Jump to content


Photo

Why no commercial "Krupa Collimator"

  • Please log in to reply
12 replies to this topic

#1 daveyfitz

daveyfitz

    Explorer 1

  • -----
  • Posts: 82
  • Joined: 02 Jan 2013
  • Loc: Utah

Posted 14 February 2013 - 01:10 PM

I was looking at the DIY plans for the Krupa collimator, Krupa Revisited
and it looks as though it would be as accurate as a barlowed laser collimator.

(yes/no? - experts invited)

You have the same dispersed lighting of the center spot, returned to a viewing screen, after all.

I was wondering, if you added a mirror at a 45° angle, attached to the outer end, could you have a "viewable from the rear" centering screen?

Why is nobody selling this thing?
It looks like it would be very inexpensive to build.

Perhaps it would undercut their profits from the more expensive collimation tools?

#2 Steve OK

Steve OK

    Viking 1

  • -----
  • Posts: 843
  • Joined: 22 Sep 2007
  • Loc: OKC, OK

Posted 14 February 2013 - 01:36 PM

I've wondered the same thing. I've made a couple of them. Easy to build, very easy to use. I haven't done the barlowed laser thing since I made my Krupa. A Krupa collimator can built for just a few dollars, or just pennies if you have a good junk drawer!

#3 Jason D

Jason D

    Fly Me to the Moon

  • *****
  • Posts: 6635
  • Joined: 21 Oct 2006
  • Loc: California

Posted 14 February 2013 - 08:09 PM

But what about the secondary mirror adjustment?
Krupa will only adjust the primary mirror!!!
If cost is the issue here then why can't you get a quality collimate cap for $5!!! It can do a decent job on the primary mirror too.
FYI: Theoretically, Kurpa is not as good as barlowed laser.
Jason

#4 daveyfitz

daveyfitz

    Explorer 1

  • -----
  • Posts: 82
  • Joined: 02 Jan 2013
  • Loc: Utah

Posted 14 February 2013 - 09:17 PM

Hey, thanks for the response, Jason.

Krupa will only adjust the primary mirror!!!



Yeah, that's true. I was kind of thinking, with a solid tube dob,
that most nights I would just check the primary and go.
Krupa looks fast and accurate to me.

Secondary adj. isnt necessary every time, is it?

FYI: Theoretically, Kurpa is not as good as barlowed laser.
Jason



Can you back that up?

Looking at what's happening, with both the krupa and a barlowed laser, we have a reflection of the center spot of the primary,
on a screen with a spot in the middle. We center the image to adjust the primary.

The scale looks about the same.

Why would the laser be more accurate?

#5 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 43398
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 15 February 2013 - 07:09 AM

Yeah, that's true. I was kind of thinking, with a solid tube dob,
that most nights I would just check the primary and go.
Krupa looks fast and accurate to me.

Secondary adj. isnt necessary every time, is it?



Sometimes the secondary shifts, sometimes it doesn't. Sometimes the primary shifts, sometimes it doesn't.

If you are not using a Paracorr, it is less likely that the secondary will shift enough to cause an issue with the collimation but it happens.

I always adjust the secondary so it is spot on, then the primary. Why do the job half way?

As far as why it is not more popular... I can only guess. One reason is that it can only be used for the primary mirror. A second reason is that if one were to build a commercial unit similar in quality to say Howie Glatters products, it would cost about as much. Lasers are cheap, cheap and there's a lot of parts in the "focused Krupa Collimator."

Jon

#6 johnnyha

johnnyha

    Fly Me to the Moon

  • *****
  • Posts: 6500
  • Joined: 12 Nov 2006
  • Loc: Sherman Oaks, CA

Posted 15 February 2013 - 08:23 AM

What's with the big separate battery pack? :confused: And correct me if I'm wrong but this seems like more suited to a two-man operation where someone can adjust the primary while the other looks in the focuser.

#7 FirstSight

FirstSight

    Duke of Deneb

  • *****
  • Moderators
  • Posts: 9767
  • Joined: 26 Dec 2005
  • Loc: Raleigh, NC

Posted 15 February 2013 - 08:32 AM

FYI: Theoretically, Kurpa is not as good as barlowed laser.
Jason


Do you mean it's theoretically less accurate in its projection of the primary center spot (and if so, why) or else do you mean it's theoretically less effective in producing an easily usable image of the primary center spot? Speaking of the original Krupa design, the underlying basic principle on which it works is the same as a barlowed laser (incoming broad-enough cone of light of roughly even illumination to cover center spot and immediately surrounding area of primary mirror reflects back up optical axis, projecting image of the "shadow" of center spot up the optical axis since it's of different reflectivity than mirror surface).

Here's my best shot at seat-of-the-pants analysis/hypothesis: it's likely that the barlow-diffused laser beam is a more efficiently, evenly spread light beam than the LED for purposes of projecting a more sharply defined, more contrasty, and more easily seen shadow of the center spot (especially under different ambient light conditions). Is there a better, more technically accurate explanation for the claimed superiority of the barlowed laser vs the Krupa than the one I gave?

#8 daveyfitz

daveyfitz

    Explorer 1

  • -----
  • Posts: 82
  • Joined: 02 Jan 2013
  • Loc: Utah

Posted 15 February 2013 - 09:58 AM

the barlow-diffused laser beam is a more efficiently, evenly spread light beam than the LED for purposes of projecting a more sharply defined, more contrasty, and more easily seen shadow



That could be a good reason to say the laser is more accurate, IF it were true.

But I'm wondering, centering a slightly fuzzy annular ring, vs centering a sharper one, does that REALLY matter as to accuracy?
I think one could center the fuzzy one just as well.

Here are some images of lasers and krupas.

To my untrained eye, compared to the better lasers, they look essentially equivalent.

Some of the lasers look pretty bad.

Attached Files



#9 daveyfitz

daveyfitz

    Explorer 1

  • -----
  • Posts: 82
  • Joined: 02 Jan 2013
  • Loc: Utah

Posted 15 February 2013 - 10:49 AM

As far as why it is not more popular... I can only guess. One reason is that it can only be used for the primary mirror. A second reason is that if one were to build a commercial unit similar in quality to say Howie Glatters products, it would cost about as much. Lasers are cheap, cheap and there's a lot of parts in the "focused Krupa Collimator."



Jon:
Thanks for your input, but I STRONGLY disagree.

The laser must be centered with precision in the tube, it must be internally machined to point straight down the tube, with GREAT precicion.
It must be mounted with precision adjustment screws to facilitate collimation,
and then must be collimated at the factory.

The Krupa needs one precision machining. The hole that holds the LED must be in the center of the focuser.

PERIOD.

The LED does not need to point with much accuracy.
No adjustment screws, no internal machining, no collimation.

"Lasers are cheap" - LEDs are 10's of times cheaper.

I gotta think the Krupa would cost a fraction of the laser collimator to produce.

#10 Steve OK

Steve OK

    Viking 1

  • -----
  • Posts: 843
  • Joined: 22 Sep 2007
  • Loc: OKC, OK

Posted 15 February 2013 - 01:02 PM

While it is true that the Krupa can only be used for collimating the primary, the same is true of a barlowed laser. When I was using a barlowed laser, I would first align the secondary using the laser alone, then remove the laser, stick it in the barlow, and insert the combo into the focuser to collimate the primary. Now, I use the laser first for the secondary, then the Krupa for the primary. Same number of steps, and both methods need two pieces of equipment. The image of my center spot is quite sharp with the Krupa, and if the open end of the scope is covered I can collimate in broad daylight if I need to.

#11 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 22548
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 16 February 2013 - 06:49 PM

Lots of people buy cheshires from FarPoint and Catseye, and they are just used to align the primary. And, they too are tools you use by looking through a central hole, not from down below.

To those of you who can collimate a primary accurately from down below the primary, using the barlowed laser technique and a Blug or Tublug, more power to you. My vision isn't good enough to adjust the image from that far away. I have to stand up and look at it from closer to evaluate the image, or put my chair half way from focuser to primary and rock back and forth from focuser to primary to see the results of moving the collimation knobs.
For someone like me, having to move back and forth from the primary to the focuser is easy because my arms are long and I can easily reach around the bottom of the scope and turn a knob (the advantage of a scope with a 62.5" focal length).

I made a Krupa a few years ago, and it worked just as well as a barlowed laser and was just as easy to read. BUT, since I always collimate while it's still light, I found it a little faint for mid-afternoon use.
As far as I can see, it is exactly as accurate as a barlowed laser or cheshire so long as it is well-machined and well-fitted to the focuser.
I can see that, at night, looking directly at the light could be deleterious to night adaptation, but that's pretty much true of lasers too and besides the damage to night vision only lasts a short time after collimation.

The difficulty of putting a battery inside the unit, or putting a 45 degree window in the tool probably has hurt the chances of its making it into production anywhere. But I see no issues with it as a collimation tool if properly made.

A good cheshire is a heck of a lot easier to make, though, so the Krupa is just a more complicated way of making a primary-only collimation tool.
A laser and a barlow can do both mirrors. So can a combination sight tube and cheshire. The Krupa is just a primary tool, but, like a barlowed laser, usable in the middle of the night to check alignment, and somewhat less complicated than the 2-piece barlowed laser.

#12 Vic Menard

Vic Menard

    Aurora

  • *****
  • Posts: 4762
  • Joined: 21 Jul 2004
  • Loc: Bradenton, FL

Posted 16 February 2013 - 07:53 PM

I suspect if the Krupa had been popular before the Cheshire, we'd see a lot more Krupa's today. But the Cheshire was the first commercially available dedicated primary mirror alignment tool, followed by modified "peep" sights with white/reflective undersides, and later, but with some fanfare, the Barlowed laser debuted. There are a lot of primary mirror collimating tools available today!

Personally, I use a simple thin beam laser with a 1mm aperture stop for most of my axial alignment tweaks. The diffraction pattern surrounding the laser dot carries the silhouette of my triangular center spot back to the target where I get results similar to a Barlowed laser. And since the laser dot is still visible, I can assess and correct the focuser axial alignment too. One tool, both axes.

I haven't checked the laser with 1mm aperture stop for balanced operation (a very cool feature of "true" Barlowed laser collimators), but I suspect it will not be balanced (like the Cheshire and the Krupa). In my case, I'm not worried about it because the registration between my focuser and laser is reliably consistent.

And I'm like Don--I have a Blug (and a TuBlug) which I rarely use with my 88-inch long Newtonian. It's just too far to see the critical alignment necessary for my f/4 optics. They're very cool accessories to use with scopes that are about 3- or 4-feet shorter where I can get a closer look, but I'm comfortable moving back and forth to get my primary mirror alignment sorted out, so for my scope, I'm set. Of course, I collimate a lot of other scopes on any given observing evening, so it's nice to have the "cool" accessories...

#13 FlorinAndrei

FlorinAndrei

    Viking 1

  • -----
  • Posts: 867
  • Joined: 28 Sep 2010
  • Loc: California

Posted 17 February 2013 - 05:50 PM

"Lasers are cheap" - LEDs are 10's of times cheaper.


As an amateur, you can buy laser diodes in bulk for about $2 a piece. I'm sure you could get even better prices if you're a manufacturer and purchase regularly. So, I doubt this is what dictates the overall price.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics