22 replies to this topic

[PhotonHunter1]

I can't believe this is the only way to achieve focus on my ES102 APO Triplet...  My optical train consists of an ASI2600MM-Pro (with no tilt plate) + EFW-7 + M42 22mm extension (to achieve proper back focus) + Hotech 2" SCA FF + focus ring + two 1.5" extension sets + focus drawtube (drawn into to just above 0).  This is the only way I am able to achieve focus!  I've tried a direct connection between the filter wheel and the focusing tube, and I've tried using the Hotech with one extension however, both scenarios failed to achieve focus.  Below is a picture of the setup:

 

 

This is an awful lot of weight hanging off the back of my scope. Is there another way to achieve focus that I am not aware of or have not considered?  Thanks in advance for your help!

 

 

Edited by PhotonHunter1, 14 May 2022 - 08:43 PM.

[t-ara-fan]

Are you focusing on infinity or a tree down the street?

It does look a little wobbly.

[PhotonHunter1]

Are you focusing on infinity or a tree down the street?

It does look a little wobbly.

I was using Arcturus when running through the different scenarios.

[ColdyNitesNC]

I've always been able to achieve focus with just one extension tube on my ES ed102, but I have a different focuser. It looks like you have a lot of travel available on your focuser as pictured; are you sure one extension won't work?

[drd715]

If the focuser draw tube is all the way in (forward) you may want to get a different extention tube that will place the focuser about halfway out in its travel.  With the draw tube front opening so far up along the light cone it could be cutting (vignetting) some of the light that illuminates the sensor. This is more problematic in smaller diameter draw tubes and lower number focal ratios.

[PhotonHunter1]

I've always been able to achieve focus with just one extension tube on my ES ed102, but I have a different focuser. It looks like you have a lot of travel available on your focuser as pictured; are you sure one extension won't work?

Unfortunately one extension gets me close but not completely focused. To drd715's point above, maybe if I tried a single 2" extension that would get me to full focus and then some.

[PhotonHunter1]

If the focuser draw tube is all the way in (forward) you may want to get a different extention tube that will place the focuser about halfway out in its travel.  With the draw tube front opening so far up along the light cone it could be cutting (vignetting) some of the light that illuminates the sensor. This is more problematic in smaller diameter draw tubes and lower number focal ratios.

 

A bit out of scope (no pun intended) regarding this topic, but is there a refractor that would be better suited to the ASI2600?

[scadvice]

There are larger diameter extension tubes that mount in between the OTA and the focuser. This moves the focuser back, then you require less focal length in between the the focuser and camera, flattener, ect.  This provides a more stable tube assembly and eliminates vignetting. On my scope when I use the flattener/reducer (F/5.2) I must remove that extension and replace the focuser. This also applies using the diagonal with an eyepiece. When I want to use my flattener (F/7.0) I must replace that large diameter extension between the OTA and focuser.

 

You may want to call Explore Scientific and talk to someone knowledgeable about your scope and the equipment your using as they may have an extension just for that purpose.

[drd715]

A bit out of scope (no pun intended) regarding this topic, but is there a refractor that would be better suited to the ASI2600?

Now that question opens a real can of worms. Yes and no.  You have a fine scope for imaging subjects that match the framing that the camera sensor size and telescope provide. Actually two composition frame sizes if using a reducer flattener and non-reduced for a narrower image.   For imaging a scope with a larger diameter focuser draw tube can be helpful in holding a long heavy camera/ flattener/ filter wheel/ auto focuser/OAG combination.   It helps prevent "droop" in the optical train and has less chance of vignetting the light cone.   A great focuser is a real plus for imaging. For F-7+ or so a 2.5 Inch focuser is OK, but low Focal numbers and large sensors can benefit from a 3 inch or 3.5 Inch focuser draw tube.  Feather touch is nice.  And then a larger diameter focal reducer or reducer/ flattener such as the 82mm threading Ricardi is helpful for the lower focal numbers in combination with a larger draw tube focuser. 

 

Now to throw a little twist into the discussion about the spacers after the focuser;  some optical telescope tubes are made with a removable spacing section in the tube ahead of the focuser to accommodate extra light path when useing a binoviewer. As an opposite concept adding an extention section onto the telescope tube would move the focuser farther from the objective and eliminate the focuser extention tubes (if the space is just extra spacing and not flattener backfocus).  It is even possible to mount the flattener inside of the draw tube on large diameter focusers.

 

Once I came across at a star party an individual with a long imaging train behind the focuser. He had assembled a very long dove tail bar that went all the way back to the camera way behind the focuser. The camera was attached to the dove bar with its own ring. The ring base attached in a slotted groove and a Teflon slide. An interesting way to support the camera. 

 

I  think your current telescope is fine for imaging. 

[pointedstick]

I briefly had the carbon fiber version of that scope, with the same focuser. It's a terrible focuser even for visual, and I would not trust it at all for astrophotography. It's a weak Crayford that you can make slip just by pulling hard on it. I'd worry that your expensive electronics will fall out. There's a reason Explore Scientific ditched this focuser design with their newer scopes. The hex focusers they use now are supposedly much better.

 

The scope itself should be fine, but for imaging, I would strongly recommend replacing the focuser with a rack & pinion that's significantly beefier. You may be able to buy the newest hex version from Explore Scientific directly, if you don't decide to go with a superfancy Moonlight or Feathertouch model.

[ColdyNitesNC]

You also have to remember that this is an "essential series" scope, Explore Scientific's entry level triplet, so entry level hardware. There is a reason that Explore Scientific includes two extension tubes with the scope, to address a situation like yours. The fact that you have the two extension tubes in place, but your focuser appears to be nearly all the way in, isn't too bad, assuming your picture is showing a more or less focused scope.

 

pointedstick indicated your focuser is a Crayford; I was thinking it was rack and pinion. Is it a Crayford? I know people have upgraded this focuser, and sometimes with considerable effort involved to remove the old focuser due to the presence of a generous amount of glue. For that reason, I haven't tried to upgrade my hex, and secondarily, I don't have the money.

 

Even though I have only had to use one extension tube on my 102, I do have to use both extension tubes on my ED80, with my focuser around the number 10 position. This focuser is definitely a rack and pinion, as is the hex on my 102.

[Alan French]

When used straight through refractors have to have a good bit of out-travel or need an extension, because they are designed to also be used with a star diagonal. Designs may add a bit more room for a bino-viewer.

 

Clear skies, Alan

[XTATDSM]

Your camera back-focus seems to be a lot more than 55mm. Its been a while but it looks like your camera has 17.5mm then your filter wheel has around 20mm. The extension you have behind the filter wheel seems to be more than what you should have in there, that extra back-focus does effect your overall focus.

 

Good luck

[scadvice]

OK your distances going from the the CMOS (camera sensor) to the rear surface mounting on the EFW is 12.5mm. The EFW is 20mm.  The picture you posted appears to have mounting adapters on the EFW so the gap MAY be a little longer but not much (3 to 8mm??). Lets ignore that observation about the mounting adapters from the picture for now as the back focus is close enough and not the focus point issue we are working on. So, the spacer you stated is 22mm. That all comes out to 54.5mm. .

 

Likely the spacers and focus ring you installed are where the problem is. If this is true then my first post (#8) is probably valid. If it were me I would see if I can unscrew the OTA from in front of the focuser. Measure the threads in millimeters and the pitch and order a larger spacer (about 50 to 55 mm if I figured it right) to take up the longer focal point but give you focuser travel and a larger non-vignetted stable image train.

 

That's why I suggested that you call Explore Scientific and talk to someone knowledgeable about your scope and the equipment your using as they may have an extension just for that purpose. I'm not particularly fond of using the two inch diameter adapters either.

[droe]

These are easy to use instead of those screw-in extension tubes.

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[ColdyNitesNC]

Even though it might seem that along with the length of your optical train with the two extensions, focal reducer, filter wheel, and adapters, that you have a lengthy optical train, it doesn't really appear to be too outside the norm. Aside from the subjective feeling of it being on the long side, what do your actual measurements tell you? If you have 55mm from your focal reducer to your imaging sensor, then that is correct. What you need to do to achieve focus from there will dictate how many extension tubes you use, and in your case it appears to be two, as supplied by Explore Scientific.

 

What matters more, I would think, is what are the results you are getting with your imaging? Are you getting results that are unsatisfactory? You have the Zwo EAF, so I would think that would help to stabilize your focus. Now, there clearly are better after market focusers available than the stock Explore Scientific, but it's possible you might already have a workable solution that is in search of a problem.

[PhotonHunter1]

OK your distances going from the the CMOS (camera sensor) to the rear surface mounting on the EFW is 12.5mm. The EFW is 20mm.  The picture you posted appears to have mounting adapters on the EFW so the gap MAY be a little longer but not much (3 to 8mm??). Lets ignore that observation about the mounting adapters from the picture for now as the back focus is close enough and not the focus point issue we are working on. So, the spacer you stated is 22mm. That all comes out to 54.5mm. .

 

Likely the spacers and focus ring you installed are where the problem is. If this is true then my first post (#8) is probably valid. If it were me I would see if I can unscrew the OTA from in front of the focuser. Measure the threads in millimeters and the pitch and order a larger spacer (about 50 to 55 mm if I figured it right) to take up the longer focal point but give you focuser travel and a larger non-vignetted stable image train.

 

That's why I suggested that you call Explore Scientific and talk to someone knowledgeable about your scope and the equipment your using as they may have an extension just for that purpose. I'm not particularly fond of using the two inch diameter adapters either.

I think I'm following you - and appreciate your help, thank you! l have some free time tomorrow and plan on contacting ES. More to come.
 

[PhotonHunter1]

Even though it might seem that along with the length of your optical train with the two extensions, focal reducer, filter wheel, and adapters, that you have a lengthy optical train, it doesn't really appear to be too outside the norm. Aside from the subjective feeling of it being on the long side, what do your actual measurements tell you? If you have 55mm from your focal reducer to your imaging sensor, then that is correct. What you need to do to achieve focus from there will dictate how many extension tubes you use, and in your case it appears to be two, as supplied by Explore Scientific.

 

What matters more, I would think, is what are the results you are getting with your imaging? Are you getting results that are unsatisfactory? You have the Zwo EAF, so I would think that would help to stabilize your focus. Now, there clearly are better after market focusers available than the stock Explore Scientific, but it's possible you might already have a workable solution that is in search of a problem.

What's interesting is I had a similar set up, i.e., two extension tubes, flattener, filter wheel, etc. when I was using my ASI1600mm-c. Stars were round, guiding was 0.3 rms and my images were really nice. When I purchased the 2600 is when the issues started. Oblong stars (see initial post) and guiding that at best is 1.2 rms! The size and weight difference between the 1600 and 2600 are significant which is why I believe the issue has to be either the increased weight sitting so far behind the scope, or the sensor on the 2600 is tilted. I have some free time tomorrow and plan to contact ES - interested to see what they will say.
 

[ColdyNitesNC]

Did you allow for the additional 11mm setback of the sensor on the 2600 vs the 1600? I'm sure you probably did, but thought it might be worth mentioning.

[scadvice]

When I was setting up my optical trains I realized that there were three configurations I was dealing with . The first was "viewing"  and getting the focus point where it could  utilized the right angle prism to the eyepiece .  The FL is not changing it's length through a mirrored diagonal but through a prism it will by increasing 33% for the distance the light travel's through the prisms glass. 

 

The second configuration was the spacing between the the FF and the camera here it's 55mm. This occurs behind the focuser. If there is no focal point within it's travel then it needs to be move fore or aft ideally to the center of the focusers travel.

 

The third was that in order to have the correct focuser travel for decreasing the F ratio to F/5.2 that focal length needs to move toward the primary lenses because it focus point has shortened. This is where I remove the larger spacer in front of the focuser.

 

All these adjustments must occur before any diagonal and eyepiece or OAG,FF/FR the EFW and camera.  Why? Because the back face of the FF or RF is the starting point of the Backfocus and when you have that correct (55mm in this case) there can be not changes to it. All focus point positioning and adjustments have to occur in front of that.

Edited by scadvice, 17 May 2022 - 02:11 AM.

[PhotonHunter1]

Did you allow for the additional 11mm setback of the sensor on the 2600 vs the 1600? I'm sure you probably did, but thought it might be worth mentioning.

 

 

When I was setting up my optical trains I realized that there were three configurations I was dealing with . The first was "viewing"  and getting the focus point where it could  utilized the right angle prism to the eyepiece .  The FL is not changing it's length through a mirrored diagonal but through a prism it will by increasing 33% for the distance the light travel's through the prisms glass. 

 

The second configuration was the spacing between the the FF and the camera here it's 55mm. This occurs behind the focuser. If there is no focal point within it's travel then it needs to be move fore or aft ideally to the center of the focusers travel.

 

The third was that in order to have the correct focuser travel for decreasing the F ratio to F/5.2 that focal length needs to move toward the primary lenses because it focus point has shortened. This is where I remove the larger spacer in front of the focuser.

 

All these adjustments must occur before any diagonal and eyepiece or OAG,FF/FR the EFW and camera.  Why? Because the back face of the FF or RF is the starting point of the Backfocus and when you have that correct (55mm in this case) there can be not changes to it. All focus point positioning and adjustments have to occur in front of that.

Here's an updated image with the measurements:

 

The filterwheel is directly connected to the camera face. According to ZWO the distance from the camera face to the sensor is 12.5mm. The EFW-7 adds another 20mm and my extension tube is 22mm. I have separate various sized spacer rings to extend the back focus distance if needed.

 

The total distance from the threads at the back of the Hotech flattener to the sensor adds up to 55mm. The two 1.5" extension tubes in front of the Hotech flattener are needed to achieve focus. The set up is the exactly the same with my SFFR flattener/reducer. ES was closed when I called (5P CST). Will be calling earlier tomorrow.
 

[ColdyNitesNC]

I was thinking the distance from the camera face to the sensor is 17.5mm. Is the 17.5 that Zwo states on their site in error?

 

Oh, I see. If you remove the tilt adapter, then it is 12.5mm.

Edited by ColdyNitesNC, 17 May 2022 - 08:38 PM.

[scadvice]

I was thinking the distance from the camera face to the sensor is 17.5mm. Is the 17.5 that Zwo states on their site in error?

 

Oh, I see. If you remove the tilt adapter, then it is 12.5mm.

No you were correct but he has removed the tilt plate which is 5mm thick.