Backfocus on an Edge HD 9.25

[StuartT]

According to the manual, the Edge HD 9.25" needs 146mm of back focus. I have a Diamond Steeltrack focuser attached to mine and that measures 130mm. My ASI2600MC has a sensor 17.5mm behind the front plate, and my OAG is 37mm thick so is this going to work?

(I find backfocus very confusing for some reason)

Edited October 26, 2022 by StuartT

[ollypenrice]

You have your focuser extended by about 20mm according to the scale. Is this 20mm extension intentional?

Olly

 

[StuartT]

46 minutes ago, ollypenrice said:

You have your focuser extended by about 20mm according to the scale. Is this 20mm extension intentional?

Olly

 

I generally try to put focusers at about half way when I have a motor focuser attached to give it leeway either side. You don't want a focuser trying to push up against a limit

(Sometimes in this hobby I feel such a dunce)

Edited October 26, 2022 by StuartT

[ollypenrice]

If the Edge does not have a moving field flattener element then the backfocus will be from the fixed back of the telescope, no? So can you ot lose the 20mm extension?

Olly

[gorann]

Why do you think you need a focuser on the back of the Edge HD? My impressions with my EdgeHD 8 and 11 is that their mirror focusers are quite good. The problem you will possibly run into with that extra focuser is that it will change the back focus distance when you focus and then also affect the flatness of your image. My Meade 14" ACF is a different story since it has an optical system that does not have a critical  fixed back focus for getting a flat field, so there I have a Steeltrack like you. But with the Edge you cannot deviate many mm before you loose your flat field. I assume you could get it very close and that extra focuser will help you temperature compensate, but the length of the image train you describe (inclusing the OAG) seems much too long for making it work, but you just have to wait for a clear night to try it out.

[gorann]

PS. I see on FLO's site that it says about the Steetrack that it is "Suitable for Celestron C14/C11/C9.25/C8/C6/C5.  Also suitable for Meade SCT/ACF telescopes with standard 2" SCT thread. " I think that may mean that is should not be used for the Edge series due to the short fixed back focus.

[ollypenrice]

10 hours ago, gorann said:

Why do you think you need a focuser on the back of the Edge HD? My impressions with my EdgeHD 8 and 11 is that their mirror focusers are quite good. The problem you will possibly run into with that extra focuser is that it will change the back focus distance when you focus and then also affect the flatness of your image. My Meade 14" ACF is a different story since it has an optical system that does not have a critical  fixed back focus for getting a flat field, so there I have a Steeltrack like you. But with the Edge you cannot deviate many mm before you loose your flat field. I assume you could get it very close and that extra focuser will help you temperature compensate, but the length of the image train you describe (inclusing the OAG) seems much too long for making it work, but you just have to wait for a clear night to try it out.

Good point. I didn't realize that the Meade and Celestron flat field scopes differed in this way.

If the moving mirror and Celestron's focus motor work as well on the Edge as they do on the RASA then there should be no problem.

Olly

[gorann]

44 minutes ago, ollypenrice said:

Good point. I didn't realize that the Meade and Celestron flat field scopes differed in this way.

If the moving mirror and Celestron's focus motor work as well on the Edge as they do on the RASA then there should be no problem.

Olly

Yes, the Meade ACF gets a flat field from the design of the corrector plate, while the EdgeHD accomplish this with a group of lenses, so a built in field flattener.

Edited October 27, 2022 by gorann

[StuartT]

14 hours ago, gorann said:

Why do you think you need a focuser on the back of the Edge HD? 

Because I need a motor focuser to be able to do astrophotography. That connects to the shaft of the Steeltrack's focuser.

13 hours ago, gorann said:

PS. I see on FLO's site that it says about the Steetrack that it is "Suitable for Celestron C14/C11/C9.25/C8/C6/C5.  Also suitable for Meade SCT/ACF telescopes with standard 2" SCT thread. " I think that may mean that is should not be used for the Edge series due to the short fixed back focus.

I'm not sure I understand this point. The Edge manual says the backfocus is 146mm. The Steeltrack is about 110mm long (I forgot I added the Baader clicklock in this photo which adds another 20mm or so. I shall remove that). So surely the focus will still be about 36mm behind (i.e. outside) the end of the focuser?

[gorann]

51 minutes ago, StuartT said:

Because I need a motor focuser to be able to do astrophotography. That connects to the shaft of the Steeltrack's focuser.

I'm not sure I understand this point. The Edge manual says the backfocus is 146mm. The Steeltrack is about 110mm long (I forgot I added the Baader clicklock in this photo which adds another 20mm or so. I shall remove that). So surely the focus will still be about 36mm behind (i.e. outside) the end of the focuser?

OK, I thought it would be like in the image, so 130 mm. My suggestion is that you put it all together and use the Steeltrack focuser to get to exactly 146 mm between the back edge of the scope and and the sensor of your camera. Then focus with the mirror-focuser until you get sharp stars. Then you can probably use the Steeltrack focuser to fine focus without loosing the flat field (the movement will probably be within the tolerance of the flattener. I hope that made sense.