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

Problems with Celestron F/6.3 Reducer-Corrector

This topic has been archived.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

#1 Alexander Vasenin

Alexander Vasenin

    Sputnik

  • -----
  • topic starter
  • Posts: 29
  • Joined: 27 Sep 2015

Posted 01 October 2015 - 06:26 PM

Hi everyone,

 

I've recently started experimenting with astrophotography. I'm using Celestron 8" SCT (not EdgeHD) on Advanced VX mount with Canon 600D attached via T-adapter and Celestron F/6.3 Reducer-Corrector #94175. For guiding I'm using Celestron GuideScope with NexGuide.

 

I've run into following troubles with reducer-corrector:

 

1. A bright stars near the edge of the frame creates big oval-shaped artifacts (crop #1). I don't see such artifacts without reducer-corrector. Is it normal? Is there any way to get rid of them?

 

2. While stars at the center of the frame are reasonably round (crop #2), near the edge of the frame have a strange, non-round shape (crops #1 and #3). I thought a corrector supposed to fix those things, while in fact it make it much worse. What's wrong with my setup? Do I need some kind of spacers as "Getting started: long exposure astrophotography" recommends or my corrector is broken?

 

get.jpg

crop #1: top left corner

 

get.jpg

crop #2: center

 

get.jpg

crop #3: bottom right corner

 

all crops are parts of the same single 60s autoguided image

 

Thanks!

 



#2 FiremanDan

FiremanDan

    Aurora

  • *****
  • Posts: 4,869
  • Joined: 11 Apr 2014

Posted 01 October 2015 - 07:05 PM

This is a problem with the focal reducers. I have the same set up. The is a starzona focal reducer that is $350 that allegedly has a flat field. 
I am not sure the cause of that artifact, but the bad stars are just part of the joy of a C8 w/ focal reducer. 
Most people just crop their images. 



#3 Bradicus

Bradicus

    Viking 1

  • *****
  • Posts: 850
  • Joined: 30 Nov 2013

Posted 01 October 2015 - 07:13 PM

Yep. That's the reducer.  Triple-check your collimation, as that will reduce the effectiveness of the reducer, but I gave up on the Celestron model and purchased the Starizona SCT corrector and never looked back. When properly collimated, the Starizona will give you good stars all the way to the corners and eliminate almost all the vignetting. 



#4 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 10,303
  • Joined: 12 Oct 2007

Posted 01 October 2015 - 07:16 PM

The correction will depend strongly on the backfocus distance from the reducer to the sensor, which I think is around 105mm but I get them mixed up.

 

You should attach the reducer directly to the back of the sct, and then set the distance so that the image scale you measure corresponds to f/6.3.  The farther out you are the greater reduction you have and it will get badly aberrated.

 

Your image shows mostly astigmatism in the corners and my guess is you are too far out.

 

I had good benefit with the reducer on my c11 operating at about f/5.7 with OAG - but that was with a smaller sensor.  For your sensor I would try different backfocus distances and see if the corners improve.  You may not be able to make it perfect, but it should be improved.

 

Once you have the spacing about right, you can then try adjusting focus and offset focus from the center a bit to improve the overall field and compensate for field curvature.

 

The starizona reducer may work better but it costs more.  But I think you can improve upon what you currently have by adjusting the setup.  A key mistake people make is not to put the reducer directly on the back of the sct.

 

Frank



#5 Bradicus

Bradicus

    Viking 1

  • *****
  • Posts: 850
  • Joined: 30 Nov 2013

Posted 01 October 2015 - 07:33 PM

In my case, I was using the Celestron directly on the back of the scope and the purpose-built Celestron t-adapter (which should have given me good spacing) but the results were never satisfactory.  I'm not sure why, but I've heard that the C11 is better corrected than the C8. 



#6 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 10,303
  • Joined: 12 Oct 2007

Posted 01 October 2015 - 08:17 PM

The astrobin image indicates a pixel scale of 0.69" per pixel - and if that is the direct image of the 0.43 um pixels I calculate it as being at f/6.4, which is pretty close to what it should be.

 

If the corners are equally bad and if it is well focused in the center, then I think all you can do is experiment with changing focus to sacrifice quality in the center for improved stars in the corners.  You may need to decide on some amount of cropping and optimize overall within that smaller cropped region.

 

I think even with the corners aberrated the overall field has been improved by the reducer - mainly by reducing coma - while also making it wider field and faster - which is the whole point.  But it isn't perfect, especially if the image is guided and focused well to reveal the defects.

 

The starizona or other reducers may work better on the c8 - I don't know.

 

Frank



#7 pedxing

pedxing

    Vanguard

  • *****
  • Posts: 2,427
  • Joined: 03 Nov 2009

Posted 02 October 2015 - 06:03 AM

The Starizona reducer works much better on the C8. You get what you pay for.

The Celestron reducer is great for visual. I've found that for photography, it alters the star shapes at the corners but it's hard to say if it actually improves them...

The downside of the Starizona reducer (aside from cost) is 10mm less backfocus which can make an OAG/DSLR combo challenging. I had to go to a thin OAG to get the distance right.

#8 kraegar

kraegar

    Apollo

  • *****
  • Posts: 1,415
  • Joined: 17 Apr 2012

Posted 02 October 2015 - 07:22 AM

I never got my images from the c8 looking good, with the exact same setup.  Very similar issues.  I jumped to a refractor - much easier to guide on the avx, too, and only use the c8 for planetary / moon AP, and visual use now.  When I have a nicer mount, I'll get a scope more  suited to AP for the longer focal lengths.



#9 FiremanDan

FiremanDan

    Aurora

  • *****
  • Posts: 4,869
  • Joined: 11 Apr 2014

Posted 02 October 2015 - 07:31 AM

Mike17 just got one. He sent me some pictures he did last night with the Starizona. HUGE difference.
Since I'm down to just a C8 and a decent refractor is about a year out for me (that kind of money doesn't come easy) I'm thinking about getting one of these Starizonas.

#10 entilza

entilza

    Soyuz

  • *****
  • Posts: 3,798
  • Joined: 06 Oct 2014

Posted 02 October 2015 - 08:26 AM

Yeah why not, and you can now autoguide.  I was even thinking to suggest just trying your old, N6 with a coma corrector even :)



#11 FiremanDan

FiremanDan

    Aurora

  • *****
  • Posts: 4,869
  • Joined: 11 Apr 2014

Posted 02 October 2015 - 08:30 AM

I have a coma corrector on the AT6IN. It's just another piece of garbage AstroTech.

#12 Alexander Vasenin

Alexander Vasenin

    Sputnik

  • -----
  • topic starter
  • Posts: 29
  • Joined: 27 Sep 2015

Posted 05 October 2015 - 09:35 AM

Hi everyone,

 

I'm a bit messed up my forum settings, so I haven't received a notifications about your answers and thought nobody answered me. I'm glad I was mistaking so badly  :lol:. Thank you all guys for your valuable information.

 

You should attach the reducer directly to the back of the sct, and then set the distance so that the image scale you measure corresponds to f/6.3.  The farther out you are the greater reduction you have and it will get badly aberrated.

It's attached directly to the back of the SCT.

 

I had good benefit with the reducer on my c11 operating at about f/5.7 with OAG - but that was with a smaller sensor.  For your sensor I would try different backfocus distances and see if the corners improve.  You may not be able to make it perfect, but it should be improved.

I would definitely want to play with it. I could easily increase backfocus distance by unscrewing T-adapter, but I don't see a how could I try decrease it. Any ideas?

 

Triple-check your collimation, as that will reduce the effectiveness of the reducer, but I gave up on the Celestron model and purchased the Starizona SCT corrector and never looked back.

I'm not expert, but the collimation looks very well - central obstruction shadow is directly in the center of the unfocused blob.

 

The is a starzona focal reducer that is $350 that allegedly has a flat field.

That looks like an interesting option (though it might be a bit tricky to get one to Moscow). I haven't heard anyone have alternatives to Celestron Reducer before. Thanks!


Edited by BlackSir, 05 October 2015 - 09:37 AM.


#13 shawnhar

shawnhar

    Cosmos

  • *****
  • Posts: 9,452
  • Joined: 25 Jun 2010

Posted 05 October 2015 - 12:23 PM

I ordered a short variable tadapter from Scopestuff and got much better stars after reducing the distance to the corrector by 10mm

#14 FiremanDan

FiremanDan

    Aurora

  • *****
  • Posts: 4,869
  • Joined: 11 Apr 2014

Posted 05 October 2015 - 12:52 PM

I ordered a short variable tadapter from Scopestuff and got much better stars after reducing the distance to the corrector by 10mm

How much did this change your FOV? 



#15 shawnhar

shawnhar

    Cosmos

  • *****
  • Posts: 9,452
  • Joined: 25 Jun 2010

Posted 05 October 2015 - 03:22 PM

Not much, probably went to F6.8 or so, not a big difference in the FOV. Not saying the stars were magically nice and round all the way to the edge, but there was definitely an improvement, they went from being comets/boulder looking things to being "mostly" round.



#16 N_DD

N_DD

    Vostok 1

  • -----
  • Posts: 192
  • Joined: 21 Oct 2012

Posted 05 October 2015 - 03:42 PM

Hi,

 

I am also using the Celestron f/6.3 reducer on a C8 and getting very similar results: bright stars creating odd reflections, curved field, comatic stars at the edges (I am using a modified 1000D/Rebel XS). Spacing is right (inferred from pixel scale I am exactly at f/6.3).

However, by carefully focusing not in the center of the FOV, but around 1/3 off-axis, I find that the compromise is still acceptable: the image is slightly softer, but still ok.

 

Which Starizona reducer are we talking about? The 0.75x or the 0.5x?

 

Nico



#17 shawnhar

shawnhar

    Cosmos

  • *****
  • Posts: 9,452
  • Joined: 25 Jun 2010

Posted 05 October 2015 - 04:02 PM

Scott Rosen has taken some awesome images with a C8 and stock focal reducer, he used to shoot 2 sets of images, one focused for the center and one for the outer and then blend the 2 so he had nice sharp focus across the field. A bit much for me personally but it works.



#18 Alexander Vasenin

Alexander Vasenin

    Sputnik

  • -----
  • topic starter
  • Posts: 29
  • Joined: 27 Sep 2015

Posted 06 October 2015 - 06:48 PM

I've measured back focus distance of my rig: given APS-C mount have 44mm flange focal distance and the length of Celestron #93633-A T-adapter with Canon adapter is 61mm, my back focus distance is 105mm. I also have Celestron #93640 Barlow T-adapter combo which gives minimum of 122.5mm back focus.

 

BTW, I found interesting article which suggest proper back focus should be about 84mm (i.e. 95mm-11mm). Thinking about getting one of Scopestuff adapters...



#19 Alexander Vasenin

Alexander Vasenin

    Sputnik

  • -----
  • topic starter
  • Posts: 29
  • Joined: 27 Sep 2015

Posted 06 October 2015 - 07:11 PM

I ordered a short variable tadapter from Scopestuff and got much better stars after reducing the distance to the corrector by 10mm

I haven't found variable T-adapters here. Which one have you bought?



#20 shawnhar

shawnhar

    Cosmos

  • *****
  • Posts: 9,452
  • Joined: 25 Jun 2010

Posted 06 October 2015 - 08:31 PM

Doh!

 I just went back and looked at my order, it was actually the short T-adapter and a 10mm extension.

SCT T-adapter for Direct Photography, Short (collar ring type)
Item #: TASS

10mm T-Thread Extension Tube / Spacer Ring
Item #: TTET

 

 I already had 5mm and 15mm extension tubes so I can swap out and change the distance. I think that short adapter plus a 10mm and a 5mm extension make 50mm. (I think, it's been a long time since I figured out all the distances but I was running 95mm sensor to focal reducer and got better stars than the 105)

 I shot with that short one and a 10mm extension last night and it comes out to around F7.8 instead of F6.3. Less reduction but better stars.



#21 Alexander Vasenin

Alexander Vasenin

    Sputnik

  • -----
  • topic starter
  • Posts: 29
  • Joined: 27 Sep 2015

Posted 07 October 2015 - 11:03 PM

Thanks a lot!




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