Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

SCT focal reducer question.

This topic has been archived. This means that you cannot reply to this topic.
9 replies to this topic

#1 LakeFX

LakeFX

    Mariner 2

  • -----
  • topic starter
  • Posts: 270
  • Joined: 02 Jan 2007

Posted 19 October 2010 - 09:25 AM

I'm a bit confused by the focal reducers available for SCT scopes. I've heard a lot of bad things and a lot of good things about them. So i have a couple questions.

I have a meade SC-8 UHTC scope.
I have two T-Adapters. One is a 2" sct-type that connects to the visual back directly. The other is just a 2" tube type.

Which Focal Reducer should I buy? Antares, Celestron or Meade. Is there a better one than these? Should i get the NexGen .5? and will i need another T-Adapter for any of them?

#2 Bachus

Bachus

    Surveyor 1

  • *****
  • Posts: 1,680
  • Joined: 25 Nov 2005

Posted 19 October 2010 - 06:42 PM

First of all...check over at the SCT/CATs Forum and do a search. The "FR" question comes up numerous times.

As to "good & bad" that depends on the user and use of I guess. As to models some time back there were issues with certain Meades but that's all I can say. I have the Celestron that's about three years old and love it. It'll take your 8" 2000mm and drop it somewhere around 1200mm which gives you a bit wider of a field for viewing. It's like having two scopes basically. Generally you want it as close as possible to your primary mirror which means it screws directly to the back. It has female treads on the backside which you can use various adapters or a sct type diagonal or whatever. From time-to-time you can snatch these up for around $80.00 used. Hope this helps a bit...


Kenny J

#3 LakeFX

LakeFX

    Mariner 2

  • -----
  • topic starter
  • Posts: 270
  • Joined: 02 Jan 2007

Posted 19 October 2010 - 08:53 PM

Thanks. I'll take a look over there and see what I can find out.

Is it possible to use the .5x reducer with an APS-C DSLR?

#4 ccs_hello

ccs_hello

    Voyager 1

  • *****
  • Posts: 10,408
  • Joined: 03 Jul 2004

Posted 19 October 2010 - 09:27 PM

I'd suggest Celestron or (now fixed) Meade 0.63x Focal reducer/Field Flattener (with SCT thread designed for SCT). 0.5x is pushing it too hard and significant vignetting will show in your APS-C sized DSLR.
Some even not feel comfortable with 0.63x and choose 0.8x FR/FF.

Clear Skies!

ccs_hello

#5 vorkus

vorkus

    Viking 1

  • -----
  • Posts: 563
  • Joined: 18 Aug 2010

Posted 21 October 2010 - 12:32 PM

I'd stick with the 6.3 reducer. I have a C8 and the Celestron reducer and it works fine with a SCT to t-thread adapter. I was hoping to use a flip mirror but no dice. It serious tunnel vision with my Canon 1000D. For now I have to unscrew my diagnol once done with alignment and then screw in the camera. For the person who invented SCT threads, "may he forever live under cloudy skies".

John

#6 anemec

anemec

    Mariner 2

  • -----
  • Posts: 245
  • Joined: 26 Jan 2010

Posted 21 October 2010 - 04:50 PM

I have been dealing with this exact issue over the past 2 weeks. I have an 8" LX200 and the Meade 4000 f/6.3 which when I first imaged with it drove me nuts because of the field rotation.

The problem was the spacing between the FF/FR and the sensor.
Pat Holland helped me out a great deal in this thread.

So after reading up on it, I went back and reduced the distance between my FF/FR and my chip sensor.

Here is an image from before I corrected the spacing issue. And here is an image from after I corrected the spacing issue.

Now, I still do get significant vignetting if I use the FF/FR coupled with my IDAS LPS. At least I did last night. Even with Flats. But I have imaged for almost a year at f/10 and to be able to pull out nice details like that M27 shot with only 45x75 sec subs was something I didn't think I'd be able to do prior to using that FF/FR.

Just my 2 cents.

#7 Falcon-

Falcon-

    Aurora

  • *****
  • Posts: 4,950
  • Joined: 11 Sep 2009

Posted 21 October 2010 - 05:01 PM

Sorry to sidetrack, just a minnor correction, the issue you had before tweaking your spacing was Coma or Field Flatness problems, not Field Rotation. Field rotation is a mechanical thing while coma or field flatness are optical problems. Field rotation occurs when you have a bad polar alignment such that improper tracking results in the whole image being rotated - the result is stars at the edge that are arcs around the center of the frame (or around the guide-star if autoguiding). This is a particularly huge limiting factor on alt/az mounts without a wegde since the alt/az tracking motion always creates field rotation.

#8 PatHolland

PatHolland

    Vanguard

  • *****
  • Posts: 2,455
  • Joined: 11 Oct 2009

Posted 21 October 2010 - 05:29 PM

I'm a bit confused by the focal reducers available for SCT scopes. I've heard a lot of bad things and a lot of good things about them. So i have a couple questions.

I have a meade SC-8 UHTC scope.
I have two T-Adapters. One is a 2" sct-type that connects to the visual back directly. The other is just a 2" tube type.

Which Focal Reducer should I buy? Antares, Celestron or Meade. Is there a better one than these? Should i get the NexGen .5? and will i need another T-Adapter for any of them?


Hello LakeFX. I am going to try and help you the best I can with this FR choice you have. The first one we'll try to help you with is the choice of reducer. The Meade and Celestron 6.3 focal reducers are very similar but have slightly different spacing for a proper F 6.3 focal ratio. The Meade is 95mm and I believe the Celestron is 105mm (someone correct me if I am wrong). Otherwise, they'll deliver the same and perform the same. You can increase or reduce the focal ratio by changing the spacing from the imaging chip to the sensor, I.E. 55mm on the Meade reducer is F7.9 (which for me is another sweet spot). I have the Antares 2" filter-type reducer that is a .5 reducer/corrector and have not done enough testing with it to comment (I need to buy more spacers for effective testing with the Antares) but the Antares F6.3 SCT reducer is not a corrector, just a reducer (according to scopestuff.com) so this is a different reducer with different properties. I do not own one so I cannot comment on it's abilities at all. The trick and the more important thing to remember about how focal reducers work, is the spacing. This is where I believe people have bad experiences with focal reducers. Spacers for every type of camera and/or optical train are avilable through scopestuff.com and even here at Astronomics. Getting to the "sweet spot" is what this is all about. Another thing to remember is that you WILL get vignetting with a FR unless you are using a very small chip camera. There are 2 ways to reduce/remove the vignetting.
1. Space the FR and chip closer together (F7.9) and there is almost no vignetting at all.
2. Take flats. This is what I do even if I image at F7.9. It works great every time.
I am frequently asked about the focal ruducer thing and I have promised many a review on reducers, how they work and best case scenarios for chip sizes and best matched reducers for them. I have resolved to get this done in the next month. In the case of larger chip imaging, I recommend the Meade or Celestron F6.3 reducer. I don't know enough about the antares SCT 6.3 to recommend it at this point. Hope this helps. Also, look at the link in Adam's thread where I go into spacing of the optical train. Rest assured, I will write a review that will include part numbers and photos of all the spacing options I use w/ the focal reducers I own. I hope this helps.

#9 Bachus

Bachus

    Surveyor 1

  • *****
  • Posts: 1,680
  • Joined: 25 Nov 2005

Posted 21 October 2010 - 06:03 PM

I think the spacing or ideal distance from FR to sensor is 103-110mm

#10 anemec

anemec

    Mariner 2

  • -----
  • Posts: 245
  • Joined: 26 Jan 2010

Posted 21 October 2010 - 06:42 PM

Sean, you are right. I keep saying that incorrectly for some reason.


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics