare it side by side, but I don't have the C6R lens with me anymore. I was quite pleasantly surprised with the reduced CA in this 8" f/8.8, so I did some searching, and found that someone else had also noted that this Istar 8" f/8.8 lens has less CA than a C6R. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Hi All. The full detail can be found in this link, whereby a forum discussion on the CA of this 8" ISTAR :- www.singastro.org/forum2/viewtopic.php?f=3&t=8763&start=20Regards James Ling |
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