Back-Focus 101: Celestron EdgeHD: C9.25, C11, and C14 Basic Back-Focus for DSLR and CCD/CMOS Cameras 

Glad you think so, thanks!

Do you have a mirrorless camera?

No. It is a Canon DSLR. I have all the correct attachments per Celestron.

@chaprick0106 I assume you have the EdgeHD T-adapter for the larger scopes and not the smaller one for the 8”. In that case, your telescope with and without reducer should be at 146mm of back-focus and you shouldn’t have any issues achieving focus. The only thing I can possibly think of is the t-adapter isn’t the right size. Other than that, if you have the right t-adapter, t-ring and a standard DSLR, focus shouldn’t be an issue, and you might be running into another problem. I also assume you have the proper t-ring.

Yes, 42mm would be too small for that. Starizona makes a T-adapter that allows you to include a filter in the imaging train. It’s a clever design and very recommended.

Barlow lens back-focus has more to do with magnification than anything. The further the sensor from the Barlow, the greater the magnification factor. If a back-focus is specified for say a 2x Barlow, at that distance you will get 2x magnification. Closer in you’ll get maybe 1.8x, further out maybe 2.2x.

Video on this subject will be coming though.

Do you mean the T-Adapter is M42 and the Drawer is M48? If so, I assume you’re using the ZWO M42-M48 adapter, but it threads in too far?

@@AstroBlender Hey Cody, I think I found the solution, if I use two M42-M48 adapters next to each other, it holds/stops the spacer at the edge of the t-adapter.

Sounds good. Just make sure the side that has the spanner wrench cut outs is facing you in case you lock the threads up.

@@AstroBlender Good Point, Thanks for the advice.

Yes, you certainly could do that. The larger aperture of 48mm actually prevents vignetting on larger cameras because it allows more of the light to get to the sensor through the larger diameter.

@@AstroBlender Thanks Cody. So all I would need to get is a 48mm T ring for my DSLR. I'd rather have 2 rings knocking around than 2 T adapters, probably cheaper, too! You have the Williams Optics one, right?

For an Edge SCT I use a Celestron Edge T-Adapter and a WO 48mm T-ring

@@AstroBlender Thanks Cody! Clear skies & thanks for all your great videos

I got lucky and got it brand spanking new from a classified ad! These back-order days are tough!

@@AstroBlender You were so lucky. Over the past 6 weeks I just went through 3 focal reducers for my C11 EdgeHD from different retailers, all with threading issues. Each had the same issue of bad threads.Celestron took them back but now are having me wait until July-Oct for a new batch. Sounds like a common issue. So bummed to go through galaxy season without it.

Really?? That’s strange, it feels like an extremely quality part. I wonder if manufacturing these days is trying to make things so fast the QA has gone down? 3 of them though, seems unusual.

You know I totally smirked in my head when I said it!

hahaha

New to all this and I feel like an idiot but… I don’t get it?

The camera doesn’t set the backfocus requirement, the telescope and its lenses do.

No you would want to use the f6.3 reducer/corrector. The flattener in the EdgeHD sets the backfocus, this is not the case in the non-edge scopes.

Im not sure if you said what size your edgehd is, but I have the 9.25 version. Also, thank you for these great videos!

Glad to hear!

You can use the Starizona EdgeHD T-adapter. It has filter capability built into the T-adapter and would work great!

Thanks Zac, it’s why I put these videos together! I’m glad you find them helpful!

End of the threads on either the main scope or reducer to the sensor of the camera. Give or take a 1mm usually. CCD inspector works great if you want as much precision as possible. If not the 146mm should be fine in general.

Thanks Chris.

Glad it was helpful!

I don’t think it’s a waste of time at all. I love MCTs, such an easy to use scope. The reality is the slow focal ratio makes long exposure imaging take more time, increasing the probability of tracking errors, etc. For EAA you’ll still always get more color/detail even with short exposures then you would your eye, so I think it is just fine, personally. I haven’t done much EAA though, so take my opinion with a grain of salt.

@@AstroBlender Thank you. I'm looking forward to trying it out.

@@jimhackney4727 May I ask, what is EAA? I am a newbie and not familiar with this term. At least I don't remember it. Thank you.

@@Big.Ron1 EAA is Electronically Assisted Astronomy. It allows a person to see much, much more than is possible with the naked eye. The short answer is it is similar to AP but without all of the post-processing. Of course you also lose the end result of a fantastic photo. EAA uses the same gear as AP, but it allows a person to acquire an image very quickly. Instead of long exposures taken over a longer period of time, you capture images with much shorter exposures anywhere from 1 to 30 seconds in length. These images are improved by using software to do live stacking. Also, the contrast, sharpness, gain, and binning can all be adjusted in near real time. The results are quite good. If seeing is good, and if it's the right time of year, a person can view 10, 20, 30, or more objects in one evening. And all of the images can be saved to your personal library for sharing. You can also save the individual frames for post processing at a later date if you so desire. Since exposure times are greatly reduced, it is possible to acquire images using alt-az mounts although a GEM is still preferred and will yield better results. The same thing applies to an auto guide scope. It's not required, but it could help. Search online for EAA images. Cloudy nights has an entire forum dedicated to EAA. Some are stunning. Again, they will never be as good as a post-processed AP image, but they can be very nice. I think the main draw is the ability to see good results that can be shared with family and friends in near real time.

EAA is awesome for outreach. I still prefer an eyepiece for outreach but many people find the view underwhelming. Pop in a camera and all of that changed. Great explanation!

Thanks, Craig.

For planetary you don’t need to worry about it so much. I have a video all planned out about back focus for barlows and power mates but haven’t gotten around to it yet.

The 8” Edge has a back-focus of 133mm, or 5.25 inches.

Your English seems good to me! I’m glad you found it helpful.

I would say it matters because most filters have different coatings. You want the reflective side to face the telescope, and the less shiny side to face the camera.

No, the 8SE doesn’t have an integrated field flattener.

Agena Astro sells one, but I just use my calipers. They do the exact same thing.

For SCTs I would always use an OAG.

I would check it out, but the ASI Air Pro makes me use their Focuser too. It’s almost like Apple fan boy, but forced upon me by ZWO. :D

@@AstroBlender BTW, for planetary, what is the change in backfocus if I use an extender (let's say 2.0x) such as Powermate or Explore Scientific?

Absolutely! Check out the Celestron EdgeHD white pages paper or their other published documentation to see the performance decline as you move away from proper backfocus.

@@AstroBlender Thanks ! I'd love to see some actual Images taken with and without proper backfocus to "see" what I'm missing . LOL ./SRK

It’s pretty obvious, the off-axis performance degrades quickly. Zoom in on the stars on the edges of your image and you’ll see the distortion. That being said, a larger sensor will see greater effects since your getting more of the outer edge of the light cone. A smaller sensor you won’t see the effects as much.

You bet!

Is your C9.25 and EdgeHD? If it’s a standard you’re good.

@@AstroBlender is standard! So I relax a bit being the case, but why is the case… and thanks so much!

The EdgeHD has a built in two-lens field flattener in the baffle tube. The field flattener requires an exact back-focus to flatten the field properly. Too much distance or too little from the specified back-focus results in aberrations.

@@AstroBlender I just see this! thanks so much for the information, Now I feel good to have chosen the standard version to begin astrophotography!