· Get a Cloudy Nights T-Shirt · Submit a Review / Article Click here if you are having trouble logging into the forums Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User You are not logged in. [Login] Entrance · Main Index · Search · New user · Who's Online FAQ · Calendar Equipment Discussions >> Video and Electronically Assisted Astronomy Previous   Index   Next   Threaded   Pages: 1 rhymeswithorange member Reged: 10/09/10 0.5x focal reducer necessary for 12" dobsonian?       #5273804 - 06/16/12 12:41 AM Edit   Reply   Quote Quick Reply   Hi, I just bought a samsung sdc-435 (s-2000) as well as all of the accessories I need for it (ie. c-adaptor and cables). The one accessory I'm missing is a 0.5x focal reducer (which apparently seems to be sold out everywhere ie. GSO and Antares brands.). However now that I think about it my dobsonian is already a f/4.9. Would it even be necessary? Edited by rhymeswithorange (06/16/12 12:42 AM) Post Extras:       GlennLeDrew Postmaster Reged: 06/18/08 Loc: Ottawa, Ontario, Canada Re: 0.5x focal reducer necessary for 12" dobsonian? [Re: rhymeswithorange]       #5273831 - 06/16/12 01:21 AM Edit   Reply   Quote Quick Reply   For brighter targets you may be able to get by at the scope's native f/4.9. But dimmer targets will really benefit from reduction because of the better signal to noise ratio. A 0.5X reducer makes the image 4X brighter. Just last night I had my Mallincam working on just this scope, which belongs to a friend. I started out with my 0.5X reducer (the Mallincam MFR-5), and am glad I did. Even working at ~f/2.5, the minimum exposure which did justice to the variety of objects I imaged was 15 seconds, with the gain (AGC) set to the halfway point on the slider. Now, the sky was quite dark, with M13 easily seen with the unaided eye. The dark background does require a longer exposure in order that it be not black. (I like my galaxies and nebulae to reveal the faint bits, and to see this some sky glow must be seem on screen.) Under light pollution, the exposure will be shorter, which will still show reasonably faint stuff--relatively speaking--albeit at rather lower contrast. Of course, a light pollution filter will darken the sky, possibly warranting an increase in exposure. The bottom line, a focal reducer is VERY handy for video work, even if you don't use it regularly. And when your camera has limited maximum exposure time, this is especially true. If you can't find such a reducer, obtain a busted 25mm aperture bino, remove a good objective and mount it in a 1.25" filter cell with silicone. Note that in any case, this, or the commercial version which uses the same lens, must be located some 45-50mm from the detector in order to work at 0.5X. Post Extras:       Lorence professor emeritus Reged: 09/15/08 Re: 0.5x focal reducer necessary for 12" dobsonian? [Re: rhymeswithorange]       #5274393 - 06/16/12 01:48 PM Edit   Reply   Quote Quick Reply   Asking if a focal reducer is necessary is the same as asking if it's necessary to put sugar in a cup of coffee. How will you know if you like it unless you try it? It's all about field of view. You will likely find yourself wanting a wider view that you will get with the .5. I suggest you get an MFR-5. It has two cells. The first one is a 0.8X reduction and the second is a 0.55X reduction. They can be used independently or together and with an optional spacer to to achieve f/ratios between .8 and .3. With the f/ratio of your telescope you will want a barlow as well. I use a 3x on an old f/6.5 C5 telephoto lens. Post Extras:       StarStuff1 Carpal Tunnel Reged: 04/01/07 Loc: South of the Mason-Dixon Line Re: 0.5x focal reducer necessary for 12" dobsonian? [Re: Lorence]       #5274470 - 06/16/12 03:01 PM Edit   Reply   Quote Quick Reply   IMHO a focal reducer is a necessary as a monitor when using an astro cam. Post Extras:       rhymeswithorange member Reged: 10/09/10 Re: 0.5x focal reducer necessary for 12" dobsonian? [Re: StarStuff1]       #5274508 - 06/16/12 03:33 PM Edit   Reply   Quote Quick Reply   Thanks for the responses you guys, it really means a lot. How would the quality of this 0.5x focal reducer compare to a GSO or Antares model? http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&item=280385585515&viewitem=#ht_865wt_932 Post Extras:       rhymeswithorange member Reged: 10/09/10 Re: 0.5x focal reducer necessary for 12" dobsonian? [Re: rhymeswithorange]       #5274597 - 06/16/12 04:50 PM Edit   Reply   Quote Quick Reply   I actually just found a GSO 0.5x focal reducer online so it looks like my setup is as good as I wanted it to be. Before I let you guys go, I was wondering which IR filter you would recommend: Baader UV/IR cut filter http://agenaastro.com/baader-1-25-uv-ir-cut-filter.html#Reviews or Astronomik IR block filter http://www.optcorp.com/product.aspx?pid=1621-3179 Edited by rhymeswithorange (06/16/12 04:54 PM) Post Extras:       StarStuff1 Carpal Tunnel Reged: 04/01/07 Loc: South of the Mason-Dixon Line Re: 0.5x focal reducer necessary for 12" dobsonian? [Re: rhymeswithorange]       #5276114 - 06/17/12 07:57 PM Edit   Reply   Quote Quick Reply   FWIW I use an Orion photo imaging filter (SkyGlow, I think). Seems to work fine. Post Extras:       ccs_hello Postmaster Reged: 07/03/04 Re: 0.5x focal reducer necessary for 12" dobsonian? [Re: rhymeswithorange]       #5276628 - 06/18/12 06:26 AM Edit   Reply   Quote Quick Reply   The OP's Newtonian OTA has a fl=1500mm. I guess the Dob is not on EQ platform or adapted for tracking. I would say SDC435's max. 8 sec integration time probably is the limit for untracked Dob at focal length in the long end, depend on which sky area it is pointing to. With half of the fl, it works out better. A mjor issue for a Newtonian with an add-on F.R. is insufficient inward focuser movement. A 0.5x F.R. (typical fl=100mm) at 0.5x setting will require half of that focal length (i.e., 50mm) inward movement. Most of the Newt can't achieve that, unless either move up the mirror or redrill the focuser mounting hole. Clear Skies! ccs_hello Post Extras:       GlennLeDrew Postmaster Reged: 06/18/08 Loc: Ottawa, Ontario, Canada Re: 0.5x focal reducer necessary for 12" dobsonian? [Re: ccs_hello]       #5276860 - 06/18/12 10:29 AM Edit   Reply   Quote Quick Reply   The 12" f/5 of my friend's is the Skywatcher version which has three sliding poles, allowing the upper 'cage' to slide well down fir more compact storage. To accommodate such in-focus issues, he made three equal-length spacers from PVC tubing, each cut so that they snap on and off the poles easily. In this fashion, a focal reducer is easily accommodated, and by happy accident the collimation is not altered sufficiently to bother with. Post Extras:       rhymeswithorange member Reged: 10/09/10 Re: 0.5x focal reducer necessary for 12" dobsonian? [Re: GlennLeDrew]       #5279751 - 06/19/12 10:21 PM Edit   Reply   Quote Quick Reply   Thanks again guys. Could you help me choose an IR filter though. I'm looking at deciding between these now: 1. Baader UV/IR cut filter $67 2. Astronomik IR block filter (I'm thinking about the astronomik ir CCD filter rather than the 742 or 807 filters) $39.95 3. OPT brand IR blocking filter $29.95 4. OPT brand uv/ir block filter. $39.95 Post Extras:       mclewis1 Thread Killer Reged: 02/25/06 Loc: New Brunswick, Canada Re: 0.5x focal reducer necessary for 12" dobsonian? [Re: rhymeswithorange]       #5279785 - 06/19/12 10:37 PM Edit   Reply   Quote Quick Reply   With an all reflective surface (mirrors only) scope like a Newtonian you may not need the IR cut/block filter. Yes the small glass elements in the focal reducer might have an effect on the focus points for IR light but I'll bet it won't be a big deal. I'm going off of theory here, it would be great if someone with a Newtonian scope could confirm this. Oh and the Astronomik 742 and 807 are IR pass filters not cut/block types, they would actually be exactly the opposite of what you'd want since they only pass (and not block) IR from the 742 or 807 angstrom points. They are used for planetary imaging where it seems our atmosphere is a bit more stable at the longer (IR vs visible) wavelengths so they pass light starting only in those deep red/IR regions. As for the choice - well for some objects you actually want some of the deep violet/UV data so I'd choose the Astronomik IR Block filter ... but perhaps only if someone can confirm that my comments above are incorrect and an IR filter will actually be beneficial with a Newtonian scope with a focal reducer. Post Extras:       GlennLeDrew Postmaster Reged: 06/18/08 Loc: Ottawa, Ontario, Canada Re: 0.5x focal reducer necessary for 12" dobsonian? [Re: mclewis1]       #5279931 - 06/20/12 12:13 AM Edit   Reply   Quote Quick Reply   Reflective optics by themselves, of course, focus all wavelengths equally, and so no 'off band' de-focusing will occur. Being a near-to-focus optic, a focal reducer should not introduce any significant de-focus of light at the extremes of the camera's spectral range of sensitivity. I believe an IR/UV cut filter should not be required. At any rate, do examine star images carefully to first ascertain the state of focus before splurging on such a filter. I would certainly not use one unless really necessary. Unless the filter removes more light pollution than it does desired light from the target? But then, an exposure-time-limited camera might well benefit more from every photon admitted through the system. Post Extras:       Pages: 1 Previous   Index   Next   Threaded   Extra information 7 registered and 4 anonymous users are browsing this forum. 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