Altair/GSO RC8 back focus & setup

[Andy274]

Hi guys, 

I've just acquired an Altair branded RC8 carbon tube. 

I'm trying to find the back fucus distance to chip from the backplate on the scope. 

I've got myself an astrophysics 0.67x telecompressor. I'm getting conflicting distances - I'm being told 254mm which is a mile long. 

If anyone has a pic of their configuration & or distances etc - that's be a massive help

Cheers

Andy

Edited April 2, 2020 by Andy274

[MarkAR]

Some confusing info lower down in the description, optimised at 85mm. Maybe that's what you need.

https://www.astro-physics.com/ccdt67

[Andy274]

Thanks but that doesn't really help. I've no idea how far away the focal point is from the rear of the scope with or without the reducer. I've seen different images of people's setup (all different) but no lengths i.e. 1"Spacer-tilt ring-focuser- reducer-spacer-camera vs similar with no tilt ring etc. 

Nobody can tell me what the distance to chip is from the backplate. We always seem to need to find the answers ourselves as it never ships with the scope from the manufacturer does it. 

The hunt continues

[Fellside]

Hi

Vlaiv has an RC8 and should be able to help. Im not sure he uses a FR though.

There a quite a few RC8 users on here.

I found this

http://www.astronomyforum.net/astro-imaging-forum/202749-problems-focusing-dslr-gso-rc8.html

If nothing else just a guess

Try two 25mm spacers native FL and one Spacer when the reducer is fitted. Get it working first and check collimation without the Reducer.

Graham

Edited April 3, 2020 by Fellside
Speeling

[Andy274]

I'll give that a go Graham thanks

Andy

[david_taurus83]

Have you asked Altair?

[Andy274]

HI David

yes, they confirmed 254mm which seems a very very long way from the back of the OTA

[david_taurus83]

That doesnt sound that much when you factor in the depth of whatever focuser your using. You could measure the distance between the mirrors and then from the secondary out the back of the OTA and tally up where the focal length lands.

[Fellside]

254mm looks about correct.  Have a look at GSO RC8 images on Google. That should give you an idea at what it will look like.

On my RC10 the Scope it is pushed forward on the mount and I have added an ADM Dovetail Side Mounting Counterweight to the front due to all the weight hanging out the back.

The reducer is not critical to spacing. I have read some users have had distortion in the corners if you push it close to 0.67. Spacing is 85mm rear flange to sensor for 0.67. So less than that should be fine.

Graham

[B4silio]

Hi,

This is my setup at focus, I'm also using the 0.67x reducer (which is pushed further inside the focuser).

I have exactly 85mm between reducer and sensor. The complete distance seems to be ~187mm for me (focus for H_a filter, pass-through would be a bit different).

I actually had to take out all the extension rings between plate and focuser as I could hardly get close enough for focus.

[Andy274]

B4silio that is exactly what I was looking for.  Much appreciated.  So if I wanted to have 0.75 I would reduce that to something like 50mm between the back of the focuser (by back of the focuser I mean the end closest to the camera) & the camera.  That 160mm from the backplate to the camera end of the focuser is crucial im guessing - but as stated by Fellside not so much between the focuser & camera - depending upon how much focal reduction I want.  The sweetspot for me would be between .75 & .63

cheers

Andy

[B4silio]

Hey Andy!

The reducer shortens the light path to the focus point from the secondary mirror to (hopefully) the sensor, so the stronger the reduction, the shorter the "real" distance. With the reducer at a distance corresponding to a .75x reduction, the distance from plate to the back of the focuser would need to be longer to compensate for the smaller shrinking happening inside the reducer. And if you have no reducer at all you'll start needing all the extender rings to get the sensor sufficiently far away from the plate (It would not surprise me if that was indeed the infamous 254mm you mentioned).

If you'll allow me some mad text-drawing skills:

• Sensor <----> 0.67x Reducer <-------------> Plate
• Sensor <--> 0.75x Reducer <---------------------------> Plate
• Sensor <------  no reducer  --------------------------------------------> Plate

The focuser itself can be anywhere on the right hand side of the schema, as long as it provides the proper overall distance. In my case I tend to push the reducer fully inside the focuser so that when screwing it in place I am not pinching the reducer lens, but this also allows me to have a bit of play with the optimal range of focus.

Hope this helps!

Basilio

EDIT: Added an actual drawing 

Edited April 4, 2020 by B4silio

[Andy274]

9 hours ago, B4silio said:

Hey Andy!

The reducer shortens the light path to the focus point from the secondary mirror to (hopefully) the sensor, so the stronger the reduction, the shorter the "real" distance. With the reducer at a distance corresponding to a .75x reduction, the distance from plate to the back of the focuser would need to be longer to compensate for the smaller shrinking happening inside the reducer. And if you have no reducer at all you'll start needing all the extender rings to get the sensor sufficiently far away from the plate (It would not surprise me if that was indeed the infamous 254mm you mentioned).

If you'll allow me some mad text-drawing skills:

• Sensor <----> 0.67x Reducer <-------------> Plate
• Sensor <--> 0.75x Reducer <---------------------------> Plate
• Sensor <------  no reducer  --------------------------------------------> Plate

The focuser itself can be anywhere on the right hand side of the schema, as long as it provides the proper overall distance. In my case I tend to push the reducer fully inside the focuser so that when screwing it in place I am not pinching the reducer lens, but this also allows me to have a bit of play with the optimal range of focus.

Hope this helps!

Basilio

EDIT: Added an actual drawing 

Hey Basilio, 

Thanks for this - I've been outside as its been clear & I've managed to get the scope in focus. Unfortunately I've got no bahtinov mask yet so can't get it dialed as precisely as id like. The collimation looks a little bit out but that's just a cursory impression looking at a de-focused star. I had to add an extra spacer between the backplate & focuser and I'm approx 50mm between focuser (which sits in the focuser) and chip. Attached is an image of the moon using my smartphone whilst it was up on screen. 

Again thanks for your help on this

Cheers

Andy

On 03/04/2020 at 15:37, B4silio said:

Hi,

This is my setup at focus, I'm also using the 0.67x reducer (which is pushed further inside the focuser).

I have exactly 85mm between reducer and sensor. The complete distance seems to be ~187mm for me (focus for H_a filter, pass-through would be a bit different).

I actually had to take out all the extension rings between plate and focuser as I could hardly get close enough for focus.

That is exactly what I'm looking for B4silio - absolute magic. 

[B4silio]

Hey Andy

Moon looks gorgeous! Even from a phone picture of a screengrab of a preview the details on the craters look like it's absolutely well focused, I suspect any difference in focussing would have less impact than the distortions due to viewing.

[Andy274]

Thanks Basilio,

Without a bahtinov mask I was getting as close as I can to focus I'm happy with.

The next challenge is checking collimation...which as we know is the big test 😱

Cheers 

Andy

[B4silio]

BTW, both APT and CCDCiel have focus aids that give you a read on FWHM so that can help you out as well.

Good luck with collimation, as you've seen I've been playing (and banging my head against a wall) with it, but I'm feeling reverse frustration ("Why on earth did I not do this 1 month ago!").

Clear skies!

Basilio

[Mannycas]

10 inches of backfocus from rear of main lens to tip of focuser.