Crayford style focuser for Vario-Finder 10x60 ; 61x250 f/4.1


Woody Schlom
 

Nick,

 

Once focused to your guide camera, that should be it.  Lock down the fine-focus with the retaining ring and call it done.

 

Woody

 

From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of Nick Iversen
Sent: Thursday, October 15, 2020 3:14 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Crayford style focuser for Vario-Finder 10x60 ; 61x250 f/4.1

 

I tried the Baader fine focus but in the end I bought a helical focuser which is much easier to use. Screwing and unscrewing the front lens to focus was a pain because it unlocks the lens which has to be locked again.

What is best practice for using the fine focus?


Mike Dodd
 

On 10/15/2020 7:07 PM, Woody Schlom wrote:
Once focused to your guide camera, that should be it. Lock down the
fine-focus with the retaining ring and call it done.
Lock it down is what I do. Some may smile at my method, but it works: <http://astronomy.mdodd.com/observatory.html#Autoguiding>

My 60mm f/5 guide scope no longer has a focuser mechanism, and the camera is bolted to a bracket on a dovetail plate. I loosen the bolt, and nudge the camera while my wife at the computer tells me if focus is better or worse.

It takes about five minutes. I tighten the bolt, and we're done for many months.

Clear skies.

--- Mike


CurtisC
 

On Thu, Oct 15, 2020 at 04:20 PM, George wrote:
It works great.
FWIW, I looked at the .fits files from projects earlier this year, when I was using the Baader Vario-Finder for guiding instead of the QSI's OAG.  The guiding results were at least as good as what I'm getting now.


M Hambrick
 

I have a question for Roland or George related to one of the previous threads that the Baader Vario-Finder works well as a guider for short focal length scopes. What would you say is the limit of focal length with this finder ? I ask because you have one of these Baader Vario-Finder scopes on your 175 F8 EDF at your Hawaii observatory. 

Will the 10 X 60 Baader Vario-Finder work with a 180 EDT ? I normally use it with a Quad TCC (effective f.l. 1166.4 mm)


Roland Christen
 

I don't know what the limit is. It's not like walking on a pier and when you get to the end you fall off into deep water. It's more like walking into a shallow beach, going further and further out until suddenly your head is under water and the shark is coming. Where that point is, is anyone's guess.

Rolando



-----Original Message-----
From: M Hambrick <mhambrick563@...>
To: main@ap-gto.groups.io
Sent: Fri, Oct 16, 2020 8:40 am
Subject: Re: [ap-gto] Crayford style focuser for Vario-Finder 10x60 ; 61x250 f/4.1

I have a question for Roland or George related to one of the previous threads that the Baader Vario-Finder works well as a guider for short focal length scopes. What would you say is the limit of focal length with this finder ? I ask because you have one of these Baader Vario-Finder scopes on your 175 F8 EDF at your Hawaii observatory. 

Will the 10 X 60 Baader Vario-Finder work with a 180 EDT ? I normally use it with a Quad TCC (effective f.l. 1166.4 mm)


M Hambrick
 

Thanks Roland

I am asking because George suggested to me that in order to avoid flexure, I should be doing my guiding with the Baader Vario-Finder attached to the imaging scope rather than to use a separate guide scope piggybacked on top. I already own the separate guide scope and dovetail plates, but I would have to purchase the Baader Vario-Finder. I don't want to buy one if if it is not accurate enough to use with the longer focal length of the 180.


CurtisC
 

If Rolando is using the Baader Vario-Finder as a guider on a 175 f/8, then I am well-reassured about resuming use of it on my TEC 140.

Incidentally, Rolando, a fellow Palomar Observatory docent told me you had bought a house in HI north of her.  She lives alternately in Waikoloa Village and Temecula, CA, and she seems to be well-acquainted with Christopher Erickson, who often posts here.  She gave me the name of the community, which I won't repeat here.  I looked in GE for a dome or roll-off, but I haven't found it :)


CurtisC
 

Mr. Hambrick: At least in earlier years, the recommendation was that the Vario-Finder bracket should be attached directly to the body of the focuser.  My TEC140ED has a Feathertouch 3545 focuser, which has screw holes for the special bracket that A-P sells.  I think A-P's focusers are similar.  The only slightly worrisome part is that the holes are very shallow, and the screws are very short.  They're fine with the Vario-Finder and bracket, but I wouldn't attach anything heavier.


Roland Christen
 

Yes, you should attach the guider to the imaging scope. I have always recommended that. A separate guide scope on top of the rings will not work because the imaging scope will be flexing inside the rings and be pointing in different directions with gravity.

Rolando

-----Original Message-----
From: M Hambrick <mhambrick563@...>
To: main@ap-gto.groups.io
Sent: Fri, Oct 16, 2020 11:40 am
Subject: Re: [ap-gto] Crayford style focuser for Vario-Finder 10x60 ; 61x250 f/4.1

Thanks Roland

I am asking because George suggested to me that in order to avoid flexure, I should be doing my guiding with the Baader Vario-Finder attached to the imaging scope rather than to use a separate guide scope piggybacked on top. I already own the separate guide scope and dovetail plates, but I would have to purchase the Baader Vario-Finder. I don't want to buy one if if it is not accurate enough to use with the longer focal length of the 180.


Worsel
 

This topic has come up on PHD2 forum.  The answer to your question involves more than just the focal lengths now that cameras and software are the norm for guiding.

The OLD rule of thumb when manual guiding was the only game in town was the guide scope focal length had to be at least 1/2 that of the imaging scope.  With the advent of autoguiding, the image scale of the two cameras comes into play.

Bruce Waddington posted some comments about requirements for guide scope and cameras on the PHD Stark Labs forum and copied to PHD2. Note the Stark Labs forum is no longer available

"Hi, this is Bruce.  I posted my response on the Stark Labs forum last night.  As I said there, what probably matters more is the relationship between the two image scales rather than two focal lengths – I think you already knew that.  The goal is for the tracking/guiding to be seeing-limited, you want to keep any centroid error well below that.  Beyond the simple stuff I described, it becomes a matter of your seeing conditions, the sensitivity of the guide camera and how faint the guide star is, the size of the star disks on your main system, etc.   I don’t think there’s any cookbook answer for this.  And if you’re imaging at long focal lengths (fine image scales), the issues of differential flexure will usually take over and most of this other stuff becomes irrelevant."

The following was part of the same thread

Bruce’s approach can be translated into a similar rule of thumb as the historical value with respect to focal length of the guide scope, if we accept a couple assumptions. This reduces the number of calcs needed.

 

GP = Guide camera pixel size in microns

GF = Guide camera focal length in mm

IP = Image camera pixel size

IF = Image camera focal length

 

Assumptions:

    1. Centroid accuracy of 0.2 pixels for the guiding software

    2. Movement of less than 1 pixel on the main image is the acceptable upper limit

 

Guider image scale = GP * 206.265/GF

Imager image scale = IP * 206.265/IF

 

Acceptable Movement on main image = 1 pixel = GP * 206.265/GF * 0.2

                                                                                       ---------------------

                                                                                 IP * 206.265/IF

 

 

 

Rule of thumb

Guider focal length > 0.2*(GP/IP)*IF

 

Bruce’s first example

 

Guider focal length > 0.2*(5.3/7.3)*1680 = 229

The 350 f.l. guidescope is OK

 

Bruce’s second example

Guider focal length > 0.2*(5.3/7.3)*2540 = 354

The 350 f.l. guiderscope is marginal

 

 Bryan


M Hambrick
 

Thanks Bryan Those are very useful formulas.

So if I plug my equipment specifications into the formulas I have:
GP = 7.4 microns (SBIG STi or ST2000-XM)
GF = 480 mm (This is my Tele-Vue Pronto that I currently use for guiding. It is piggybacked on top of the 180 EDT rings)
IP = 6 microns (SBIG STXL16200)
IF = 1166.4 mm (180 F9 EDT with 0.72X Quad TCC)

Guider focal length > 0.2*(7.4/6.0)*1166.4 = 287.7 mm

This would suggest that the 250 mm focal length of the 10 X 60 Vario-Finder is not long enough to use as a guide scope for the 180 EDT. So this raises yet another question; Which is the least bad option; The 250 mm Vario Finder mounted directly on the main imaging scope, or the 480 mm Pronto piggybacked on the rings of the 180 EDT ?


Sébastien Doré
 

Wouldn't adding a barlow to the Vario train get you the best of both world ?

Sébastien

_


CurtisC
 

Rolando has definite opinions about this.  As I understand it, the main scope is (usually) mounted in rings lined with felt.  As the scope moves, it moves around a tiny bit inside the felt lining.  But the guide scope, in the setup you describe, is mounted rigidly on top of the rings.  The main scope is moving within the rings.  The guide scope is not moving.  This introduces errors in the guiding.  The focuser itself, which is holding your camera, might move a bit.  The A-P bracket for the Baader mounts rigidly to the main casting of the imaging scope's focuser.  Now, in my case, using the Baader on my TEC140, I worry that the drawtube of the main scope's focuser may sag a bit as the main scope moves.  So the drawtube is moving, but the guide scope is not.  I haven't seen anything that I can definitely attribute to a sagging focuser drawtube.  But it's a worry.  


Mike Dodd
 

On 10/18/2020 1:48 AM, CurtisC via groups.io wrote:
Rolando has definite opinions about this. As I understand it, the main
scope is (usually) mounted in rings lined with felt. As the scope
moves, it moves around a tiny bit inside the felt lining. But the guide
scope, in the setup you describe, is mounted rigidly on top of the
rings. The main scope is moving within the rings. The guide scope is
not moving. This introduces errors in the guiding.
When I was tracking down differential flexure, I considered EVERYTHING that might move, summarized here: <http://astronomy.mdodd.com/flexure.html#FlexureTable>

* Replaced the ring felt with solid styrene.

* Moved the guide scope/camera from atop the OTA rings, because a ring itself can flex with that extra weight.

* Bolted the guide camera to a dovetail clamp so it didn't hang on the guide scope focuser (in fact my guide scope no longer has a focuser).

* Machined a 1/2" aluminum "backstop" for the ST-402 guide camera because its case flexed when attached with a single 1/4"-20 bolt in its tripod socket.

Almost certainly a lightweight CMOS camera hanging on a robust focuser will have little, if any, flexure. But the only way to be sure is to test various things until the flexure disappears.

I even bought from Optec a tube to temporarily replace my Pyxis rotator, in case the rotator was flexing (it was not).

FWIW.

--- Mike


Roland Christen
 

Piggyback on rings is not recommended for such a long heavy main scope.

Rolando



-----Original Message-----
From: M Hambrick <mhambrick563@...>
To: main@ap-gto.groups.io
Sent: Sat, Oct 17, 2020 9:45 pm
Subject: Re: [ap-gto] Crayford style focuser for Vario-Finder 10x60 ; 61x250 f/4.1

Thanks Bryan Those are very useful formulas.

So if I plug my equipment specifications into the formulas I have:
GP = 7.4 microns (SBIG STi or ST2000-XM)
GF = 480 mm (This is my Tele-Vue Pronto that I currently use for guiding. It is piggybacked on top of the 180 EDT rings)
IP = 6 microns (SBIG STXL16200)
IF = 1166.4 mm (180 F9 EDT with 0.72X Quad TCC)

Guider focal length > 0.2*(7.4/6.0)*1166.4 = 287.7 mm

This would suggest that the 250 mm focal length of the 10 X 60 Vario-Finder is not long enough to use as a guide scope for the 180 EDT. So this raises yet another question; Which is the least bad option; The 250 mm Vario Finder mounted directly on the main imaging scope, or the 480 mm Pronto piggybacked on the rings of the 180 EDT ?