9 replies to this topic

[Randy Blair]

With a set of Denkmeier II's (2") already in use for my dob, I just received a Christmas Meade LX10.  It has an SCT 2" Mirror Diagonal (Antares 2 MDS) that comes with it. 

My question is based on some comments for set up that infer that a Barlow and a short light path to the binoviewers is ideal for optimal mirror position in the SCT.

For those familiar with Denks, can I just use the 2x OCS as a barlow to achieve this?

 

 

[Randy Blair]

Using a Barlow on an SCT is going to be for very high magnification.  A focal reducer on this scope would be useful for large more spacious objects like beehive, pleides, Andromeda, m81 and 82, double cluster etc. My eyepiece collection is now fairly complete and I enjoy each pair listed in my signature.  I don’t wear glasses when viewing.  Looking to acquire a Richfield Star Sweeper.  That should do it.  Also am making a dew heater from pipe heating wire while waiting for a clear night

[ayadai]

My question is based on some comments for set up that infer that a Barlow and a short light path to the binoviewers is ideal for optimal mirror position in the SCT.

 

A moving mirror does not have an "optimal position", IMHO. For visual, that's one advantage of an SCT; you can reach focus with just about any combination of barlow, eyepeice, binoviewer, etc...

[Eddgie]

A moving mirror does not have an "optimal position", IMHO. For visual, that's one advantage of an SCT; you can reach focus with just about any combination of barlow, eyepeice, binoviewer, etc...

If you are new to binoviewer use, you may not realize that with a standard SCT, the more light path you add, the greater thr the focal length of the instrument becomes, and at some point, the system experiences excessive spherical aberration and aperture loss. 

[Eddgie]

With a set of Denkmeier II's (2") already in use for my dob, I just received a Christmas Meade LX10.  It has an SCT 2" Mirror Diagonal (Antares 2 MDS) that comes with it. 

My question is based on some comments for set up that infer that a Barlow and a short light path to the binoviewers is ideal for optimal mirror position in the SCT.

For those familiar with Denks, can I just use the 2x OCS as a barlow to achieve this?

The 2" OCS will preserve your full aperture and will help control spherical aberration.  

 

I do not have ray traces for the LX10, so there is no definitive answer on when aperture reduction starts, but with a 2" diagonal, standard visual back, and binoviewer, most SCTs will lose as much as 1.5" of aperture.

 

Some is contingent on whether you  are using a Denk II with power switch. If so, the aperture loss  (if any) without an OCS would be greatest in low power mode, and depending on the scope, would have the least or none in high power mode. The good news though is that the low power mode would give you a much wider field of view than you could get otherwise and the power switch system makes for an efficient observing session. 

 

To best understand your own system, consider measuring the actual effect of the binoviewer by measuring the aperture with a laser. To do this, bring the system to best focus on a star with the binoviewer in place.  Don't touch the focuser now until after measuring. Place the OTA maybe a foot from a wall and square to it with the corrector facing the wall. Shine the laser through one of the eyepieces. This will project your entry pupil on to the wall and you can measure the size. This is the only real way to know how much (if any aperture) you would be losing. 

 

Based on that, you can make your decision on whether you want to spend the money to convert to a short light path, or use the OCS full time.

 

If you are viewing planets or the moon, I would always recommend that you use the OCS because the system can have meaningful spherical aberration without it.

 

Without knowing the exact behavior though, the only thing I can say is that in general, shortening the light path is always good when using a binoviewer in an SCT unless you are using some form of Barlow, Amplifier, or powerful GPC. As I mentioned earlier though, I do not have a ray trace of your instrument, and this information is based on the general behavior of commercial SCTs, and not specifically to your model. 

[Randy Blair]

The 2" OCS will preserve your full aperture and will help control spherical aberration.  

 

 

If you are viewing planets or the moon, I would always recommend that you use the OCS because the system can have meaningful spherical aberration without it.

 

Without knowing the exact behavior though, the only thing I can say is that in general, shortening the light path is always good when using a binoviewer in an SCT unless you are using some form of Barlow, Amplifier, or powerful GPC. 

Thank you Eddgie.  My 2" diagonal mounts right on to the back of the scope thus keeping a shorter light path.  I may mostly use the low power OCS to correct spherical aberration etc.  I am ok swapping eyepieces ranging from 32mm to 12mm (i do not have a power switch version). Then I can swap the low power OCS for the Richfield star sweeper reducer for more expansive targets like double cluster, beehive etc etc.

[Randy Blair]

The 2" OCS will preserve your full aperture and will help control spherical aberration.  

 

 

 

If you are viewing planets or the moon, I would always recommend that you use the OCS because the system can have meaningful spherical aberration without it.

 

Without knowing the exact behavior though, the only thing I can say is that in general, shortening the light path is always good when using a binoviewer in an SCT unless you are using some form of Barlow, Amplifier, or powerful GPC. As I mentioned earlier though, I do not have a ray trace of your instrument, and this information is based on the general behavior of commercial SCTs, and not specifically to your model. 

Will the Richfield Star Sweeper (Focal Reducer) reduce Spherical aberration just as a 2x OCS or Barlow?

[Eddgie]

Will the Richfield Star Sweeper (Focal Reducer) reduce Spherical aberration just as a 2x OCS or Barlow?

Focal reducers in general will increase spherical aberration in SCT or MCT with moving mirror focusing, but the question is always about how much it will introduce. 

 

This is from my own use of a reducer used with a C14. It was the Binotron low power reducer coupled used with a 2" diagonal.

 

In low power mode in the C14 (again, 2" diagonal and standard suppled 2" visual back) the aperture was reduced to something like 12.6" and the spherical aberration was nearly 1/2 wave. Now this is not the end of the world for low power observing, and the extra field of view for many might be considered a worthwhile tradeoff, but it was a lot of SA. 

With the high power mode in the same configuration, the aperture was (as I recall) reduced to 13.6 inches and he SA was around 1/6th wave. Again, this is going from memory.

 

I went to a short light path (Custom adapter to go from the power switch to the top of a 1.25" Baader Prism, and a short light path visual back and 15mm SCT to T2 adapter) and the improvment was very good, with (as I recall) about 13" in low power and 13.9 inches in high power with only a small a touch of SA. 

 

The best way to know how a configuration will change is to measure it yourself. You can use the laser method to measure the aperture, and you can model the spherical aberration using Aberrator 3.0

[Randy Blair]

Focal reducers in general will increase spherical aberration in SCT or MCT with moving mirror focusing, but the question is always about how much it will introduce. 

 

 

I went to a short light path (Custom adapter to go from the power switch to the top of a 1.25" Baader Prism, and a short light path visual back and 15mm SCT to T2 adapter) and the improvment was very good, with (as I recall) about 13" in low power and 13.9 inches in high power with only a small a touch of SA. 

 

The best way to know how a configuration will change is to measure it yourself. You can use the laser method to measure the aperture, and you can model the spherical aberration using Aberrator 3.0

Thank you very much.  Short light path will be what I will try.

[Randy Blair]

Using the low power ocs with the star sweeper on the bottom seems to be a good combo for my eyepieces.  Tack sharp and well corrected.