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Opinions on Denk Standard OCA vs 45mm OCA

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#1 Eddgie

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Posted 12 January 2014 - 01:20 PM

I have limited experience with the Denk in a Newtonian, but my question is about the 45mm OCA.

The last time I used a Denk in my 12" f/4.9 dob, even with the 45mm OCS, it seems like there was a bit of field illumination falloff in low power mode, however at the time, I needed a lot of extension on the OCA because my focuser did not have the necessary in-travel to reach focus otherwise.

Since then, I have replaced the factory focuser with a lower profile focuser, and I am confident that I could now reach focus with the OCA screwed fully in.

I am looking to buy a either a Denk II or a new Binotron system, but I am torn on the extra cost of the 45mm OCA.

I will likely use Explore Scientific 20mm 68 degree eyepecies for now (maybe add D21s later).

Has anyone compared the standard OCS and the 45mm OCS when used in this manner, with no extension required?

How was the standard 38mm OCS? Did you feel that light falloff was acceptable, or was it noticable? I live under fairly bright skies, so this means that severe illumination falloff is detecatble due to the fact that the sky will appear much brighter at the center of the field than the outside. I can tolerate a little dimming, but not a lot.

I know it is subjective, but I would like to hear opinions.

#2 Allan Wade

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Posted 12 January 2014 - 06:17 PM

Eddgie, I can't give a comparison of the 2 because I bought the OCS 45 straight up. But I can say in my 12" Dob with D21's that the field appears uniform right across. That's with no extension of the OCS and my backyard sky is about magnitude 5 or slightly better.

I decided it was worth paying the extra $149 to upgrade to the OCS 45 when I bought the Binotron, because I didn't want to pay $249 later if I decided I needed to make the upgrade.

#3 Eddgie

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Posted 12 January 2014 - 06:58 PM

Are you using the factory focuser? Did you have to extend the OCS by about 3/4th inch?

#4 Allan Wade

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Posted 12 January 2014 - 07:51 PM

No, I put the Feathertouch on it and just got real lucky with the height of the focuser. I can reach focus in all 3 power switch positions, with the OCS screwed all the way in.

My setup is a bit non standard. I have a Moonlite filter slider that attaches through a Moonlite focuser base plate. I then had Starlight machine up a 0.5" spacer that attaches to the base plate. The Feathertouch focuser then attaches to the spacer. It's the 1.5" focuser because I use it with the SIPS when I am mono viewing.

#5 faackanders2

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Posted 12 January 2014 - 07:56 PM

I have limited experience with the Denk in a Newtonian, but my question is about the 45mm OCA.

The last time I used a Denk in my 12" f/4.9 dob, even with the 45mm OCS, it seems like there was a bit of field illumination falloff in low power mode, however at the time, I needed a lot of extension on the OCA because my focuser did not have the necessary in-travel to reach focus otherwise.

Since then, I have replaced the factory focuser with a lower profile focuser, and I am confident that I could now reach focus with the OCA screwed fully in.

I am looking to buy a either a Denk II or a new Binotron system, but I am torn on the extra cost of the 45mm OCA.

I will likely use Explore Scientific 20mm 68 degree eyepecies for now (maybe add D21s later).

Has anyone compared the standard OCS and the 45mm OCS when used in this manner, with no extension required?

How was the standard 38mm OCS? Did you feel that light falloff was acceptable, or was it noticable? I live under fairly bright skies, so this means that severe illumination falloff is detecatble due to the fact that the sky will appear much brighter at the center of the field than the outside. I can tolerate a little dimming, but not a lot.

I know it is subjective, but I would like to hear opinions.


I have a Denk II and two dual arm power switches (one is a reducer/multiplier) with my 17.5" f4.1 dob. I own 4 OCSs the original multipurpose (38mm), new multipurpose (45mm), newtonian (38mm), and multiplier (38mm).

The new 45mm multipurpose is definitely better than the 38mm multipurpose, because as you mentioned this OCS requires the most infocus or extension(s) to move the OCS inward. Moving the OCS inward pushes it deeper into the cone of light and in LL (0.87x) power mode observing the full moon light cone the 38mm OCS clips the light cone; whereas the 45mm version allow all or more of the cone of light in the larger opening. The advantage of the multipurpose OCS is lower power than the other OCSs. If I only had the original dual arm power switch the powers would have been 1.2x(L)/1.8x®/2.3x(M). This OCS in LL (0.87x) power mode has so much infocus that I need to remove my Astrocrumb filter slide with denk adapter that pushes it inward, but if I choose to not use LL power mode but rather use all other 8 out of the 9 powers I can use the Astrocrumb filter slide.

The Newtonian OCS has less infocus but is higher power. 1.0x LL power mode requires no infocus extensions. This was the original OCS I got with my Denk II, and I believe it does work best in my f4.1 scope. If I only had the original dual arm power switch the powers would have been 1.4x(L)/2.0x®/2.5x(M).

The Multiplier OCS requires alot of out focus, especially in HH highest power mode. If I only had the original dual arm power switch the powers would have been 1.8x(L)/2.6x®/3.2x(M). The best view of Jupiter I ever has was with this OCS. I normally use the lower 6 out of 9 powers with this OCS, due to the fuzziness of excessively high powers (est 3.3x3.2/2.5 highest power).

Note with the Newtonian and Multiplier OCSthe focus position is aft enough to not clip the lightcone with 38mm. But the Multipurpose OCS definitely has an advantage with the 45mm opening due to its' infocus.

Hope this helps in your decision.

Ken ;)

#6 faackanders2

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Posted 12 January 2014 - 08:02 PM

24mm panoptics are the best eyepieces (widest TFOV in 1.25") for binoviewing since newtonians always(MOSTLY) REQUIRE OCS which increase power. I rarely use my 14mm 82AFOV ES backup eyepieces (when the pans get iced/dewed up).

#7 GlennLeDrew

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Posted 12 January 2014 - 10:40 PM

Vignetting will depend on:
- BV front aperture.
- OCS diameter
- Distance between OCS and BV front aperture.
- F/ratio of light entering the OCS.

And for the sake of completeness:
- BV rear aperture.
- Field stop diameter.
- Distance between BV rear aperture and field stop.

The first list of dimensions impacts the size of the circle of full illumination, if there is no aperture reduction occurring, and the light fall/off immediately therefrom. The second list impacts edge-of-field darkening, having no impact on the central field at all.

The geometry is that of a two-baffle system. The OCS either has no effect on vignetting, thus leaving the BV's front aperture as the first baffle, or it becomes the new front baffle. (In between these two conditions is the very particular state whereby the OCS has exactly the aperture which just accommodates that same light bundle just accommodated by the BV's front aperture, for a particular f/ratio.)

It all might seem complicated, but it's really very simple geometry.

#8 a__l

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Posted 13 January 2014 - 04:58 AM

I have both OCS. Photo. Travel focuser on my 24" f/3.3:
1. DenkII+OCS-45. Eyepiece Pan24. Minimum space for diopters on bino. Without extend the OCS by about 3/4th inch. Negative component switch (1.3x) - 0 mm.
2. DenkII+OCS-38. Eyepiece Pan24. Minimum space for diopters on bino. Negative component switch (1.5x). Travel + 18 mm.
3. For reference. Paracorr-2 + Ethos (21,17,13,8). Travel + 23 mm.
In this part of bino with OCS45 placed inside the telescope tube about 30 mm (switch 1.3x)
4. For users Mallincam.
MFR-6+20 mm spacer (used 2" camera adapter). Travel +13 mm.
MFR-3+20 mm spacer (used 2" camera adapter). Travel + 2 mm.
I have myopia. For comparison eyepieces and Mallincam add 2..3mm to eyepiece travel for normal vision.

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#9 faackanders2

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Posted 14 January 2014 - 08:21 PM

I have both OCS. Photo. Travel focuser on my 24" f/3.3:
1. DenkII+OCS-45. Eyepiece Pan24. Minimum space for diopters on bino. Without extend the OCS by about 3/4th inch. Negative component switch (1.3x) - 0 mm.
2. DenkII+OCS-38. Eyepiece Pan24. Minimum space for diopters on bino. Negative component switch (1.5x). Travel + 18 mm.
3. For reference. Paracorr-2 + Ethos (21,17,13,8). Travel + 23 mm.
In this part of bino with OCS45 placed inside the telescope tube about 30 mm (switch 1.3x)
4. For users Mallincam.
MFR-6+20 mm spacer (used 2" camera adapter). Travel +13 mm.
MFR-3+20 mm spacer (used 2" camera adapter). Travel + 2 mm.
I have myopia. For comparison eyepieces and Mallincam add 2..3mm to eyepiece travel for normal vision.


I am surprised. Both OCS are labeled "Multi-Purpose" so I do not understand why one would be 1.3x and the other 1.5x. They should be the same power and focus at the same point (with the same eyepiece and powerswitch position). The major/only difference between these OCSs should be the 45mm vs. 38mm clear aperture, and the benefit is at the lowest power with most infocus (preventing/reducing light cone clipping).

P.S. I also have both these OCSs (Asked numerous times for Russ to make/market 45mm version, but gave up waiting years and bought the 38mm 1st [when Russ convinced me to stop waiting it wouldn't happen], and then the 45mm 2nd when it became available later that year). Maybe I was the catalyst that convince him ther was a market for it afterall?

If you have different powers, could Russ have made different power versions for you?






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