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View Full Version here: : Do I really need to shorten my 10" Dob


Newbie79
31-08-2009, 08:49 PM
Hi,

I am going to be mounting my 10" Dob on a Eq6 mount with a view to getting into astrophotography. I have been told that I will need to shorten my Dob tube in order for a DSLR to be able to focus properly. Is this correct? Is there away around this?

I don't really understand why this is. Is someone able to explain it to me in very simple terms?

Thanks heaps

Matt

erick
31-08-2009, 08:56 PM
Matt, when you are using eyepieces, you look at a focussed image which is at the focal length from the primary mirror. It is basically somewhere inside the focusser draw tube, sort of below or just inside the bottom of your eyepiece. When you want to photograph, you want that focussed image to be at the CCD of the camera. However you mount your camera, that CCD will normally be some distance above the top of the focusser draw tube. Houston, we have a problem! Hence why people shorten the tube, meaning that the focal point moves further away from the central axis of the tube - ie. further out through the focusser draw tube. One issue that may arise is that the secondary mirror is no longer big enough to capture all the converging light cone and turn it 90 deg - you may lose some of the available light.

Options that may work - you have to experiment. 1) Use the primary collimating screws to push the primary mirror further up the tube - maybe even replace them with longer screws. You might get into all sorts of issues around the support springs. 2) Replace the standard focusser with a low profile focusser (GSO make them and Bintel/Andrews sell them - or ask here in IceTrades for a second-hand). Both those might get you there, depending on your camera and how you mount it.

You can still use the scope for eyepiece observing, but will have to add an extension tube to the top of the focusser draw tube.

stephenb
31-08-2009, 09:05 PM
Okay, here's what I know....

When you focus with an eyepiece in the focuser, you adjust the focuser up and down to acheive focus for your eye.

Your telescope has a fixed focal length, say for example, 1250mm (if it is an f5). So the distance of the light path from the surface of the primary mirror - to the secondary mirror - up the focuser - to the focal point at eyepiece is 1250mm...

Your focuser will have a fair bit of travel on both sides of focus, but...

because you want to attach a DSLR camera on the focuser in place of an eyepiece, some focusers are not able to give enough "travel" in their mechanism to allow the image to come into focus on the CCD chip of the DSLR. Sometimes all it needs is an extra 15-20 mm, at on end of the focuser travel.

There are 2 main ways to get around this:

1. Replace the focuser with a "low profile" focuser. This type of focuser has a reduced height on top of the OTA, allowing the attached DSLR to come into focus.

2. Move the primary mirror up the tube, closer to the secondary mirror. This causes the focal point to "shift" further out of the focuser, aloowing you to bring the DSLR into focus.


What you need to do is establish how much extra travel you need in your focuser, and in which direction. Do you need the focuser to go lower or higher, but you run out of travel in the mechanism?

Most likely, if you have a GSO/BinTel Dob, replacing the focuser with a low profile focuser will easily rectify the problem - and all that takes is undoing 4 screws.

Others please chime in here, but I hope I have helped in some way...

stephenb
31-08-2009, 09:07 PM
Eric can also type faster than I can :lol: that's sort of what I was trying to say.

erick
31-08-2009, 09:12 PM
But then I have to spend several edits correcting the spelling mistakes :D

DavidU
31-08-2009, 09:17 PM
Years ago I had a Russian Newt and I replaced the collimation screws with 40mm longer ones and used longer much stronger springs. This worked a treat and didn't have to cut the tube. I also cound easly return it to standard when I sold it.
Has any one done this?

Newbie79
31-08-2009, 09:18 PM
My DOB is a 10" Skywatcher with a focal length of 1200mm. Is there a way to work out if a low profile focuser will do the job before I go and purchase purchasing a DSLR. The thought of having to move the mirror scares me !!!!!!

DavidU
31-08-2009, 09:23 PM
A member on IIS Alchemy wrote this great artical.
http://www.iceinspace.com.au/63-441-0-0-1-0.html

stephenb
31-08-2009, 09:39 PM
David's quite correct. If you only need 15-20mm more travel in the focuser, then your primary mirror collimation screws need only be 15-20 mm longer.

Cutting the tube and moving the whole primary mirror and the mirror cell (the holder) is a last resort.

I did this on an 8" tube once. I had the choice of a low profile focuer of using longer screws. The longer screws were cheaper.

Tandum
31-08-2009, 11:06 PM
Matt,
I have a new GSO 10" dob sitting here in plastic. I have a new focuser coming from starlight, if I don't get focus with it I will cut the tube, there is no option. I'm not prepared to screw around with longer screws and changing retaining springs, It's easier to trim the tube.

If you have a dslr with live view just hold near the focuser and move it in / out to see how far away you are.

Newbie79
02-09-2009, 08:42 PM
Do these same problems with focus occur with a CCD camera as opposed to a DSLR

Ian Robinson
02-09-2009, 09:28 PM
I'm in the throws of rebuilding/upgrading my old 10" newt (f4.66) and it will go onto my New Atlux.
I'll also use my setup for imaging at prime focus with my 40D body as well as visual use.

I've used this model to calculate my upgraded OTA's design specs for astrographic and visual mode.

http://www.catseyecollimation.com/designie5.html (http://www.catseyecollimation.com/designie5.html)

You'll need to make the necessary measurements of your OTA and input these to the model , it'll spit out the answers you need , which you can then examine .
You'll also need to know registry distance for your camera and how much your camera adapter plate adds to that).

You can then investigate what happens when you change focusor , and move the mirror up and essential play with the model to get the design you think will be your best compromise for multi-purposes.


In my case my minimum tube lengths are :

VISUAL USE length = 49.381in = 1253mm

Spider depth is 95mm (min) from front of OTA (can be shortened if needed by cutting threadbar length to suit).

PRIME FOCUS w/ MPCC ONLY length required = 48.746in = 1238mm

PRIME FOCUS w / Esyguider + Lumicon CCFF length required = 47.866in = 1216mm

I have opted to build my OTA to suit the astrograph mode with Esyguider attached to the camera body., and the Crayford racked in fully less maybe 3 to 5mm (my focusor has provision to adjust my racked in height if I need to).
If I decide to use juist the MFCC attached to the camera body - I'll not need to fully rack in.
When I'm jsut usng eyepieces , I expect to be able achieve focus ,if I need to have more focusor travel up ,I'll simply buy a 2" x 1" extension tube for those eyepieces.

I have made provision to move the mirror cell up an inch or so if I need it too (difference between the Esyguider mode and MFCC mode).

No you do not need to male you tube shorter, but you may need to move the primary closer to the secondary to achieve focus at the camera chip, if you don't have enough prime focus point placed in a range of draw tube travel that you can get with your current focusor.

Another solution is maybe to install second focusor at convenient point on the same circumference line as your existing focusor that has a very low profile , maybe a nice 2" helical (if money is tight) ..., then you simply rotate the secondary to poiut at the second focusor , collimate and insert the camera attached to the coma corrector (a Baader is probably the best for this available now (that Lumcon discontinued making there great 2" coma corrector (designed to be used with their 2" Newt Esyguider) .

I have a 2" helical focusor stashed somewhere if you are interested that I bought from Lumicon many years ago and never used..

If you can't reach focus on the camera with your existing OTA and focusor , then you either need a lower profile focusor and / or to move the mirror closer.

Newbie79
03-09-2009, 08:07 PM
Thanks everyone for the info. Can someone tell me if the problems with achieving focus with a DSLR also apply to a CCD camera?

Regards Matt

DavidU
03-09-2009, 08:15 PM
I think you would find the CMOS sensor is 20mm or so further back because of the shutter mechanism mirror etc. So an astro CCD would be more forward somewhat

Newbie79
03-09-2009, 08:32 PM
Does this mean it is less likely that that tube would have to be altered if i were to use a ccd camera instead of a dslr?

DavidU
03-09-2009, 08:40 PM
In some instances.

Ian Robinson
03-09-2009, 09:20 PM
If you measure up all the distances between the key components of your ota , and know the registry distance from the bayonet to the sensor plain for your camera , or CCD , and model your ota you will quickly know what needs to be done to reach focus when imaging.

bmitchell82
04-09-2009, 01:39 PM
ill chime in here.
i have a SW 10"
I reach focus with the standard SW focuser
DO NOT USE A LP FOCUSER ON THIS SCOPE
why you will have it almost racked all the way out to gain focus.
your eps will not focus at all......
-experience speaks loudest.

i have had to modifiy the components that interact with the focusser but it depends on the camera you mount to the tube.

if you have a DSLR aka 40d is my exp, a Low profile gso/bintel will not work properly the dslr is too heavy for the focuser i cannot speak about the normal profile focuser

*how to reach focus with your dslr and your normal SW focuser...???

- The adaptor for the 1 1/2" ep's unscrew the 1 1/2" it will leave you with a canon t2 thread. purchase a canon eos adaptor (~30 bucks) this will then screw into where the nose piece was.

- connect the camera to the focuser and BAM WITCHA SPICE WHEEEZLE.

the camera will focus approximately 4-10 mm from the focuser which is perfect as the focuser housing is holding the most of the weight of your camera equipment hence your collmination holds true.

If you need anyhelp just send me a message and ill clear it up for you

Regards

Brendan.:thumbsup:

Newbie79
08-09-2009, 11:50 AM
Well after much advice from many people (Thanks to all of you) I have just ordered my NEQ6W from York Optical. Now i just have to wait its arrival.....

Thanks again for all the advice

Matt

DavidU
08-09-2009, 12:04 PM
Nice one Matt !