Interfacing an EOS 350D problem

[Scorpius]

Hi. I have a Canon EOS 350D and a SW880 10" with a 2" crayford focuser which I replaced the R&P .

I have the appropriate T rings adapters etc, however although the focal length with the crayford has been reduced by 10mm, The Canon is still out of the f range with the focuser fully in by about 25mm. By holding the camera face against the focuser face (no T ring/adapters) I can just focus in a 5mm range. (prime focus mode)

The distance between the sensor and the camera face is app 35mm which does not allow for the fitting of the T ring and adapter which adds app 25mm. (a total focal addition of 60mm)

On my homemade 8" I overcame this by raising the primary mirror 60mm, that in turn required the EPs to be fitted to an extension tube. I don't want to mess around with mirrors on the 10"
Any suggestions please short of buying a low profile focuser (Moonlite) ??:(

Happy New Year to all.

[iceman]

Hi Dave.

I've got a 10" newt and will be wondering the same thing in the near future (when I've got my newt EQ-mounted) (I also have a 350D). I also can't get my DMK to focus at prime focus for the same reason.

I've got the moonlite focuser, with I think a 1" spacer. I haven't actually experimented to find out how much in-focus i'm missing to be able to bring it to focus, so I can't actually help you at the moment regarding my moonlite. I'll do some experiments over the next week or so (when I get a chance to get the scope out again) and let you know.

I also want to avoid moving the mirror up the tube if possible, because I want a long focal length for planetary imaging, so I don't want to shorten it at all.. plus, the hassle of extensions for eyepieces etc, i'd rather avoid.

[asimov]

I had the same trouble with the 8" GSO newt (with standard R&P focuser) I use.

A bit hard to explain but I had to unscrew the 2" section off the top of the focuser, make an aluminium spacer ring to fit inside the drawtube & then fit a standard 2" - 1-1/4 reducer. I believe the previous owner Eric fitted a moonlite (or something similar) to overcome the problem.

I wouldn't have a clue if you can apply the same method to a crayford.

[Scorpius]

I have been doing some serious thought and calculations based on how one reaches the focal length in this case 1200mm for the SW880 The theory applies to both the R&P and Crayfords The average 10" dob R&P has a profile of 4" The GSO type Crayford has a profile of 3" and Moonlight's Low Profile a max of 2", based on the following calculations which are copied from an email I sent to Roger a local member. The outcome would necessitate the use of a longer drawtube (2.75") and a 50mm extension for the EP.

Quote

Apparently the problem I am experiencing is common with dobs, the owner of IIS forum has a sticky on the subject and seems that apart from raising the primary as I did on the 8" there is not much one can do. In my case a low profile Moonlite may resolve it with a few mm to spare. The focal length of the 10" with the R&P is as follows (total 1200mm)
Primary to secondary 915mm
Secondary to Tube 140mm
Tube to focal point, 145m = Total 1200mm
The R&P has a 4" (103.0mm) profile The Crayford GSO - has a 3" (76.0mm profile) this from the tube to the face of the 2" socket and does not include 1.25adapter

The CMOS sensor to the camera face is 35mm and the T-Ring and T-Ring adapter 25mm which gives me 60mm with the present setup which puts it 60mm beyond the focal point.

The T-Ring has an 1.25" insert held by 3 set screws which can be removed and by replacing it with a 2" barrel to go directly into the 2" focuser socket.
This would give me a camera profile of 35+15mm = 50mm

A Moonlite low profile 2" focuser has a profile of only 50mm to the 2" face fully in. and 100mm fully extended Thus with the focal point being 145mm from the tube the camera adding 50mm "should" focus around the center. "theoretically" It would mean of course adding an extension for the EP's to compensate for the loss.

Unquote
Late Update 6.1.07
I have since been in touch with Steve at JMI in CO USA http://www.jimsmobile.com/data_ev_preview_frame.htm
and the do an EV Focuser and one can choose the options in the "Build your own" section where you choose which features you want or wish to leave out, like reducers etc.

They make a PWM controller and software is available for around $140US.

But the motors can be be easilly controlled with a home brew 555 IC circuit which I have used for many years to test model servos and speed controllers on my scale model ships. With some good soldering and etching skills, the controller which will give a PWM output from <1RPM to >10,000RPM (a tad fast for the focuser) for around $25 of bits from Jaycar etc Its also possible to add limit switches and preset positioning if you are into electronics.
Cheers:D

[iceman]

I'm not sure i'll do anything about mine..

When I get an EQ6, i'll probably (in the future) get an ED80 (or equivalent) to do widefield DSO stuff with my 350D, as opposed to using the newt for DSO imaging.

[Scorpius]

Well here it is. No more bits of string and chewing gum. My engineer mate opened up the base hole of the old R&P mounting plate and drilled and tapped 4 x 4mm holes. Now the barrel of the GSO crayford slides inside and should now focus OK. I did a test on my favorite focusing tree and could just get it to focus with the drawtube fully racked out. Which tells me I should focus in space with about 5 to 10mm racked out.

Here is the picky of the mod

[iceman]

Excellent work Dave! Well done.

[Scorpius]

For step by step pictures of the mod please check http://members.optusnet.com.au/~mar.dav/dls006.html

Dave

[Eternal]

You could have also purchased a low profile Bintel 10:1 crayford focuser to solve the problem.

[Scorpius]

If you had looked at the information I have given. I did purchase a Bintel (GSO) so called low profile crayford. which has a racked in height of 3 inches to the 2" drawtube face. These are called lo profile, but at 3" they are not. Only Moonlite sells a low profile focuser which has a racking in height of 1.25 inches to the 2" drawtube face. Even the JMI lo profile focuser has a racked in height 2.5" and cost 3 X the price of a GSO

Remember also the original SW880 10" scope came with an R&P focuser which had a 4" fully racked in profile and focusing at 15mm out with a 25mm plossl. This monstrosity was used to compensate for the short tube length, the distance from the Primary to the secondary being only 915mm - The radius of the tube 140mm The total fl being 1200mm (I will leave you to work out the rest of the dimensions needed to achieve this)

This has been a long standing problem with dobs and camera interfacing (see other posts and stickies by the site administrator Iceman on this very subject) as the camera has, from the body face a focal length of 35mm to the sensor. By applying the formula in determining the focal point on a 10" F5 scope this means that the Canon 350D focuses with the focuser fully racked in and the camera face directly on the drawtube. Not allowing for T Ring and 2" adapter. (which adds 25mm) You will see how the figure of 60mm compensation is needed.

Changing from a 4" R&P to a Bintel GSO 3 only reduces the focal length by 25.5mm
The method I have used has lowered the focal point for prime focus by a total of 60mm This allows the camera with T ring and 2 " adapter to focus at a racked out position of 10-15mm above the focuser body.

I had a similar problem with an 8" dob I constructed and can assure you that other than moving the primary mirror up by 60mm. This is the only feasable alternative. It also requires that you have to extend the focal length by 60mm to bring the focal point back for conventional EPs'

I would also repeat that this is for Prime Focus Photography and not Afocal, which presents no problem at all. The solution was worked out mathematically, experimentally and in practise and it works I am sure you will appreciate that this has been thoroughly examined and confirmed by more knowledeable people than my humble self

Did you check my website for detailed photos and comment?

Have a nice day :)

[Eternal]

My bad!

[Scorpius]

Not all eternal, We are here to help guide and assist anyone who joins this forum. You will find many real willing and helpful people. Some of the members are professionals and also specialised in many fields which all have a bearing and useful input into this wonderful hobby.

As we say here, the only dumb or stupid question is the one that is never asked.