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Experiences Using Compound Focal Reducers with a Lodestar

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#1 alphatripleplus

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Posted 22 September 2016 - 11:20 PM

There have been a number of recent posts on stacking focal reducers to create a compound reducer with more reduction than a single reducer. For example, using a 1.25inch or 2inch 0.5x reducer with either a Meade or Celestron f/6.3 reducer. In addition, Jim Thompson has produced a number of reports where he has tested a variety of combinations of reducers.  After reading Jim's reports, I decided to experiment to see if using a compound reducer would allow me to get more aggressive  focal reduction (than a single reducer) without introducing too much aberration towards the edge of the field. I'll try to summarize my results below - apologies for the length of the post.

 

For these tests I used a C8 and Lodestar X2 mono which has a 8mm diagonal sensor.  As the Lodestar is only a 1/2 sensor, my results in terms of aberrations at the edge of the field may not be valid for cameras with larger sensors. I used several combinations of 3 reducers:

 

A  1.25inch Antares 0.5x reducer (focal length 95mm)

Two Meade (Japan) f/6.3 reducer/flatteners (focal length 240mm)

(If you are wondering why I own two identical Meade f/6.3 reducers (I call them the 'twins'), bear with me for a moment, and I will explain.)

 

To keep this post from getting too long, I will show the results from just 3 set-ups that illustrate what can be done. In each case, I used NGC6946, the Fireworks Galaxy, as my target as the field has lots of background stars, so it easy to see aberrations in the corner of the field (plus I like the Fireworks Galaxy smile.gif ).

 

SET-UP I - 0.5x reducer and one Meade f/6.3 reducer - f/3.6 overall

(Sensor to 0.5x distance approx 42mm; reducer separation 30mm in spacers + SCT/T-Thread adapter)

 

NGC6946_16x15s_f3.6_CS_2016.9.7_20.34.06.jpg

 

In Set-Up 1,  both the vignetting and aberrations in the corners of the field are obvious. In order to mitigate the vignetting, the separation of the reducers should be increased, since vignetting may have resulted when the light cone from the f/6.3 reducer meets the much smaller aperture of the 1.25inch reducer. Set-Up 2 has an increased focal reducer separation:

 

SET-UP 2 - 0.5x reducer and one Meade f/6.3 reducer - f/4.3 overall

(Sensor to 0.5x distance approx 25mm; reducer separation 102mm in spacers + SCT/T-Thread adapter)

 

NGC6946_17x15s_ND_f4.3a_CS_2016.9.11_21.00.11.jpg

 

In Set-Up 2, the overall focal reduction is less aggressive and both vignetting and aberrations are reduced, although I still see some aberrations in the upper right corner of the field.

 

As a final example, I used the twin Meade (Japan) reducers, as I suspected that the Antares reducer might have been responsible for the aberrations in both Set-ups 1 and 2. I used almost zero separation between the Meade reducers - screwing one into the other:

 

SET-UP 3 -  Meade f/6.3 reducer + Meade f/6.3 reducer - f/3.3 overall

(Sensor to 1st f/6.3 reducer distance approx 73mm + SCT/T-Thread adapter; reducer separation 0mm in spacers)

 

NGC6946_12x15s_ND_f3.3_CSx_2016.9.22_19.59.17.jpg

 

Interestingly, although the twin Meade reducer combo gives the most aggressive focal reduction - effectively f/3.3 - this combination yields the least noticeable aberrations in the corners of the field. There is, however, a little uneven field illumination which may be from reflections between the almost in contact reducers, and some minor vignetting noticeable in the right corners.

 

I think that using the twin f/6.3 Meades is a potentially viable alternative to using the old Meade f/3.3 reducer. It is possible to get slightly more reduction out of the twin Meades than I have shown, but I ran out of focus on the C8 approaching f/3.0 with this combination. So no hyperstar competition here.

 

I'm not sure if the current Chinese made f/6.3 reducers are as aberration free as these older Japanese reducers, but perhaps someone will try them in combo.


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#2 roelb

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Posted 23 September 2016 - 09:51 AM

Wow Errol, thanks for sharing those tests!
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#3 alphatripleplus

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Posted 23 September 2016 - 10:40 AM

Thanks, Roel. At some point, I thought about trying all three reducers in series to get more reduction, but that is a lot of glass. Also, I suspect aberrations from having 3 achromats in the optical train might be a problem. It would be interesting to hear if anyone has a similar experience using just twin f/6.3 reducers.


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#4 jimthompson

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Posted 23 September 2016 - 01:55 PM

Hi Errol,

 

Very nice test, well done!  I have not tried stacking two f/6.3 reducers but I have stacked two 2" 0.75x focal reducers from Mallincam (OEM GSO) and had very good results.  I have also had good luck simply increasing the spacers between a single f/6.3 FR and the camera to get down to around f/4.5.  In your image with the stacked f/6.3 it looks like your stars are a little strange in shape our around the outside of the image.  Perhaps this is due to over correction of the field?...there is some field flattening built into the FR as well.  Also, a heads up that if you were to now use your stacked FR configuration with a camera that has smaller pixels (higher resolution), you may see additional aberrations that were not visible with your Lodestar.  That has been my experience at least.

 

cheers,

 

Jim T.


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#5 alphatripleplus

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Posted 23 September 2016 - 06:22 PM

Hi Jim,

 

Thanks for your insights and the great reports on focal reducers. Yes, I did notice the slightly strange shapes (particularly with the brighter stars) near the edges in the stacked twin f/6.3 reducer image. Overall, I thought the aberrations were worse in the other two set-ups, but you are right that #3 still suffers from some aberrations. I might be able to test if it is an issue of field flatness by seeing if there is a differential focus between the center and the edge.

 

If I still had an ASI224 I could see if more aberrations are noticeable with the smaller pixel ASI224. Perhaps I will try to compare images of a few targets taken previously when I owned that camera and the combo 0.5x/f/6.3 reducer, with those using the Lodestar to see if my experience is the same as yours.


Edited by alphatripleplus, 23 September 2016 - 06:24 PM.

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#6 alphatripleplus

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Posted 25 September 2016 - 10:47 PM

One more example of the twin stacked Meade reducers from this evening - but this time I reduced the sensor distance to the 1st f/6.3 reducer by about 11mm, kept both reducers stacked and almost in contact as in Set-Up 3. This increased the effective f/ratio from f/3.3 to f/3.9:

 

SET-UP 4 -  Meade f/6.3 reducer + Meade f/6.3 reducer - f/3.9 overall

(Sensor to 1st f/6.3 reducer distance 62mm + SCT/T-Thread adapter, [i.e.approx 78mm total]; reducer separation 0mm in spacers)

 

NGC6946_15x15s_ND_f3.9_CS_2016.9.25_20.47.35.jpg

 

I think Set-Up 4 is perhaps a slight improvement in terms of corner and edge aberrations relative to Set-Up 3, but still shows slightly uneven illumination across the field.

 

To illustrate the slightly uneven illumination, I decided to shoot a fainter target, NGC 7640 an 11th magnitude galaxy in Andromeda. I had to stretch this image, a stack of 32 x15s exposures, a bit more to show any detail and the residual vignetting is easier to see than in the NGC6946 image

 

NGC7640_32x15s_ND_f3.9_CS_2016.9.25_21.06.33.jpg

 

I will continue to experiment with the twin f/6.3 reducers, as I think the aberrations at f/3.3 to f/3.9 are acceptable for my EAA usage at least.

 

Edit: I corrected an earlier error - In Set-Up 4, the sensor to 1st f/6.3 reducer distance was reduced by 11mm (not 9mm) from Set-Up 3. Also note that  in Set-Up 4 the thickness of the SCT/T-thread adapter I have results in a TOTAL sensor to  1st f/6.3 reducer distance of 78mm (= 62 + T adaptor thickness). Hope that makes sense.


Edited by alphatripleplus, 22 May 2019 - 08:21 AM.

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#7 Dwight J

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Posted 26 September 2016 - 01:09 AM

Intriguing results, who would of though stacking 0.63 reducers would provide these good results.  The last, set up #4, looks the best and still plenty fast.  


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#8 GlennLeDrew

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Posted 26 September 2016 - 06:48 AM

To better see unevenness of illumination, simply use longer exposures so as to bring up the sky glow. Even a single frame at a fairly long exposure will do this very handily. Make it *bright*, like a full Moon has suddenly lit up the sky.

 

Assessing vignetting when the very center *barely* shows sky glow is rather uncertain.

 

Is the Lodestar sensor of a type used in video cameras? Does it have stripes along two two adjacent sides which are used for reference purposes, not contributing to the image? If so, this can explain an offset to illumination. While the sensor package might be perfectly centered, the effective offset resulting from the unused edge stripes would have the the illumination pattern, and perhaps also the optical axis, depending on how collimation was performed, displaced with respect to the image center.

 

Another possibility. Reflections among the lens elements, and/or of the sort related to pupil ghosts, might be slightly displaced due to small amounts of tilt or sagging in the system as a whole or in some part.


Edited by GlennLeDrew, 26 September 2016 - 06:52 AM.


#9 alphatripleplus

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Posted 26 September 2016 - 08:36 AM

Dwight - Yep, I agree that Set-up 4 @ f/3.9 looks the best overall.

 

Glenn - You are right that with the sky glow visible, it is easier to see the variation in illumination across the field. In fact, before capturing these images when adjusting the black level to the left of the histogram, I could see the effects more clearly. Probably should have included more sky glow in the captures to illustrate the point, rather than trying to show the most aesthetically pleasing view :) .

 

I'm not sure about the source of the slightly uneven illumination. However, it does seem that by changing the sensor to 1st focal reducer distance (going from Set-up 3 to Set-up 4) did have an effect on the slightly uneven illumination: In Set-up 3 (f/3.3) the right-side of the field of NGC6946 seems a little brighter, whereas in Set-up 4 (f/3.9) the right-side of the field seem darker. My feeling is that it does not result from edge stripes on the sensor, as one might expect the uneven illumination would not vary with sensor to reducer distance if this were the case. Anyway, if I'll continue to play around a little bit more with my available spacers. Thanks for your comments.



#10 roelb

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Posted 26 September 2016 - 10:43 AM

Stupid thought: left <--> right uneven illumination due to Lodestar assembly orientation?



#11 alphatripleplus

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Posted 26 September 2016 - 11:17 AM

Stupid thought: left <--> right uneven illumination due to Lodestar assembly orientation?

Roel,

 

Not sure that is the cause because when I move the sensor a bit closer to the 1st reducer (going from f/3.3 to f/3.9), the uneven illumination seemed to reverse from slightly brighter on the right side (f/3.3) to slightly darker on the right side (f/3.9).



#12 GlennLeDrew

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Posted 26 September 2016 - 05:47 PM

This apparent 'flipping' of the illumination pattern independent of the chip orientation is becoming more strongly suggestive to me of asymmetric reflections off the glass surfaces. A telling test would be to chuck the reducer(s), singly or in combination, on a lathe, true up the housing, and observe a laser spot's reflection(s) as the spindle is turned. In lieu of a lathe, a V-block and flat glass plate could serve to constrain lateral and longitudinal motion while the unit is spun in place.

 

Not that I expect the average amateur to go to such lengths! ;)


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#13 GlennLeDrew

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Posted 26 September 2016 - 05:52 PM

I should add that the Meade f/6.3 reducer is notorious for generating a pupil ghost in its nominal configuration. A pair ganged up could possibly exacerbate reflections. While likely altering the distribution of the back reflections, if there is any summation of them this might well make for a worse result.


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#14 alphatripleplus

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Posted 26 September 2016 - 06:15 PM

Thanks for these insights, Glenn. The slightly uneven illumination does not really bother me, but as I am having so much fun I will try a few more configurations down the road :)



#15 alphatripleplus

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Posted 27 September 2016 - 10:41 PM

One final set of images I took at f/3.7 with the twin reducers (posted in the image gallery) seems to be fairly aberration free.

 

http://www.cloudynig...82#entry7457047



#16 Rickster

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Posted 28 September 2016 - 09:25 AM

Interesting experiment.  Thank you for posting.


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#17 roelb

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Posted 28 September 2016 - 05:24 PM

Just for comparison.
All with Evolution 8" @ f/5.

NGC 6946 - H IV-76 - Arp 29 - C 12 - "Fireworks" Mixed Spiral Galaxy in Cygnus: 10 x 25 s

NGC.6946.-.H.IV-76.-.Arp.29.-.C.12.-.Fireworks.Mixed.Spiral.Galaxy.in.Cygnus_2016.8.24_03.09.02.jpg

NGC 5907 - H II-759 - "Splinter-Knife" Spiral Galaxy in Draco: 7 x 30 s
(6 SE must be Evo 8)

NGC 5907 - H II-759 - Splinter-Knife Spiral Galaxy in Draco.jpg

NGC 6503 - Dwarf Spiral Galaxy in Draco: 10 x 30 s

NGC.6503.-.Spiral.Galaxy.in.Draco_2016.9.6_22.55.10.jpg
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#18 alphatripleplus

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Posted 28 September 2016 - 05:39 PM

Very nice detail, Roel, particularly in NGC 5907. I think for smaller objects like these galaxies the 1000mm focal length you are using works well. I haven't tried f/5 yet with the Lodestar X2M, but I will.


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#19 roelb

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Posted 28 September 2016 - 07:31 PM

In the following posts I give an overview of my "focal reducer" test results with the Atik Infinity Color version.

I want to carry out some further tests, but at the moment I don't have enough different spacers to achieve this.

All done with SCT Evolution 8" using a 1.25" visual back.

In rather dark environment but sometimes suffering from dew while heater is defect.

 

Following tests where done:

1. f/6.3 @ 91 mm      => f_cal/6.62  (l_train = 158 mm => ALT_max ~ 75°)

2. f/6.3 @ 110 mm    => f_cal/6.20  (l_train = 177 mm => ALT_max = 63°)

3. f/6.3 + 50 mm spacer + f/5 (10 mm width) + 41 mm spacer: total of 101 mm to CCD => f_cal/3.03  (l_train = 168 mm => ALT_max = 65°)

4. f/5 @ 66 mm         => f_cal/3.49  (l_train ~ 109 mm => base mount cleared)

 

Scheduled: f/5 @ 41 mm => f_cal/5.6

 

Notes: f_cal   = calculated as per Astrometry.net

           l_train = length of the image train measured from OTA baffle to end of Atik Infinity.

                        To clear the Evo 8 mount base, l_train must < 155 mm.


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#20 roelb

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Posted 28 September 2016 - 07:35 PM

1. f/6.3 @ 91 mm      => f_cal/6.62

 

NGC 6946 "Fireworks":

 

Fireworks.jpg

 

M 36:

 

m36.jpg

 

M 81  (dew on corrector plate)

 

m81 - DEW !.jpg


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#21 roelb

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Posted 28 September 2016 - 07:36 PM

2. f/6.3 @ 110 mm    => f_cal/6.20

 

NGC 6946 "Fireworks":

 

Fireworks.jpg

 

M 74:

 

M 74 - 2.jpg

 

M 81:

 

M 81 - 8x20s - 2.jpg

 

M 82:

 

M 82 - 8x20s - 2.jpg

 

NGC 7814:

 

NGC 7814.jpg


Edited by roelb, 28 September 2016 - 07:39 PM.

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#22 roelb

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Posted 28 September 2016 - 07:42 PM

f/6.3 + 50 mm spacer + f/5 (10 mm width) + 41 mm spacer: total of 101 mm to CCD => f_cal/3.03

(couldn't image "Fireworks" due to obstruction early evening), but vignetting is obvious)

 

IC 342 - 2 x binned:

 

IC 342 - 2 x binned.jpg

 

NGC 147:

 

NGC 147.jpg

 

 


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#23 roelb

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Posted 28 September 2016 - 07:46 PM

4. f/5 @ 66 mm         => f_cal/3.49

 

NGC 6946 "Fireworks":

 

Fireworks 10 x 30 s.jpg

 

M 31:

 

M 31 - 9 x 15 s.jpg

 

M 74:

 

M 74 - 9 x 20 s.jpg

 

M 81:

 

M 81 - 9 x 20 s.jpg

 

M 82:

 

M 82 - 8 x 15 s.jpg

 


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#24 Anduin

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Posted 28 October 2016 - 10:11 AM

One more example of the twin stacked Meade reducers from this evening - but this time I reduced the sensor distance to the 1st f/6.3 reducer by about 9mm, kept both reducers stacked and almost in contact as in Set-Up 3. This increased the effective f/ratio from f/3.3 to f/3.9:

SET-UP 4 - Meade f/6.3 reducer + Meade f/6.3 reducer - f/3.9 overall
(Sensor to 1st f/6.3 reducer distance approx 64mm; reducer separation 0mm in spacers + SCT/T-Thread adapter)

attachicon.gifNGC6946_15x15s_ND_f3.9_CS_2016.9.25_20.47.35.jpg

I think Set-Up 4 is perhaps a slight improvement in terms of corner and edge aberrations relative to Set-Up 3, but still shows slightly uneven illumination across the field.

To illustrate the slightly uneven illumination, I decided to shoot a fainter target, NGC 7640 an 11th magnitude galaxy in Andromeda. I had to stretch this image, a stack of 32 x15s exposures, a bit more to show any detail and the residual vignetting is easier to see than in the NGC6946 image

attachicon.gifNGC7640_32x15s_ND_f3.9_CS_2016.9.25_21.06.33.jpg

I will continue to experiment with the twin f/6.3 reducers, as I think the aberrations at f/3.3 to f/3.9 are acceptable for my EAA usage at least.


Following Alphatripeplus experience with the twin f/6.3 reducers, I tried this combination yesterday and was very pleased with the results. Much better than a 0.63 plus 0.5 , a combination I wasn't sucessful at all.

I used a Celestron and an Antares 0.63x reducers directly connected. Here the results. Thanks again Alpha for the thread and suggestions , really helpful.

4c3b6937d23a199ccd1266457c8a66b9.png

55d372038671fa0d3f444b1a1f3acc33.png




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#25 alphatripleplus

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Posted 28 October 2016 - 12:28 PM

Anduin, I think it is very encouraging that the twin f/6.3 reducer concept  works with what I presume are the newer Chinese made f/6.3 reducers that you are using. Looks like there is no need to stick with the old Japan f/6.3 reducers that I have.




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