5 replies to this topic

[mayhem13]

So I’ve never owned a goto Dob and haven’t had a chance to get this out for first light with this weekend being no exception (we’re having a blizzard as i type). Not sure how accurate and smooth the goto and tracking will be at 4000mm. The scope needs a finder and I haven’t decided what to do…..would a simple 9x50 RACI up front do the job or should I mount an old 80mm frac on the bottom tube with a diagonal and crosshair eyepiece?

 

Also on Barlows?…..I’ll be using the ASI224mc with 3.7u pixels so with the 5x guidance I’m at F18.5. The native Fl and ratio of this scope is 1650 at 4.6. Equipped with a 4x Barlow gives me an f ratio of 18.4….right on the theoretically ideal for my camera……..is this ok or will it only work in excellent seeing? 

[CPellier]

When you try to choose a barlow for a particular setting, it is unavoidable to take into account the real amplification brought by all the optical train, not the nominal one. Most of the time barlows produce a noticeable increased amplification. The Powermate 4x from Televue will not so it should be fine. But the 5x will, as an example.

If you use an ADC - something which is highly recommanded! You will have to place it between the barlow and the camera, so it will increase again the real amplification, if the barlow/powermate is behaving like this.

[RedLionNJ]

In my opinion, the term "goto Dob" is disrespectful to the memory of John Dobson. A Dobsonian is a deliberately simple telescope design, no knobs or whistles. Your scope would more appropriately called a goto altaz Newtonian. I know very few people will agree with me on this, so moving on.... 

 

As Christophe implies, every Barlow is made a little differently. In your position, I'd be looking for 3 or 4 times amplification. It's really tough to figure out, in advance of purchase, how to achieve this, for the reason Christophe points out. Amplification in most Barlows is related not only to the specific optical design, but also to the spacing between the Barlow and the sensor.  If I was to use a traditional Barlow (not a PowerMate), I'd probably be looking at a nominal "2x" Barlow. There is a very good chance the spacing introduced by the ADC may increase the amplification to 3x or even more.

 

I don't think you're often going to get the maximum possible resolution out of a 14-inch scope here in the north-east. Maybe once or twice a year, for a few minutes each time.

 

A far better bet would be to end up at a focal ratio somewhere around f/13 to f/15.  On the rare occasions everything comes together to make a higher resolution possible, just upscale via drizzle-stack in AS!3.

[mayhem13]

Thanks fellas…..,the TV 4x is the Barlow i was considering it it appears that with the 30mm of additional distance created by the ADC, I’ll be right at the rated 4x mag so perfect for those rare nights when the seeing is just right.

[Sarciness]

You mean the 4x PM or 4x Barlow? The PM will be close to 4x but Barlow likely won't.

[mayhem13]

You mean the 4x PM or 4x Barlow? The PM will be close to 4x but Barlow likely won't.

Yes....the 4x powermate.